ICAM 2013 Programme and presentations

Contents

Programme

Monday, June 3

  • Opening and Invited lecture; Chair: Jože Rakovec
    9:30-10:15 Evelyne Richard HyMeX SOP1, the field campaign dedidicate to heavy precipitation and flash-floods in Northwestern Mediterranean
    (Short Abstract) ( Presentation; 7.0M)
  • Waves and rotors chair; Chair: Branka Ivančan Picek
    10:45-11:00 Miguel A. C. Teixeira Trapped lee wave drag in two-layer atmospheres
    (Short Abstract) ( Presentation; 1.0M)
    11:00-11:15 Johannes Sachsperger Dynamical aspects of the atmospheric wake of Madeira Island
    (Short Abstract) ( Presentation; 12M)
    11:15-11:30 Jeanne Colin A meteorological numerical model adapted to laboratory experiments : application to trapped orographic waves
    (Short Abstract) ( Presentation; 2.3M)
    11:30-11:45 Laurence Armi Stratified flow response to virtual topography
    (Short Abstract) ( Presentation; 3.8M)
    11:45-12:00 Matthew Hills Quantifying moisture perturbations leading to stacked lenticular clouds
    (Short Abstract) ( Presentation; 770K)
  • Foehn, Bora and windstorms; Chair: Michael Sprenger
    14:00-14:15 Kristian Horvath Downslope windstorms over the very complex orography: formation and development of pulsations
    (Short Abstract) ( Presentation; 4.7M)
    14:15-14:30 G.W.K. Moore The Novaya Zemlya Bora
    (Short Abstract)
    14:30-14:45 Sebastian W. Hoch METCRAX II - An upcoming field investigation of downslope-windstorm-type flows on the inner sidewall of Arizona's Meteor Crater
    (Short Abstract) ( Presentation; 4.7M)
    14:45-15:00 Georg J. Mayr Foehn diagnosis goes probabilistic
    (Short Abstract) ( Presentation; 1.7M)
  • Foehn, Bora and windstorms; Chair: Branko Grisogono
    16:30-16:45 Alexander Gohm Characterizing regimes of strong terrain-induced winds in the vicinity of the Hong Kong International Airport
    (Short Abstract) ( Presentation; 3.4M)
    16:45-17:00 Andrew Elvidge What really causes föhn warming? - A quantitative evaluation of the causes of leeside warming using the case study of the Antarctic Peninsula
    (Short Abstract) ( Presentation; 6.4M)
    17:00-17:15 Željko Večenaj Separation of the turbulence from the mean bora flows at the NE Adriatic coast
    (Short Abstract) ( Presentation; 2.4M)
    17:15-17:30 Manuela Lehner Initial simulations of flow over a small crater basin in preparation of an upcoming field experiment
    (Short Abstract) ( Presentation; 24M)
    17:45-18:00 Sebastjan Tičar Presentation of VNET

Tuesday, June 4

Wednesday, June 5

Thursday, June 6

Friday, June 7

   

Poster section

  • Air quality and Fire
    Haraldur Ólafsson Dust storms in the Icelandic highlands
    (Short Abstract)
    Rahela Žabkar Understanding and forecasting air pollution episodes in Slovenia
    (Short Abstract)
  • Boundary layer
    Daniel Leukauf Time scales of the atmospheric boundary layer evolution in an idealized valley
    (Short Abstract)
    Haraldur Ólafsson A remotely piloted aircraft for investigation of mountain flows
    (Short Abstract)
    Haraldur Ólafsson Inside the Reykjavík shelter
    (Short Abstract)
    Kristian Horvath Near-surface wind shear over the complex terrain: observational analysis
    (Short Abstract)
    Lavinia Laiti An investigation of the Ora del Garda wind in the Alps by means of Kriging of airborne and surface measurements
    (Short Abstract)
    Lavinia Laiti Numerical simulations of the Ora del Garda wind in the Alps: a comparison with surface and airborne measurements
    (Short Abstract)
    Maja Telišman Prtenjak Impact of mesoscale meteorological processes on the anomalous propagation conditions
    (Short Abstract)
  • Climate change
    Imran Nadeem Calculation of Snowline Climatology over the Alpine Region from ECMWF ERA-Interim reanalysis
    (Short Abstract)
    Jan Kysely Climate change scenarios of precipitation extremes in mountain regions of Europe based on ENSEMBLES regional climate models
    (Short Abstract)
    Marian Melo Daily air temperature range analysis for the Slovak Tatra Mountains part and scenarios up to the year 2100
    (Short Abstract)
    Massimiliano Fazzini Winter 2012-2013, exceptional snowfall in Trentino Dolomites: the example of Fiemme Valley
    (Short Abstract)
    Massimiliano Fazzini RECENT SNOW AND SYNOPTIC SITUATIONS BEARERS OF HEAVY SNOWFALL ON THE MOUNTAINS OF SICILY ISLAND
    (Short Abstract)
    Mojca Dolinar Climate change in Slovenia in the period of 1961–2011
    (Short Abstract)
    Nauman Khurshid AWAN The relevance of cut-off low systems to manifestation of large scale extreme precipitation events in the Alpine region
    (Short Abstract)
    Pavel Chernous LONG-TERM VARIATION OF LARGE-SCALE CIRCULATION AND ITS RELATION TO CLIMATE CHANGES AT THE KOLA PENINSULA
    (Short Abstract)
    Pavel Chernous AN INCREASE IN THE UPPER TREE LIMIT IN THE KHIBINY MOUNTAINS (KOLA PENINSULA, RUSSIA) AND CLIMATE CHANGE
    (Short Abstract)
  • Cold air pools
    A.N.Ross High resolution simulations of cold air pooling in small-scale valleys
    (Short Abstract)
    Allison Charland Idealized simulations of canyon exit jets in Utah
    (Short Abstract)
  • Convection
    Phillip Scheffknecht The Interaction of Convective Storms with Complex Terrain: A Case Study of an Alpine Supercell
    (Short Abstract)
    Stjepan Ivatek-Šahdan Extended intensive rainfall in Rijeka due to local orography
    (Short Abstract)
  • Field campaigns and Measurement
    Antonio Stanesic The impact of data assimilation of different observation types on forecast of severe precipitation in the northern Adriatic during Hymex IOP 16
    (Short Abstract)
    Manfred Dorninger Comparison of NWP-model chains by using novel verification methods
    (Short Abstract)
    Sebastian W. Hoch Slope- and Valley Winds and their interaction – Observations with Doppler Wind LiDARs during MATERHORN
    (Short Abstract)
    Sebastian W. Hoch Surface Energy Balance Observations during MATERHORN
    (Short Abstract)
  • Foehn, Bora and windstorms
    Andrew Elvidge What type of föhn event causes the highest melt rates on the Larsen C Ice Shelf, Antarctica?
    (Short Abstract)
    Danijel Belušić Longitudinal vortices in the bora wind
    (Short Abstract)
    Haraldur Ólafsson A new windgust record in a downslope windstorm in Iceland
    (Short Abstract)
    Haraldur Ólafsson A nice orographically generated wintertime temperature record
    (Short Abstract)
    Haraldur Ólafsson The Reykjavík 2012 downslope windstorm
    (Short Abstract)
    Haraldur Ólafsson On the downslope nature of wind extremes in Iceland and their variability
    (Short Abstract)
    Katarzyna Nurowska Very high resolution idealized simulations of foehn wind conditions in Tatra mountains
    (Short Abstract)
    Željko Večenaj On the turbulence integral scales for the bora flows at the NE Adriatic coast
    (Short Abstract)
  • Hydrology and Snow
    Florjana Ulaga M.Sc. Changes in surface water amounts considering climate change in Slovenian mountain region
    (Short Abstract)
    Grünewald Thomas Can a point measurement represent the snow depth in its vicinity? A comparison of areal snow depth measurements with selected index sites.
    (Short Abstract)
    Iztok Sinjur Characterisation of snowpack for better understanding of water balance in Julian Alps (Slovenia)
    (Short Abstract)
  • Mountain weather forecasting
    Guðrún Nína Petersen An Icelandic wind atlas
    (Short Abstract)
    Guðrún Nína Petersen Applications of cross-sections from operational high-resolution simulations for weather forecasting in mountainous areas
    (Short Abstract)
    Haraldur Ólafsson Visualization of real-time simulations of the atmosphere for education
    (Short Abstract)
    Kristian Horvath Background error matrix in the Adriatic region: Characteristics and seasonal variability
    (Short Abstract)
    Michael Sprenger Lagrangian Perspective of Orographic Blocking
    (Short Abstract)
    Vanja Blažica Balanced and unbalanced kinetic energy spectra in the limited area model ALADIN
    (Short Abstract)
  • Novel modelling
    Dieter Mayer A very high resolution real time diagnostic temperature model over the Alps
    (Short Abstract)
    Hálfdán Ágústsson Bridging the Meso-gamma and Micro-scales in complex orography using a modified version of the AR-WRF atmospheric model
    (Short Abstract)
    Stefano Serafin Boundary-layer phenomena in the vicinity of an isolated mountain: A climatography based on an operational high-resolution forecast system
    (Short Abstract)
  • Precipitation
    Annette Miltenberger Simulations of precipitation at a 2D mountain with the COSMO model - dependence on microphysical schemes and initial conditions
    (Short Abstract)
    Antonio Cardillo EXCEPTIONAL SNOWFALLS IN THE REGION OF MOLISE (CENTRAL ITALY) IN A CONTEXT OF EXTREMES OF CLIMATE
    (Short Abstract)
    Gregor Gregorič Application of limited area numerical model for regional drought monitoring
    (Short Abstract)
    Gregor Skok Analysis of movement of hail events and analysis of model indicators of hail in Slovenia
    (Short Abstract)
    Hálfdán Ágústsson The Öræfajökull precipitation experiment - ÖREX
    (Short Abstract)
    Hálfdán Ágústsson Comparison of observed and simulated precipitation with the mass balance of Mýrdalsjökull ice cap, South-Iceland
    (Short Abstract)
    Mario Marcello Miglietta Numerical simulations of sheared conditionally unstable flows over a mountain ridge
    (Short Abstract)
    Samantha Smith A new field experiment to observe orographic rain enhancement in the UK.
    (Short Abstract)
  • Turbulence
    Nevio Babić Evaluation of vertical eddy fluxes divergence for multiple bora events
    (Short Abstract)
    Željko Večenaj On the turbulence local isotropy during MAP IOP 15 bora event
    (Short Abstract)
  • Waves and rotors
    Alexandre Paci Topographically induced waves in a pycnocline: internal solitary waves and trapped orographic waves in the Toulouse stratified water flume
    (Short Abstract)
    Brigitta Goger Large eddy simulations of lee-wave interference over double mountain ridges
    (Short Abstract)
    Guðrún Nína Petersen Mountain waves encountered on 4 March 1974
    (Short Abstract)
    Johannes Sachsperger Dependence of boundary-layer separation regimes on stability, wind speed and surface friction: An analysis based on large-eddy-simulations
    (Short Abstract)
    Victoria Smith A spectral analysis of gravity waves above the Antarctic Peninsula
    (Short Abstract)

   

List of short abstracts

HyMeX SOP1, the field campaign dedidicate to heavy precipitation and flash-floods in Northwestern Mediterranean
Véronique Ducrocq CNRM/GAME, Météo-France, Toulouse, France
Evelyne Richard Lab. d'Aerolgie, CNRS & University of Toulouse, Toulouse, France
and the HyMeX EC-ISC Team

 

Corresponding author: Evelyne Richard
Section: Field campaigns and Measurement

The Mediterranean region is frequently affected by heavy precipitation events associated with flash-floods, landslides and mudslides each year that cost several billions of dollars in damage and causing too often casualties. Within the framework of the 10-year international HyMeX program dedicated to the hydrological cycle and related processes in the Mediterranean (http://www.hymex.org), a major field campaign has been dedicated to heavy precipitation and flash-floods from September to November 2012. The 2-month field campaign took place over the Northwestern Mediterranean Sea and its surrounding coastal regions in France, Italy and Spain. The observation strategy aimed at documenting four key components leading to heavy precipitation and flash-flooding in that region: (i) the marine atmospheric flow that transport moist and conditionally unstable air towards the coasts; (ii) the Mediterranean Sea as a moisture and energy source; (iii) the dynamics and microphysics of the convective systems; (iv) the hydrological processes during flash-floods.

During the field campaign about twenty precipitation events were monitored, including mesoscale convective systems, Mediterranean cyclogenesis, shallow-convection orographic precipitation. Three aircraft performed about 250 flight hours for a survey of the upstream flow, the air-sea fluxes and the convective systems. About 700 additional radiosoundings were launched either from HyMeX sites or from operational RS sites in Europe, as well as about 20 boundary layer balloons were launched to monitor the low-level flow over the Mediterranean Sea and the ambient atmospheric conditions. Gliders, Argo floats, drifting buoys and ocean soundings from vessels
monitored the Mediterranean Sea during the field campaign. Atmospheric and hydrological instruments such as radars, LIDARS, radiometers, wind profilers, lightning sensors, were deployed over 5 regions in France, Italy and Spain.

The presentation will present the general observation strategy and instrumentation deployed during the campaign, as well as the weather forecast component of the field operations coordination. An overview of the Intensive Observation Periods (IOP) will be then presented, together with first highlights on some observations and events.

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Trapped lee wave drag in two-layer atmospheres
Miguel A. C. Teixeira
Department of Meteorology, University of Reading, UK

Jose L. Argain
Department of Physics, University of Algarve, Portugal

Pedro M. A. Miranda
Instituto Dom Luiz, University of Lisbon, Portugal

 

Corresponding author: Miguel A. C. Teixeira
Section: Waves and rotors

Orographic gravity wave drag must be parametrized in large-scale weather and climate prediction models. But the parametrization schemes currently employed even in the most advanced models (for example, the ECMWF global model) only account for the drag produced by hydrostatic mountain waves and a bluff body drag associated with flow blocking. This is presumably done because simple formulas exist for these two drag components. However, a sizable amount of the existing mountain waves are not hydrostatic, and they will progressively become a larger fraction of the gravity waves not resolved explicitly by these models as their resolution increases. Among the non-hydrostatic waves, lee waves trapped either within a very stable layer near the surface (for example, associated with a nocturnal boundary layer) or at a temperature inversion (for example, capping a convective boundary layer) feature prominently, because they are inherently resonant, and thus may give important contributions to the drag exerted on the atmosphere at low levels.
Although trapped lee waves have been studied for simple flow configurations, such as two layer atmospheres, surprisingly the associated drag force has not been calculated for those configurations. The present study fills this gap, for the two-layer atmospheres considered by Scorer (1949), where the static stability is higher in a lower layer and lower in a higher layer, and by Vosper (2004), where the static stability is neutral in the lower layer and positive in the upper layer, but a temperature inversion separates the two layers. In both cases, it is possible to obtain closed-form analytical expressions for the trapped lee wave drag. While in the first case (lee waves trapped within the lower layer), the waves are internal and may have more than one mode, in the second case (lee waves trapped at the inversion), the waves are interfacial and only one mode is possible. This means that the drag is given by a sum of contributions from all modes in the first case, but by only one term in the second. The behaviour of the drag is explored as a function the key input parameters in these two cases, and it is found that the trapped lee wave drag may be comparable to, or even exceed, the drag associated with the internal waves that propagate towards infinity in the upper layer. It may also be substantially larger than the drag that would be produced if the atmosphere only had a single semi-infinite layer.

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Dynamical aspects of the atmospheric wake of Madeira Island
Johannes Sachsperger, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Vanda Grubišić, Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA & Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Rui Caldeira, CCM-Center for Mathematical Sciences, University of Madeira, Funchal, Portugal

 

Corresponding author: Johannes Sachsperger
Section: Waves and rotors

The mountainous island of Madeira lies approximately 850 km offshore from the southwestern tip of Portugal in the subtropical Atlantic and is well known for the formation of one of the most prominent atmospheric wakes. Under the influence of the Azores High, wake favoring conditions, such as strong marine boundary layer (MBL) inversions that lie well below the mountaintop at Pico Ruivo (1862 m) and sustained trade winds from NE (perpendicular to the mountain ridge), occur particularly during the summer months. During this period, an impressive wake resembling a Von Karman vortex street can be observed quite frequently in satellite images.

As part of an air-sea interaction study, an airborne measurement campaign (I-WAKE) was carried out in Madeira (Aug/Sep 2010), during which high quality in situ and remotely sensed data was collected in the upstream and downstream region of the island. This study focuses on the data from two research flights, flown on 2 September 2010, during which a strong unsteady vortex street was present in the MBL. Distinctive wake signals, such as sharp wind jumps in the wake shear zones and warm and dry lee eddies, were detected. In addition, the measurements in the wake reveal a strong anti-correlation of sea surface temperature (SST) and surface wind speed, due to reduced vertical mixing above the sea surface. High-resolution numerical simulations carried out with the Weather Research and Forecasting (WRF) model V3.3 are used to study the dynamics of the wake. The comparison with the observations shows that WRF reproduces the wake remarkably well, allowing us to use the model for further investigations.

The model results show the wake to be confined below the trade wind inversion. Strong potential vorticity (PV) is concentrated along horizontal shear lines, which originate from the flanks of Madeira and break up into an unsteady wake with vortex shedding further downstream. The shedding period is 7 hours. Above the trade wind inversion, gravity waves are generated in the continuously stratified flow. In addition to the wake in the MBL, we find another wake and a PV dipole due to wave breaking underneath a critical level at 4000 m a.s.l.

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A meteorological numerical model adapted to laboratory experiments : application to trapped orographic waves
Jeanne Colin, CNRM-GAME, Météo-France
Fabien Stoop, CNRM-GAME, Météo-France
Alexandre Paci, CNRM-GAME, Météo-France

 

Corresponding author: Jeanne Colin
Section: Waves and rotors

The meteorological research model Meso-NH is adapted to the simulation of flows obtained in hydraulic tanks. The main goals of this model is to offer a complementary tool to laboratory experiments and to promote collaborations between the meteorological research community and the fluid mechanics one.
Meso-NH is a non-hydrostatic atmospherical meso-scale model, typically used at resolutions ranging from 100 m to 100 km. The main changes we implemented here are the adding of the viscous diffusion forces and an explicit no-slip condition on the bottom surface.
This « adapted » version is validated against simple analytical solutions and laboratory experiments results. And as a first application of this tool, we study the impact of a surface boundary layer on trapped orographic waves.
For this matter, the simulations set-ups match those of a series of laboratory experiments carried out in the CNRM-GAME (Météo-France) hydraulic tank, where an axisymetric gaussian mount was towed in a two layers stratified fluid for different Froude numbers. These experiments were inspired by the findings of Elser, Rump and Johnson (2007) and their theorical formulation of moutain waves' shape and induced drag. A good qualitative agreement between the theory and the experiments was found, but with some quantitative discrepancies. This might be explained by differences between the ideal framework of the theory (perfect two-layers flow, no friction on the mountain surface) and the experimental -- and atmospheric -- reality. The numerical model is used to further explore this hypothesis. We focus here on the effect of the surface friction on the mountain waves. To this purpose, couples of simulations (with and without ground friction) were performed for different Froude numbers -- the no-slip condition is applied only on the mountain to reproduce the experimental set-up in the tank were the moutain was towed in still water.
More realistic results are obtained when the ground fiction is taken into account. It allows the developpement of a thin boundary layer which can grow on the lee-side of the mountain, thus affecting the shape of the orographic waves and the value of the induced drag.
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Stratified flow response to virtual topography
Laurence Armi, Institute of Geophysics and Planetary Physics, Scripps Inst. of Oceanography, University of California San Diego, La Jolla, California
Georg J. Mayr, Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria

 

Corresponding author: Laurence Armi
Section: Waves and rotors

A combination of real and virtual topography, as opposed to the real or actual topography alone, will be shown to describe the essentials of stratified flow over mountain ranges and lee side valleys. As viewed from the stratification of the troposphere downward, the strong stratification at the top of a cloud capped neutral layer can act as virtual topography. The virtual topography associated with the interface at the top of this layer, the bottom of which is in contact with the real topography, will dictate the response in the stratified flow above. Data from the T-REX field program will be used to elaborate this along with the important role of density differences upstream and downstream of the Sierra Nevada in establishing the descending downslope flow.
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Quantifying moisture perturbations leading to stacked lenticular clouds
Matthew Hills, University of Washington
Dale Durran, University of Washington

 

Corresponding author: Matthew Hills
Section: Waves and rotors

Lenticular clouds of any significant depth usually display a layered structure in the vertical. Several potential mechanisms that may generate these layers have been suggested in the past, but there has been no definitive description of their formation, quantitative analysis of the potential mechanisms, or observations of the environments that these layers form within. Here, we investigate the source of the layering using high-resolution 2D numerical simulations of stacked lenticular clouds, based on an event of these clouds that occurred over the Pennines, England, in December 2011.

Model results show that the addition of layered moisture perturbations in the upstream model sounding leads to a similarly layered structure in the cloud mass. Relative humidity perturbations within these upstream layers only 0.5% drier than the surrounding environment are sufficient to generate striations of a similar size to those commonly observed in nature (creating an indentation of approximately 160m into the lateral edges of the cloud). Increasing the magnitude of the humidity perturbations increases the scale of the layering, with 400m indentations forced with a 1.5% relative humidity decrease in the drier layers.

These layers are generated without any associated layering of the static-stability profile. Further, the cloud lies within a smooth vertical displacement field, implying that the layers are not caused by variations in vertical ascent, and are instead solely due to the layered moisture field. These simulations confirm an earlier hypothesis by Richard Scorer as to the source of the layering, and apply a quantitative estimate as to the magnitude of the moisture perturbations required.

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Downslope windstorms over the very complex orography: formation and development of pulsations
Kristian Horvath, Meteorological and Hydrological Service, Gric 3, 10000 Zagreb, Croatia
Zeljko Vecenaj, Geophysical Institute Andrija Mohorovičić, Horvatovac bb, 10000 Zagreb, Croatia
Branko Grisogono, Geophysical Institute Andrija Mohorovičić, Horvatovac bb, 10000 Zagreb, Croatia

 

Corresponding author: Kristian Horvath
Section: Foehn, Bora and windstorms

While severe northern-Adriatic downslope windstorms are since long in the focus of interest, strong bora winds in the hinterland of mid-Adriatic coast are much less studied, yet frequent and eqaully severe phenomena. The predictability of these events is considerably lower than for its northern counterpart due to the inflow complexity induced by the upwind chain of secondary orographic steep mountain sub-ranges and deep valleys.

A strong late-winter anticyclonic bora event at the mid-Adriatic was analyzed with the use of ultrasonic measurements, a SODAR and numerical sensitivity experiments carried out with the WRF model. The three-dimensional bora flow was characterized by a shallow bora layer, a pronounced directional vertical wind shear, and interaction with valley circulations in deep valleys. Two regimes of pulsations were found: i) Regime A – pulsations observed predominantly during the night and morning hours with periods of 5 – 8 minutes and ii) Regime B – pulsations observed predominantly during the afternoon with periods of 8-11 minutes. According to the model simulation, pulsations of regime A propagated far away from the point of origin, while pulsations of regime B quickly dispersed and dissipated during the stable nighttime conditions. The roles of gravity-wave breaking, Kelvin-Helmholtz instability and surface fluxes were analyzed to study the formation and development of pulsations. Furthermore, the secondary orography appeared as essential for the propagation of the bora flow away from the primary mountain range, by promoting the hydraulic-jump type of flow recovery. Finally, main differences in the bora subtle structure, there and over the northern areas, the latter pertaining to more known bora cases, are pointed out.

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The Novaya Zemlya Bora
G.W.K. Moore
Department of Physics
University of Toronto

 

Corresponding author: G.W.K. Moore
Section: Foehn, Bora and windstorms

Novaya Zemlya is a large and mountainous island in the Eastern Arctic that separates the Barents and Kara Seas. Weather station data indicates that surface wind speeds in excess of 15m/s occur approximately 50% of the time during the winter months. The air-sea interaction that occurs within a polynya that forms along the eastern shore of the island is thought to play an important role in Arctic thermohaline circulation and the water mass transformation of the incoming Atlantic water that passes by the island enroute to the central Arctic Ocean.
Although it has been proposed that a bora is responsible for these high winds, there have been no quantitative analysis of these winds and their impact on the environment.
Here we use the recently completed Arctic System Reanalysis (ASR) with its 30km spatial resolution to provide the first detailed high-resolution climatology of the surface wind field in the Novaya Zemlya region. The highest surface wind speeds are found on the western side of the island during easterly flow that is associated with a low-pressure system centered over the western Barents Sea. The high wind events are associated with a reversal in the zonal wind direction with height.
We show that the vertical structure of these high wind events shares many characteristics with idealized models of downslope windstorms associated with environmental critical layers as well as observations of the Yugoslavian Bora. In this regard, the high static stability of the upwind flow over the ice covered Kara Sea acts to increase the effective height of the topographic barrier thereby contributing to the acceleration of the flow that on the lee side of the island.
The highest wind speeds are most commonly found in the region where dense water is observed to form and we show that during high wind events, there is an approximate doubling, as compared to winter mean values, in the magnitude of the turbulent heat transfer from the ocean to the atmosphere. It is therefore proposed that the winds associated with this bora and the concomitant intense air-sea interaction contributes to the dense water formation in the Barents Sea.

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METCRAX II - An upcoming field investigation of downslope-windstorm-type flows on the inner sidewall of Arizona's Meteor Crater
C. David Whiteman, University of Utah
Sebastian Hoch, University of Utah
Manuela Lehner, University of Utah
Allison Charland, University of Utah
Matt Jeglum, University of Utah
Rich Rotunno, National Center for Atmospheric Research
Tom Horst, National Center for Atmospheric Research
Steve Semmer, National Center for Atmospheric Research
Bill Brown, National Center for Atmospheric Research
Ron Calhoun, Arizona State University
Norbert Kalthoff, Karlsruhe Institute of Technology
Bianca Adler, Karlsruhe Institute of Technology
Roland Vogt, University of Basel

 

Corresponding author: Sebastian W. Hoch
Section: Foehn, Bora and windstorms

The recently initiated METCRAX (Meteor Crater Experiment) II research program has as its main goal the improvement of understanding of hydraulic-analog atmospheric flows that produce downslope-windstorm-type events. The overall research program will combine modeling with field research to improve understanding of these flows. This presentation will focus on the design of a field program to investigate katabatically driven hydraulic-type flows at Arizona's Meteor Crater in a one-month experiment scheduled for October 2013.

Arizona's Meteor Crater is a near-circular basin with a diameter of 1.2 km and a depth of 170 m. The crater's rim projects 30-50 m above an extensive surrounding plain, which is tilted upward to the southwest. During clear undisturbed nights, a shallow mesoscale drainage flow comes down this plain from a collection of plateaus and mesas (the Mogollon Rim) to interact with the crater topography. Hydraulic flows over the crater's rim lead to occasional downslope-windstorm-type events on the inner southwest sidewall of the crater, and a hydraulic jump sometimes forms locally over the sidewall. These katabatically driven events were discovered serendipitously in a previous field program at the crater, but the characteristics of the flows were not well observed with the instruments deployed during those experiments.

The METCRAX II field measurements have been designed on the basis of initial observations and model simulations, and will determine the changes in the approaching katabatic flow that lead to the intermittent downslope-windstorm-type events. Two tall instrumented towers, a radar wind profiler/RASS, a Doppler SoDAR, a vertically pointing wind lidar, rawinsondes, and tethersondes will measure the changing dynamic and thermodynamic characteristics of the approaching flow. The channeling of the flows around the crater and over the rim will be measured with a Doppler lidar, tethersondes and automatic weather stations. The downslope-windstorm-type flows that form intermittently in the lee of the crater rim will be sensed remotely with two additional Doppler lidars, and lines of temperature and pressure sensors. Much of the meteorological instrumentation will be operated continuously during the 1-month-long experimental period. Nighttime Intensive Observational Periods (IOPs) will provide additional detailed flow information.
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Foehn diagnosis goes probabilistic
Georg J. Mayr, Institute of Meteorology and Geophysics, University of Innsbruck
David Plavcan, Institute of Meteorology and Geophysics, University of Innsbruck
Achim Zeileis, Department of Statistics, Faculty of Economics and Statistics, University of Innsbruck

 

Corresponding author: Georg J. Mayr
Section: Foehn, Bora and windstorms

Diagnosing the occurrence of foehn at a particular has evolved from manually searching time series of temperature, relative humidity and wind speed and direction to objective methods (Vergeiner, 2004). What remains particularly difficult is to distinguish nocturnal downvalley or downlsope flow from relatively weak foehn winds. We will describe an objective scheme that can also deal with such situations by ascribing a probability to the occurrence of foehn.
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Characterizing regimes of strong terrain-induced winds in the vicinity of the Hong Kong International Airport
Alexander Gohm, Institute of Meteorology and Geophysics, University of Innsbruck, Austria
Lukas Umek, Institute of Meteorology and Geophysics, University of Innsbruck, Austria
Victoria Fetz, Institute of Meteorology and Geophysics, University of Innsbruck, Austria

 

Corresponding author: Alexander Gohm
Section: Foehn, Bora and windstorms

The Hong Kong International Airport (HKIA) is frequently affected by strong terrain-induced winds that develop downstream of the mountains on Lantau Island. These winds occur when the large-scale lower tropospheric flow is from the wind sector east to south west, often with a capping inversion located slightly above crest height and a less stable layer below. The resulting flow pattern in the vicinity of HKIA is rather complex and may be characterized by downslope windstorms, gap flows, rotors, vortex formation and shedding. Associated turbulence and rapid change in the mean wind speed and direction along the flight path are aviation hazards to the aircraft flying into and out of HKIA. In recent years, a series of case studies have been conducted based on the dense observational network at HKIA as well as high resolution numerical simulations to better understand these phenomena. However, a systematic study that relates various possible flow regimes to the atmospheric background state is missing. Such a study could be the basis for a simple and efficient forecasting tool that predicts a certain flow regime based on the predicted background state represented in a coarse resolution numerical weather prediction model and a regime diagram complied a priori by a series of high resolution semi-idealized simulations.

Our goal is to develop such a regime diagram. In a first step, we apply a single-layer shallow-water model (SWM) to the terrain of Lantau Island. Compared to a conventional three-dimensional numerical model, the SWM is less computationally intensive and allows for a higher number of different simulations. Hence, the effect of a change in the upstream background state – represented by the upstream Froude number (Fr), the layer depth (H), and the flow direction – can be explored in more detail. We find that the highest flow variability in space and time is associated with vortex shedding and occurs especially for southerly flow (within the range Fr=0.15 to 0.6 and H=0.25 to 1.2 km) and less for easterly flow. Shedding occurs only downstream of the two highest mountain peaks for moderate Froude numbers (approx 0.2 to 0.4) and layer depths of about 1 km. For higher Fr and lower H the predominant vortex forms at the southwestern edge of the island. The vortex size (between about 2 and 10 km) and the shedding period (between about 10 and 60 min) is a function of the background state. Strong gap winds and associated jets and wakes near HKIA occur for Fr between 0.2 and 0.6 and H smaller than 0.8 km. In a second step, we will evaluate our SWM results for selected regimes with three-dimensional large eddy simulations.
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What really causes föhn warming? - A quantitative evaluation of the causes of leeside warming using the case study of the Antarctic Peninsula
Andrew Elvidge, UEA
Ian Renfrew, UEA
John King, BAS
Andrew Orr, BAS
Mark Weeks, UK Met Office

 

Corresponding author: Andrew Elvidge
Section: Foehn, Bora and windstorms

Leeside warming during westerly cross-barrier flow across the Antarctic Peninsula is investigated and, for the first time, a thorough quantitative comparison of the importance of the various mechanisms responsible for föhn warming is presented. Three föhn events (A, B, C) during the Austral summer of 2010/11 have been observed in aircraft observations and simulated at high resolution by the UK Met Office Unified Model. Warming contributions are calculated using a novel approach which makes use of data derived from a Lagrangian model (Lagranto). In Case A, isentropic drawdown (due to the differential advection of air from aloft, as a result of low-level upwind flow blocking) constitutes the dominant warming mechanism for 5 out of 6 sample regions. In Case B, due to the sourcing of föhn air from low level, moist regions upwind (in association with a relatively linear flow regime), latent heat provides the greatest warming contribution. For two Case C sample regions and the remaining Case A sample region, sensible heat due to turbulent mixing – a mechanism that has previously been overlooked or underestimated – provides the greatest contribution due to the combination of a relatively low upwind source region and a relatively dry air mass. The implication is that there is no dominant föhn mechanism. Which mechanism dominates depends upon the relative humidity of the approaching air mass and the linearity of the cross-barrier flow regime.
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Separation of the turbulence from the mean bora flows at the NE Adriatic coast
Željko Večenaj, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Danijel Belušić, School of Mathematical Sciences, Monash University, Melbourne, Victoria, Australia
Branko Grisogono, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia

 

Corresponding author: Željko Večenaj
Section: Foehn, Bora and windstorms

Bora is a downslope windstorm that blows at the Eastern Adriatic coast from the northeastern quadrant, most often during winter seasons. It possesses a wide spectrum of average wind speeds, and due to its gustiness the speed maxima may surpass 60 m/s. During a bora event, the turbulence is strongly developed in the lee of the mountains. So far the bora wind has been intensively studied in terms of its spatio-temporal statistics as well as macro- and meso-scale dynamics, but its turbulence characteristics still remain relatively unexplored.

In order to investigate turbulence, a suitable time/space averaging scale has to be deployed in order to define turbulent perturbations which are then used for calculation of turbulence statistical moments. This study addresses the turbulence averaging scale determined by applying several different approaches on the four cases of bora events using three different data sets. These four events are: winter bora case that occurred in November 1999 and was recorded offshore above the Adriatic Sea by the National Center for Atmospheric Research Electra aircraft, two winter bora cases that occurred in February 2005 and January 2006 and were recorded by the single point ground based measurements in Senj and Vratnik Pass, and summer bora case that occurred in July 2010 and was recorded on the three-level tower on Pometeno brdo in the hinterland of the city of Split. According to the results obtained from those four typical bora episodes, it appears that there is no unique averaging scale for determination of the bora turbulence statistics.

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Initial simulations of flow over a small crater basin in preparation of an upcoming field experiment
Manuela Lehner (University of Utah)
Richard Rotunno (National Center for Atmospheric Research)
C. David Whiteman (University of Utah)

 

Corresponding author: Manuela Lehner
Section: Foehn, Bora and windstorms

Measurements taken during the METCRAX (Meteor Crater Experiment) field campaign in 2006 revealed the occurrence of nocturnal downslope-windstorm-type flows in the almost circular and approximately 1-km wide and 170-m deep basin of Arizona's Meteor Crater. The plain surrounding the Meteor Crater slopes slightly upward to the southwest toward the Mogollon Rim. This sloping surface leads to the regular development of a drainage flow on the plain during clear-sky nights. The southwesterly drainage flow impinges on the crater rim, which extends about 30-50 m above the plain, causing the build-up of a cold-air pool upstream of the crater rim. Under certain conditions the interaction of the drainage flow with the crater topography causes downslope-windstorm-type flows in the lee of the upwind crater rim, with increased wind speeds, an intrusion of warmer air from above the plain, and a hydraulic jump in the upwind half of the crater basin.

Model simulations are run using CM1 in preparation for a second field campaign (METCRAX II) at the Meteor Crater in fall 2013, which will focus on the formation and evolution of these downslope-windstorm-type flows in the crater basin. The primary goals of the simulations are to study the impact of the crater topography, of the wind and temperature structure within the approaching drainage flow, and of the temperature structure in the crater on the development of the downslope-windstorm-type flows and to provide guidance for the deployment of the field equipment. In this presentation we will show preliminary results from these simulations, including the flow behavior upstream of the crater. As the drainage flow interacts with the crater topography and with the cold-air pool upstream, part of the flow splits around the crater, while part of it goes over the crater rim. The simulations show that the flow in the lee of the crater rim depends strongly on the vertical wind and temperature profiles within the approaching flow, indicating the importance of correctly modeling the drainage flow and the surface inversion over the plain.
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Boundary-layer characteristics over complex terrain observed by remote-sensing systems during HyMeX
Bianca Adler: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT)
Norbert Kalthoff: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT)

 

Corresponding author: Bianca Adler
Section: Boundary layer

The evolution of the convective boundary layer (CBL) in complex terrain varies significantly from the evolution over homogeneous terrain. Thermally-induced circulations and advective processes impact the CBL growth and cause interactions between the boundary layer and the layers above. As part of the HyMeX field campaign in the Western Mediterranean Sea 2012 the KITcube observation platform was installed at the mountainous island Corsica. KITcube combines various in-situ and remote-sensing systems that allows to measure the interactions of different thermally-induced circulations and the CBL growth.

During the day upslope winds caused strong subsidence in the valley atmosphere, which suppressed the CBL growth. Furthermore, they advected moisture from the lower part of the valley atmosphere along the slopes towards the ridges into the free atmosphere and resulted in the formation of clouds. Above the CBL vertical movements were measured by remote-sensing systems. Occasionally, the movements got coupled with CBL convective cells which resulted in a effective vertical mixing of moisture from the CBL into the free atmosphere above. A conceptual model of the typical diurnal evolution of the valley atmosphere and exchange processes is presented.

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Sensing the thermal PBL evolution in complex terrain using a passive microwave profiler
Ivana Stiperski, University of Innsbruck
Giovanni Massaro, University of Innsbruck
Mathias W. Rotach, University of Innsbruck

 

Corresponding author: Ivana Stiperski
Section: Boundary layer

Boundary layers in non-flat terrain are characterized by complex spatio-temporal evolution connected with diurnal variations due to valley- and slope-wind systems, layers of different stability caused by secondary circulations, inhomogeneities due to local terrain characteristics etc. Such complex boundary layer evolutions are impossible to resolve with standard daily or at best three-hourly radio-soundings. Airplane or tethered balloon measurements during specific field campaigns also give only limited information due to their finite duration and scope.
A passive microwave temperature/humidity profiler provides continuous measurements of mean temperature and humidity profiles at high temporal resolution able to resolve sub-synoptic scale phenomena and the rapid temperature/humidity evolution associated with them. With its high resolution and vertical reach it out-performs radio-soundings and other types of measurements in this respect.
As part of the Innsbruck Box (i-Box) project, a platform for studying boundary layer processes in complex terrain, such a passive microwave temperature/humidity profiler (HATPRO) is operated for the first time, in very complex terrain over a longer period of time (spanning several years). In line with the governing philosophy of i-Box, the profiler is operated continuously, mapping the changing PBL vertical structure at very high temporal resolution and adequate vertical resolution. It also provides information in the middle of the valley atmosphere where measurements are scarce or non-existent. Given that the profiler measures instantaneous volume-integrated temperature and humidity structure directly above the measurement location it is also an especially valuable tool for model validation. However, a particular challenge lies in the fact that statistical algorithms HATPRO relies on for retrieving the profiles are limited to the range of atmospheric conditions for which they have been trained. In this contribution we will address the advantages and especially challenges of using HATPRO in complex terrain. The instrument was trained on the basis of a long database of nearby radio soundings (over 4000) and accuracy for different height ranges of the profile measurements was compared to that resulting from the instrument’s “standard calibration”. In particular, we will address the instrument’s ability to detect (multiple) inversions and show novel strategies to improve this accuracy and assess their potential. First results of measurements in Innsbruck (i.e., probing the Inn Valley boundary layer) started last in autumn of 2012 will be presented.

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Flow Interactions Observed by a Very High-Resolution Surface Weather Station Network in Complex Terrain
Matt Jeglum, University of Utah; Sebastian Hoch, University of Utah; C. David Whiteman, University of Utah; Frank W. Gallagher, US Army Dugway Proving Ground

 

Corresponding author: Matt Jeglum
Section: Boundary layer

Dugway Proving Ground, host to the observational portion of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program, operates a permanently installed array of 51 automated weather stations (miniSAMS) aligned on a roughly rectangular grid with a grid spacing of one mile. The dataset includes 1-minute averages of temperature, pressure, wind speed, wind direction and solar radiation at 2 m height and wind speed and direction at 2 m and 10 m heights. The array sits in a broad, flat-floored basin surrounded by mountainous terrain with relief on the order of 500 m in all directions except northwest. To the northwest lies nearly flat terrain, including unvegetated playa (dry alkali flats which fill with water seasonally to form shallow lakes).

The relatively high spatial and temporal resolution of the array allows investigation of atmospheric boundary layer structures that are either poorly observed or unobserved using typical surface networks. Data from the array shows signatures of synoptic cold fronts, diurnal slope and valley flows, and turbulent wakes, often interacting with one-another in complex ways.

The array data was analyzed for the MATERHORN Fall campaign period (25 Sept. through 21 Oct. 2012), but one illustrative day will be investigated as a case study. On 23 October 2012 a synoptic cold front associated with a 700-mb shortwave trough crossed the miniSAMS grid in the late afternoon, leaving moderate northwesterly winds in its wake. Shortly thereafter a second, weaker boundary propagated across the grid from the northeast, with northeasterly winds behind it. This boundary displayed traits common to downslope windstorms. This boundary was followed by the return of the pre-frontal southwesterlies, with a coincident rise in temperature despite the nighttime hours. Finally, northwesterlies again propagated across the grid, accompanied by a drop in temperature. Through the remainder of the night a confluence zone was maintained over the array, with northwesterly winds on the west side and northeasterly winds on the east side. As the confluence zone wavered back and forth, individual stations sifted from NW to NE and back.

The high spatial and temporal resolution of the miniSAMS observations offers a unique opportunity to characterize and study the contribution of differently forced flows and their interactions. The knowledge gained can be applied to meso- and micro-scale processes that occur elsewhere in the world.

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Reconstruction of high-resolution meteorological fields from airborne observations: a comparison of different techniques
Lavinia Laiti - University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy
Dino Zardi - University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy
Massimiliano de Franceschi - Major Seminary, Diocese of Bolzano-Bressanone, Bressanone, Italy

 

Corresponding author: Lavinia Laiti
Section: Boundary layer

Light aircrafts represent valuable measurement platforms for atmospheric research, for they are able to provide high temporal and spatial resolution observations of the atmosphere. In particular, they are very suitable tools for the observation of atmospheric boundary-layer (ABL) structures in complex terrain, which are typically characterized by a fine-scale, fully 3D variability. Indeed, thanks to their good maneuverability, they allow to perform both vertical profiles and horizontal surveys. Various techniques have been used in the literature to remap data taken along flight trajectories onto regularly spaced, high-resolution 3D grids. In the present contribution the application of a geostatistical interpolation technique called Residual Kriging (RK) is proposed for the mapping of airborne measurements of scalar quantities. In RK the dominant (vertical) drift component underlying the original data is first extracted from the original data to filter out local anomalies, then the residual field is separately interpolated through an Ordinary Kriging algorithm and finally added back to the drift. In RK the determination of interpolation weights relies on the estimate of the characteristic covariance function of the residuals through computation and modeling of their semivariogram function. RK implementation inherently accounts for the characteristic anisotropy of the target field, and it also allows for an estimate of the interpolation error. A dataset from flights of an equipped motorglider, exploring ABL structures in the valleys near the city of Trento (in the southeastern Italian Alps) on fair-weather summer days, is adopted as test-bed database. RK method is used to reconstruct 3D high-resolution fields of potential temperature and water vapor mixing ratio for specific vertical slices of the valley atmosphere, integrating also measurements from the nearest surface weather stations (when present). From RK-interpolated meteorological fields, fine-scale local features of the valley ABL developing in connection with the occurrence of thermally-driven slope and valley winds are detected. RK’s performance is tested against the performances of other mapping methods commonly adopted for this application, i.e. methods based on Inverse Distance weighting or on Delaunay triangulations. A comparative evaluation, carried out by means of cross-validation procedures and in terms of physical plausibility of results, identifies RK as best-performing technique.
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Numerical simulations of boundary-layer phenomena and urban-scale processes in the Alpine city of Trento
Giovannini Lorenzo, Atmospheric Physics Group, Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento, Italy
Dino Zardi, Atmospheric Physics Group, Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento, Italy
Massimiliano de Franceschi, Atmospheric Physics Group, Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento, Italy and Major Seminary, Diocese of Bolzano-Bressanone, Bressanone, Italy
Fei Chen, National Center for Atmospheric Research, Boulder, CO

 

Corresponding author: Lorenzo Giovannini
Section: Boundary layer

Simulations with the Weather Research and Forecasting (WRF) model coupled with an urban parameterisation scheme are performed to evaluate the alterations caused by the urban area of Trento on boundary-layer processes in the Alpine Adige Valley. Simulations, with 500-m grid spacing, focus on a typical summer sunny day, when both valley winds and the urban heat island are well developed. Specific gridded datasets of urban morphology parameters and anthropogenic heat flux releases were created to provide high resolution input information to the urban scheme. Validation of numerical results against measurements from surface weather stations shows that the model is able to simulate reasonably well the development of valley winds, as well as the complex interaction occurring north of Trento between the local up-valley wind of the Adige Valley and an incoming lake breeze from a tributary valley. The urban heat island of the city is also well captured by the model, with strong intensities at night and low values in the central hours of the day. It is found that the city inhibits the development of the ground-based thermal inversion at night, especially in the city centre, displaying a denser urban morphology and higher buildings. Furthermore comparisons with an idealised simulation, where all the urban land use grid points are replaced by cropland, suggest that the city also affects the development of valley winds, modifying both the typical down-valley wind in the early morning, and the interaction between the up-valley wind flowing in the Adige Valley and the lake breeze. Finally sensitivity tests are performed to analyse in detail the impact of the gridded datasets of urban morphology and anthropogenic heat flux releases on the near-surface temperature and wind field.
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Flow over partially forested ridges
Andrew N Ross, Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, UK
Tim Baker, Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, UK

 

Corresponding author: Andrew N Ross
Section: Boundary layer

Many areas of complex terrain are at least partially covered by forests. Interactions between the flow in the forest-canopy and in the boundary layer above are important on the local scale for understanding transported process in forest canopies and interpreting measurements from flux towers over forests. They are also important on the larger scales as the canopy flows impact on orographic drag. Previous theoretical and modelling work has focussed almost exclusively on fully forested hills with uniform canopies. Here we present results from numerical simulations, supported by theory, to study the effect of partial forest cover on the local flow and the orographic drag caused by flow over idealised two dimensional hills. The placement of the forest on the hill can have a large impact on the local flow and transport over the hill. This will have important implications for placement and interpretation of flux tower measurements over complex, heterogeneous terrain. The placement of the forest can also lead to significant differences in the orographic drag caused by the hill. For large scale hills where there is a scale separation between the horizontal scale of the topography and the lengthscale over which the flow adjusts at the forest edge that the effects of the hill and the forest edge can be considered separately. This gives a simple method of predicting the drag over a hill with an arbitrary forest cover and could form the basis of a new drag parametrisation including the effects of partial forest canopy.

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A climatological analysis of the “Ora del Garda” wind in the Alps
Lorenzo Giovannini (1,2), Lavinia Laiti (1,2), Dino Zardi (1,2) and Massimiliano de Franceschi (1,2,3)
(1) University of Trento, Department of Civil, Environmental and Mechanical Engineering, Atmospheric Physics Group, Trento, Italy
(2) National Consortium of Universities for Atmospheric and Hydrospheric Physics (CINFAI), Rome, Italy
(3) Diocese of Bolzano-Bressanone, Major Seminary, Bressanone, Italy

 

Corresponding author: Dino Zardi
Section: Boundary layer

The “Ora del Garda” is a coupled lake and valley breeze, which flows very regularly, during clear-sky days in the warm season, from the northern shorelines of Lake Garda. Hence it propagates into the valleys nearby, until it reaches, through an elevated saddle, the River Adige Valley north of the city of Trento, where it interacts with the local up-valley winds, producing a strong and gusty flow.
Typical features of this wind are outlined by means of a climatologic analysis of data from two representative surface weather stations – one located on the shores of lake Garda, and one 30 km inland – covering a 10-year period. Furthermore, to assess the impact of the synoptic wind on the development of the lake breeze, wind speed and direction at 850 hPa level from reanalysis are analysed.
Clear lake-breeze days are identified in the period April-September by means of a set of objective criteria, based on the analysis of surface radiation and observations of wind speed and direction at the two surface stations.
It is found that the Ora del Garda develops on the shores of the lake in about 70% of the days from April to September, with higher values in summertime. Moreover in about 90% of days in which the lake breeze develops on the lake shores, it also reaches the inland weather station. Average wind strengths on Lake Garda shores are in the order of 4-6 m s-1, with maximum intensities reaching frequently 10 m s-1. The wind typically starts blowing on average at 1100-1200 LST (but the onset time is progressively delayed from spring to summer), and lasts until 1700-2000 LST (with earlier cessation times occurring more frequently in April and September). The lake breeze takes on average 3.5 h to get to the inland weather station, where it lasts until 2000-2100 LST.
The analysis of wind speed and direction at 850 hPa from reanalysis highlights that the lake breeze is on average stronger and its duration is slightly longer when the synoptic wind blows onshore. Moreover the lake breeze propagates faster, and arrives earlier at the inland weather station, under synoptic winds blowing onshore.
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Scaling the quasi-steady along-valley wind
Juerg Schmidli, Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland
Richard Rotunno, National Centre for Atmospheric Research (NCAR), Boulder, USA

 

Corresponding author: Juerg Schmidli
Section: Boundary layer

The quasi-steady-state limit of the diurnal valley wind system is investigated over idealized three-dimensional topography. Although this limit is rarely attained in reality due to ever-changing forcings, the investigation of this limit can provide valuable insight, in particular on the mass and heat fluxes associated with the along-valley wind. We derive a scaling relation for the quasi-steady-state along-valley mass flux as a function of valley geometry, valley size, atmospheric stratification, and surface sensible heat flux forcing. The scaling relation is tested by comparison with the mass flux diagnosed from large-eddy and mesoscale simulations of the valley wind system. Good agreement is found. The results also provide insight into the relation between surface friction and the strength of the along-valley pressure gradient.

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The impact of valley geometry on thermally driven flows and vertical heat fluxes
Johannes Wagner(1), Alexander Gohm(1), Mathias Rotach(1), Daniel Leukauf(1), Christian Posch(1)
(1) Institute of Meteorology and Geophysics, University of Innsbruck, Austria

 

Corresponding author: Johannes Wagner
Section: Boundary layer

In this project thermally driven flows in complex terrain are studied. The overall goal is to investigate the role of different valley geometries on slope and valley winds and on horizontally averaged valley bulk heat flux profiles. The study is based on idealised large-eddy-simulations (LES) of heated valley topographies, which are performed with the Weather Research and Forecasting (WRF) model. In a first step a simplified setting with a constant surface heat flux forcing and an infinitely long valley is used to compare the simulation results to another LES-study, which used the Advanced Regional Prediction System (ARPS) with the same domain set-up (Schmidli, J. submitted). The model comparison shows that both models agree well in the averaged and resolved-scale turbulent flow structures and in the heating of the valley atmosphere. In a second step the valley depth and width is varied to examine the impact of valley shape on slope winds and bulk fluxes. It is found that the well known three-layer thermal structure with two superposed circulation cells only occurs in deep valleys and that bulk vertical turbulent heat fluxes exhibit a secondary maximum near ridge height. In a third step the simulation is extended to a valley plain topography to allow for the development of along valley flow.
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High resolution modeling of katabatic wind and field campaign on an alpine slope
Sébastien Blein : LEGI, Grenoble, France
Christophe Brun : LEGI, Grenoble, France
Jean-Martial Cohard : LTHE, Grenoble, France
Maxime Litt : LTHE, Grenoble, France

 

Corresponding author: Sébastien Blein
Section: Boundary layer

High resolution numerical simulation is used to analyze the stable atmospheric boundary layer dynamic in mountainous region. The 3D spatial resolution is of 10m in the horizontal direction and 2m in the vertical direction above the ground surface. A 1D vertical model is coupled to the 3D model to access the flow dynamic in the region between the ground and the first atmospheric grid point. It allows a better description of momentum and temperature fluxes at this level. Within the air quality context, we focus on katabatic processes and their contribution on meso-scale motion and mixing for high polluted event during the winter on the Grenoble valley, France. The study is based on MesoNH realistic simulations (Meteo-france & Laboratoire d'Aerologie, Toulouse, France). To complete the numerical study, a field campaign (KACOSONIC, KAtabatic wind at Grand COlon mountain and SONIC anemometer data) was organized on November 2012. Measurements were performed during a strong particle pollution event. The field campaign consisted on 2 main sites. First, a Sodar was located at the slope bottom to catch the meso-scale behavior (mean wind profiles, shear layer...). It performs 600m high profiles with 6m resolution. Secondly, a 6m mast was located at the middle of the slope, were well established katabatic winds were observed. 4 sonic anemometers (20Hz) were used at 1m, 2m, 4m and 6m above the ground to describe the low level jet. Meteorological data were also stored. At the mast location (katabatic source location), the down-slope jet is of 10m high and the maximum is located between the 2m and the 4m mast point. With this configuration, the momentum and temperature fluxes are well described above and below the wind maximum height. The strong complementarity of numerical simulations and field campaign led us to study both approaches. The field campaign is temporally well defined and used to be easily trusted but lacks of spatial description, especially on the horizontal direction. This lack of spatial description is filled by the realistic numerical results which are themselves supported by in-situ data. This self-sufficient data-set permits boundary layer parametrization studies and to examine their applicability to realistic therefore complex configuration.

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Weakly-nonlinear Prandtl model for simple slope flows
Branko Grisogono, Univ. of Zagreb, Fac. of Sci., Department of Geophysics, Zagreb, Croatia
Toni Jurlina, University of Zagreb, Faculty of Science, Department of Geophysics, Croatia
Željko Večenaj, Univ. of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Ivan Guettler, Meteorological and Hydrological Service, Zagreb, Croatia

 

Corresponding author: Branko Grisogono
Section: Boundary layer

Prandtl model couples, perhaps in the most succinct way, basic boundary-layer dynamics and thermodynamics for simple anabatic or katabatic flows over inclined surfaces. This 1D analytic model assumes steady-state balance between buoyancy (positive for anabatic, negative for katabatic flows) and turbulent friction. While the classical Prandtl model is linear with a priori assigned vertically constant eddy diffusivity and heat conductivity, in this analytic work we partly relax both of these assumptions.

Focusing on katabatic flows, one of the main weaknesses of Prandtl model is that strong, near-surface flow-induced finite-amplitude potential temperature gradient does not feed back to the assigned environmental potential temperature gradient in the thermodynamic equation, even though the induced gradient below the katabatic jet can be 20 to 50 times stronger than the related environmental gradient. This issue is tackled by using a weakly-nonlinear approach where a small parameter controls feeding of the flow-induced potential temperature gradient back to the environmental potential temperature gradient. An appropriate range of values for the small parameter controlling the weak nonlinearity is provided. In this way, the near-surface potential temperature gradient becomes stronger and the corresponding katabatic jet somewhat weaker (at a slightly lower height) than that in the classical Prandtl solution.

The other issue of constant eddy coefficient K in the classical Prandtl model is treated by a given gradually varying K with the height above the surface, all within the validity of the zero-order WKB approximation. The new model is compared to data from PASTEX-94, Austria. Besides other potential applications, this modified Prandtl model may help in parameterizing inclined boundary layers in climate models since there is only little confidence on the projected changes in future wind fields and the corresponding extremes.
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The influence of a depth of a very shallow cold air pool lake on the nocturnal cooling
Jože Rakovec,Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia and Center of Excellence SPACE‐SI, Ljubljana, Slovenia
Gregor Skok, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia and Center of Excellence SPACE‐SI, Ljubljana, Slovenia

 

Corresponding author: Gregor Skok
Section: Cold air pools

A five-layer thermodynamic model for a very shallow (few meters deep) Cold Air Pool (CAP) is presented. The model has two stationary layers with constant temperature (an atmosphere layer above the CAP and a deep soil layer) and three non-stationary layers: one thin soil layer at ground surface and two very shallow air layers. The model is applied either to a shallower CAP (only one layer 3 m deep) and a deeper CAP (two 3 m layers, together being 6 m deep) CAP. The model shows that air in the deeper CAP cools more than the air in the shallower CAP. The difference between the equilibrium temperature of deeper and shallower CAP is shown to be strongly dependent on the humidity of the air inside the CAP and humidity of the atmosphere - more humidity of the air inside CAP or the atmosphere results in greater difference in cooling. The difference is also somewhat dependent on the soil heat conductivity and on temperature deep in the ground – an increase in heat conductivity results in smaller difference. In the end the model results are compared with measurements from a 10-month field experiment which consisted of simultaneous measurement of temperature and humidity at three locations inside a very large shallow CAP. The measurements confirm the importance of depth of CAP on equilibrium temperature but also hint that other factors, especially local concavity (position in a local depression), might play an equally important role in the strength of nocturnal cooling.
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Observed and modeled nocturnal wind and temperature oscillations in the lower Inn Valley
Irene Schicker, Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna
Petra Seibert, Department of Meteorology and Geophysics Vienna, University Vienna

 

Corresponding author: Irene Schicker
Section: Cold air pools

The propagation of sound and the dispersion of pollutants in basins and valleys is influenced by the local wind systems as e.g. slope and valley winds. Another parameter influencing is the location and height of the cold air pool layer. Nocturnal wind and temperature oscillations measured during an intensive observation period (IOP), November 20 2005 to March 10 2006, in the Austrian Inn Valley are investigated using the observations taken during the IOP and a meteorological model. Several events of sudden drops of the nocturnal temperature were observed during the IOP. During such events temperature decreases of more than 3 K within less than 5 – 10 minutes were found. These events occurred when the wind direction changed from downslope to upslope thus advection the cold air of the valley floor towards the observation sites indicating an swashing forth and back of the cold air pool. Also, changes in the sign of the sensible heat flux were observed. These temperature oscillations are observed at all three sites but not necessarily at the same time. At the lowest site, the temperature drops are observed more often than the ones located at the slightly higher sites. Here, a microscale change of the air masses is observed which are not only relevant for the transportation of pollution and noise but also relevant for e.g. growth of fruits.

Three measurement sites close to Jenbach, located 5 – 30 m above the valley floor along the northern and southern slope are used. At each site measurements of temperature and wind at two levels at small towers were carried out. At one site and ultrasonic anemometer was used additionally. Results and findings of an earlier case study were some basic MM5 simulations were carried out were used in addition. Besides the observations a very high resolution simulation using the WRF ARW v3.3.1 is carried out to investigate the evolution of selected events. The WRF simulations shown here use re-mapped CORINE land-use data and high resolved DEM data for the innermost modeling domain.

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Modelled variation of cold air pooling processes at various grid resolutions
Paul Burns (1)
Charles Chemel (1,2)

1 Centre for Atmospheric & Instrumentation Research, University of Hertfordshire, UK
2 National Centre for Atmospheric Science, Centre for Atmospheric & Instrumentation Research,
University of Hertfordshire, UK

 

Corresponding author: Paul Burns
Section: Cold air pools

Elucidating cold air pooling processes forms part of the long-standing problem of parameterising the effects of complex terrain in larger-scale models. The Weather Research and Forecasting model has been setup and run at high resolution over an idealised alpine-valley domain of order 10 km, to investigate the four dimensional variation of key cold air pooling drivers, under de-coupled stable conditions. Model results indicate that downslope flow characteristics are sensitive to model grid resolution, and a convergence of solutions enables a strategic model grid resolution selection. Three regimes of long-wave radiative flux divergence contribution, C_rad, to total average valley-atmosphere cooling have been identified. Starting about one hour before sunset, there is an initial 40 min period of C_rad dominance, as the flow establishes itself and when the other energy balance terms generally sum to a heating contribution. A period of instability follows, lasting for approximately one hour. Finally, there is a gradual reduction of C_rad over a period of 380 min from 60% to a final contribution of 32 %. The simulation average C_rad is close to 50 %, but is 38 % for the period of gradual decline, with maximum and minimum values, occurring at the start and close to the end of the simulation, of 233 and 16 %, respectively. The C_rad values for the final period and its average contribution are in the range of values reported in the literature. The contribution of the other energy balance terms will be discussed, together with the dependency on model resolution.
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Numerical modelling of valley cold air pools
Peter Sheridan, Met Office, UK
Simon Vosper, Met Office, UK

 

Corresponding author: Peter Sheridan
Section: Cold air pools

Valley cold air pools which form on calm, clear nights in complex terrain represent a significant
problem, enhancing the likelihood of road-ice formation, crop damage and harmful pollution episodes. Cold air pools form in valleys over a wide range of scales and geometries, and may have a very different character in different locales, presenting different forecast problems. In small UK valleys, for instance, cold pools are diurnal and the scale is less than the grid spacing of operational numerical weather prediction (NWP) models, while in many large US basins, cold pools may persist for days, and the complex interplay of processes involved in their evolution may be difficult for NWP models to forecast accurately. Here, high resolution idealised model simulations will be used to examine how cold air pool behaviour depends on factors such as valley scale and geometry as well as upstream wind and temperature profile, in order to investigate
different regimes in a controlled manner.
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Tropospheric Circulation Features During Wet and Dry Years over West Africa
Prof. Shou Shaowen, Nanjing University of Information Science & Technology, College of Atmospheric Sciences, Nanjing, China.

 

Corresponding author: Dr Ewanlen Rufus
Section: Precipitation

This paper describes a composite study comparing some features of the tropospheric circulation over the region of West Africa in the monsoon season for dry (1981-85) and Wet (1998-2004) selected years. Zonal and meridional components of the monthly mean wind are computed directly with NCEP reanalysis data for the summer months limited to May, June and August. The potential temperature and moisture variables are also examined for the periods. The results of our analysis show major contrasts between the characteristics of tropical troposphere in West Africa in the dry and wet years. In 1981-85, the upper and lower troposphere are characterized by weaker flows in coexistent with stronger Africa easterly jet (AEJ) and a southward shift of the major circulation patterns as well as numerous dynamical parameters. The observed weak tropical easterly jet (TEJ) and vertical alignment of the AEJ and TEJ axes enhance the rainfall deficit in dry years.

In contrast, during 1998-2004, the troposphere is dominated by stronger flows at lower and upper levels in concomitant with a weaker AEJ and intense northward shift of the zonal and meridional cells circulations. The observed strong vertical coupling between the zonal component anomalies at lower levels (monsoonal flow) and upper levels (TEJ), reflect the opposition in direction of mean winds at these levels. Over the region, the latitudinal location and intensity of the AEJ seems to be imperative, and tend to regulate the instability mechanisms. The instability is observed to be greater in August than in May in all the years. The contrasting convergence of monsoonal flow and the upper divergence associated with TEJ is linked with the activeness of the walker-type cell during the periods.

At 925hPa lower level, the Inter-Tropical Discontinuity (ITD) surface positions in the region generally appear to be almost in same locations for both dry and wet years. As consequence, the length of the rainy season becomes unvarying. Instead, the 1998-2004 (wet) years are well dominated by more extreme rainfall. Finally, it is evident that the convective process over the tropical region of West Africa is dynamically influenced by strong upper-level divergence at TEJ level associated with ascent in the lower troposphere, where the disturbances build-up. Contrasts in the monsoon layer between wet and dry years may be related to these fluctuations. The results from this study serve as bases for further work on the tropospheric circulation features over the region of West Africa for an enhance understanding of the regional climate.

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Time-step precipitation across an island in southern Norway – modeled and observed.
Idar Barstad,
UniComputing, UniResearch, Bergen, Norway

Giulio Caroletti,
Universita di Cagliari, Dipartimento di Ingegneria Civile, Ambientale e Architettura, Piazza d’Armi, 09123 Cagliari, Italy.

 

Corresponding author: Idar Barstad
Section: Precipitation

Observations of high spatio-temporal resolution from a precipitation network across Stord Island, located off the west coast of southern Norway, are compared to state-of-the-art numerical model simulations. The 12 week long observation period shows a clear orographic precipitation signal across the 10–15 km wide island (peak elevation 750 m). The model experiment designed to capture this signal is run with 9–3–1 km nested grid. Observations at different accumulation levels are uses for comparison.

When running the model with a convective scheme activated in 9 and 3 km domains, the total amount of precipitation over the 12 week period is underpredicted – for all nests. The maximum precipitation intensity, however, is slightly overpredicted. Time-step (5 s) precipitation from the model is also compared with observed intensities at the highest possible temporal resolution permitted by the rain collection method. At this level, the simulated precipitation at the 3 km grid did not reproduce at the correct intensities. The 1 km grid showed improvements, but had too high maximum intensities. The observations indicate that most of the precipitation is formed at intensities from 5 to 20 mm h−1 . A smaller fraction of the precipitation is formed with intensities >20 mm h−1 .

Running the model with a deactivated cumulus scheme at 3 km, showed improvements of the intensity at the two inner nests. It is suggested that treatment of the buoyancy production over the relatively warm ocean may explain the discrepancies.
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Development of an orographic rain parametrization
Simon Vosper (Met Office), Paul Field (Met Office)

 

Corresponding author: Samantha Smith
Section: Precipitation

The ability of the operational MetUM to correctly predict orographic rain enhancement at various horizontal resolutions has been assessed against rain gauge observations, by comparing rain accumulations averaged over a large number of times when a warm sector was present over the hills of the UK. The 1.5km forecasts perform extremely well over both the Lake Distict and Wales. Lower resolution forecasts are unable to reproduce the observed amount of orographic rain enhancement, partly due to the representation of the orography and partly due to the larger grid spacing.
A large number of highly idealised 2D simulations of neutral flow over a Witch of Agnesi hill of various dimensions and for various upstream humidities have been performed using the Kinematic Driver Model (KiD). The winds are prescribed using a linear model, and the KiD model allows advective transport and particle sedimentation while avoiding the complexity caused by feedbacks between dynamics and microphysics. These simulations are being used to guide the design of the subgrid orographic drag parametrization for use in low resolution NWP models.

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Orographic flows over an active volcano
Alexandros P. Poulidis, University of East Anglia
Ian A. Renfrew, University of East Anglia
Adrian J. Matthews, University of East Anglia

 

Corresponding author: Alexandros P. Poulidis
Section: Precipitation

Orographic flows over and around an isolated mountain are studied through a series of idealised numerical model experiments. The mountain top has a heated surface, thus can be thought of as an “active volcano” that is not undergoing an eruption. A series of simulations with different atmospheric conditions and control parameters for the volcano will be presented. The simulations make use of an idealised configuration of the Weather Research and Forecast (WRF) model. The study is based on the Soufriere Hills volcano, located in the island of Montserrat in the Caribbean. This location was chosen as Soufriere Hills is a dome-building volcano, leading to a sharp increase in the surface skin temperature at the top of the volcano – up to tens of degrees higher than ambient values. The simulations use an idealised topography, meaning the results have general applicability to similar-sized volcanoes located in the tropics. The model is initialised with soundings from representative days of qualitatively different atmospheric conditions from the rainy season in the tropics.
The simulations reveal that the volcano’s surface temperature anomaly can change the orographic flow response significantly, depending upon the atmospheric conditions and the size of the temperature anomaly. The flow regime and and characteristic features such as gravity waves, orographic clouds and orographic rainfall patterns can all qualitatively change. Orographic rainfall is generally enhanced with increased temperature anomaly. The implications for the eruptive behaviour of the volcano and resulting secondary volcanic hazards will also be mentioned.
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A CONTRIBUTION OF UNDERSTANDING THE CAUSES OF A HEAVY PRECIPITATION EVENT IN THE SOUTHEASTERN ADRIATIC AREA
Branka Ivančan-Picek, Kristian Horvath, Nataša Strelec Mahović and Marjana Gajić-Čapka

Meteorological and Hydrological Service, Grič 3, Zagreb, Croatia

 

Corresponding author: Branka Ivancan-Picek
Section: Precipitation

The event analyzed in this paper is geographically focused on the southern part of the eastern Adriatic region, which is prone to relatively frequent heavy precipitation events in the autumn and winter. This area is one of the rainiest in Europe. The annual amounts of precipitation higher than 5000 mm can be expected in the mountainous hinterland. The aim of this study is to identify the main mesoscale features and mechanisms responsible for the generation of an extreme precipitation event as a contribution to understanding the factors which cause the climatic maximums of the annual precipitation amounts located over this region. The event occurred during the morning hours on 22 November 2010 over the Dubrovnik coast, Croatia and the hinterland mountain range of the southern Dinaric Alps. A peak intensity of 145.5 mm was registered in the four-hour period. The rain gauge in Dubrovnik measured daily amount of 161 mm on 23 November 2010 that was more than the half of the monthly total. This event caused severe flash floods, landslides, interruption of traffic and electricity supply, as well as other infrastructural damage.
The mechanisms responsible for the formation of convection have been analyzed through synop measurements, satellite data and numerical experiments performed with the WRF model, which was set up at the convection-permitting resolution in the innermost domain. Satellite data were used to identify the precipitation systems and to estimate the intensity of the precipitation during the period of interest. The analysis of these products provided valuable information, especially over the sea where no other observational data were available.
The development of the precipitation system was connected by the strong large-scale ascent over the southern Italy and southern Adriatic due to positive temperature advection and positive PV advection growing with height. The maximum of the upward motion extended vertically from the surface through the entire troposphere, indicating an extremely strong synoptic process. The numerical simulations highlighted the essential role of southerly low-level jet stream (LLJS) in the transport of warm and moist air towards the affected area. The convergence of two branches of low-level marine air favored convection triggering over the coast and sea. Persisting for about four hours, the convergence line contributed to large precipitation amounts in the Southern Adriatic area. Furthermore, numerical sensitivity experiments suggested that the orography of the Dinaric Alps has an essential role for precipitation maximum over the mountainous hinterland, but also that the orography was not the crucial factor for the heavy precipitation near Dubrovnik. This study has highlighted the need for a dense network of observations, especially radar measurements, to validate the simulated mechanisms and improve the numerical forecasts through data assimilation.

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Stable water Isotope concentrations across the Southern Alps of New Zealand
Tim Kerr - National Institute of Water and Atmospheric Research, Christchurch, New Zealand
Roddy Henderson - National Institute of Water and Atmospheric Research, Christchurch, New Zealand
Abha Sood - National Institute of Water and Atmospheric Research, Wellington, New Zealand

 

Corresponding author: Tim Kerr
Section: Precipitation

The water resources of the South Island of New Zealand are dominated by the extreme orographic precipitation resulting from the Southern Alps lying across the Southern Hemisphere westerly wind belt. The distribution of that precipitation is poorly constrained by a limited number of rain gauges. Stable water isotope analysis provides an alternative method to characterise the precipitation distribution. The isotopic composition of a stream in recession is determined by the precipitation that has fallen into the contributing catchment over a period of time prior to the sampling. The isotopic concentration of the sampled stream water may be interpreted in terms of the amount of precipitation that has occurred between the water vapour source (the Tasman Sea in this case) and the stream catchment. In addition, an estimate of the altitude at which the contributing precipitation was condensed may be made.
Water samples from small streams in a transect across the Southern Alps were tested for concentrations of Deuterium and Oxygen 18. The depletion in these heavy isotopes from windward to leeward sites clearly describes the drying effect of the mountains on the water vapour in the atmosphere. Delta Oxygen 18 (δ18O) concentrations lead to an estimate of 38 % of the moisture being removed from the atmosphere by the mountains. The deuterium concentrations lead to an estimate of 34 %. Comparison of adjacent sites shows strong atmospheric drying from the windward side of the mountains until 10 km downwind of the main orographic barrier. From there the drying continues but at a lower rate until a second orographic divide where a second water vapour source (from the South) begins to dominate.
The concentrations obtained provide a rapid method of characterising the precipitation distribution across the Southern Alps and provide additional validation/calibration data for precipitation models independent of the six rain gauges that, at best, can only provide point measurements.

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Microphysical Timescales of Orographic Clouds in Stable Conditions
Annette K. Miltenberger, Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstr. 16, 8092 Zurich, Switzerland
Hanna Joos, Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstr. 16, 8092 Zurich, Switzerland
Heini Wernli, Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstr. 16, 8092 Zurich, Switzerland

 

Corresponding author: Annette Miltenberger
Section: Precipitation

A critical issue for state-of-the-art conceptual orographic precipitation models like the extended upslope model is the quantification of a microphysical timescale. So far the microphysical timescale has been used mainly as tuning parameter to fit observations. We propose a new approach, which by adopting a Lagrangian perspective on orographic precipitation formation allows to investigate the microphysical evolution of single air parcels. Thereby we can directly quantify the microphysical timescale of single air parcels. We used a boxmodel employing a standard two-moment microphysical parameterization to perform simulations with a sinewave-like vertical velocity pattern, which can be viewed as highly idealized representation of flow over a hill. To include the role of vertical temperature and humidity gradients and gravity wave dynamics, in addition simulations of stable flow past a two-dimensional bell-shaped hill are analyzed from a Lagrangian viewpoint. In this study different possibilities to define a microphysical timescale are evaluated and compared in their amplitude and physical meaning. It is found that the microphysical timescale is quite sensitive to the definition mostly depending on which processes are depicted. This also impacts the usefulness of the timescale for the formulation of a scaling relation of precipitation efficiency. By a quantitative comparison of the derived timescales to those found by tuning of the upslope model in simulations of real orographic precipitation, some insights on the most relevant microphysical processes in orographic precipitation formation can be gained.
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Throughfall in different forest types during extreme precipitation event in November 2012
dr. Urša Vilhar, Gozdarski inštitut Slovenije – Slovenian Forestry Institute, Večna pot 2, SI-1000 Ljubljana
Iztok Sinjur, Gozdarski inštitut Slovenije– Slovenian Forestry Institute, Večna pot 2, SI-1000 Ljubljana
dr. Primož Simončič, Gozdarski inštitut Slovenije– Slovenian Forestry Institute, Večna pot 2, SI-1000 Ljubljana

 

Corresponding author: Urša Vilhar
Section: Hydrology and Snow

Extreme precipitation events in the mountainous regions are risk factors with consequences such as flooding of populated valleys, erosion, avalanches, debris flow and landslides. Because their effects could be significantly buffered by forest cover, therefore forest management practices should aim towards decreased surface runoff and soil erosion. In Central Europe, many pure Norway spruce stands, established on primary beech sites, were converted back into mixed stands over the last decades. The conversion of forest management from spruce monocultures into mixed deciduous-coniferous forests changed the forest structure dramatically. These changes could influence the hydrological processes on the catchment scale, associated with major river flooding following extreme precipitation events.
In this study, which focuses mainly on extreme precipitations with devastating floods in November 2012, the effect of forest management on the partitioning of rainfall into throughfall and stemflow in coniferous and mixed deciduous-coniferous stands on Pohorje mountains (NE Slovenia) were investigated. Four spruce Picea abies (L. Karst) stands were compared to four mixed spruce-beech Fagus sylvatica (L.) stands. In order to estimate the throughfall under forest canopies, among these results the monthly throughfall from totalisators and half-hourly throughfall from automated rain gauges in growing seasons from 2008 till 2012 were analyzed. In the mixed spruce-beech stands the monthly stemflow on beech trees was also measured.
There were small differences in throughfall during growing season between the coniferous and mixed deciduous-coniferous stands. Seasonal stemflow on beech trees was 4 to 5 % of the bulk precipitation amount. An illustration using half-hourly throughfall data during extreme precipitation events also showed small differences in throughfall intensities among the coniferous and mixed deciduous-coniferous stands. We assume that differences in cumulative throughfall during extreme precipitation event in November 2012 are more related to topography than to different forest types. The implementation of hydrology-oriented sylvicultural measures via a more accurate prediction of the impacts of tree species conversion on throughfall in this type of mountain forest is discussed.
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The influence of climate change on river discharge in Austria with application to hydropower production
Robert A. Goler - Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna, Austria; alpS Centre for Climate Change Adaptation Technologies, Innsbruck, Austria.
Simon Frey - alpS Centre for Climate Change Adaptation Technologies, Innsbruck, Austria; Institute of Water Management, Hydrology and Hydraulic Engineering, University of Natural Resources and Life Sciences, Vienna, Austria.
Herbert Formayer - Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna, Austria.
Hubert Holzmann - Institute of Water Management, Hydrology and Hydraulic Engineering, University of Natural Resources and Life Sciences, Vienna, Austria.

 

Corresponding author: Robert A. Goler
Section: Hydrology and Snow

Electricity produced from hydropower represents a primary source of renewable energy within the Alpine region. Approximately 55% of Austria's electrical energy is produced from hydropower. The utilisation of this energy source depends strongly on the spatial and temporal distribution of water in both its solid and liquid forms. Climate change is expected to result in an increase in temperature across the Alpine region. However, the effect that climate change will have on yearly precipitation totals still remains uncertain, although changes in the seasonal distribution of precipitation seem to be consistent. Thus it is highly probable that climate change will affect river discharge and hydropower production within the Alps.

In this study the river discharge characteristics in several catchment areas within Austria are modelled using a conceptual hydrological model. Processes which are represented by the model include meltwater from snow and glaciers; surface, subsurface, and groundwater flows; and evapotranspiration. The model is driven using daily climate data generated from three regional climate models (ALADIN, RegCM3, REMO) over the time period 1951-2100 using the A1B emissions scenario. This data has been bias-corrected and downscaled to a spatial resolution of 1 x 1 km over Austria. The model is calibrated using Monte Carlo simulations in the time period 1996-2005 over which observations of the river discharge were available. Validation of the model is done using observational data from 2006-2010.

The model results show that towards the end of the 21st century changes in the river discharge will be significant. For catchment areas whose discharge is dependent on water stored in snow and glaciers, there will be a general shift in the time of maximum river discharge to earlier in the year as the snow and ice melt earlier. During the winter months the discharge is forecast to be higher than at present, which would be advantageous for hydropower production as the number of days of low discharge will be reduced. However, due to the earlier snow melt the available water for the summer months will decrease, leading to lower discharges than present, which would be disadvantageous for hydropower production.
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Orographic effects on snowfall patterns in mountainous terrain
Rebecca Mott, WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
Danny Scipion, Ecole Polytchnique Federale de Lausanne (EPFL), School of Architecture and Civil Engineering (ENAC), Environemntal Remote Sensing Laboratory (LTE), Lausanne, Switzerland
Marc Schneebeli, Federal Office of Meteorology and Climatology MeteoSwiss - Radar, Satellites and Nowcasting, Locarno, Switzerland.
Nicholas Dawes, WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
Alexis Berne, Ecole Polytchnique Federale de Lausanne (EPFL), School of Architecture and Civil Engineering (ENAC), Environemntal Remote Sensing Laboratory (LTE), Lausanne, Switzerland
Michael Lehning, WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland; Ecole Polytchnique Federale de Lausanne (EPFL), School of Architecture and Civil Engineering (ENAC), Laboratory of Cryospheric Sciences (CRYOS), Lausanne, Switzerland.

 

Corresponding author: Rebecca Mott
Section: Hydrology and Snow

Orographic lifting of air masses and other topographically modified flows induce cloud formation and preferential deposition of precipitation. In this study, we examine orographic effects on small-scale snowfall patterns in Alpine terrain. A polarimetric X-band radar was deployed in the area of Davos (Switzerland) to determine the spatial variability of snowfall. In order to relate measured snowfall fields to flow dynamics, we modeled flow fields with the atmospheric prediction model Advanced Regional Prediction System (ARPS). Additionally, we compared radar reflectivity fields with snow accumulation at the surface as modeled by Alpine3D. We investigated the small-scale precipitation dynamics for one heavy snowfall event in March 2011 at a high range resolution of 75 m. Polarimetric radar data suggest orographic snowfall enhancement near the summit region at the transition between the updraft and downdraft zone. Radar reflectivity increased in the presence of flow acceleration at windward slopes and decreased in the presence of flow deceleration at the leeward slopes. Measurements and numerical analysis also suggest preferred snow deposition on leeward slopes. Measurements show that the temporal variation of the location of maximum concentration of snow particles is strongly dependent on the magnitude of the horizontal wind velocity. For situations with strong horizontal winds, the concentration maximum is shifted from the ridge crest towards the leeward slopes. Qualitatively, we discuss the relative role of cloud micro-physics such as seeder-feeder mechanism versus atmospheric particle transport in generating observed snow deposition at the ground.

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Dependency of convection on environmental conditions over complex terrain during HyMeX
Norbert Kalthoff: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT)
Bianca Adler: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT)

 

Corresponding author: Norbert Kalthoff
Section: Convection

In mountainous areas various thermally and dynamically produced processes occur simultaneously and interact on different scales. Beside the large-scale atmospheric conditions these processes influence the evolution of convection. As part of the HyMeX field campaign in the Western Mediterranean Sea 2012 the KITcube observation platform was installed at the mountainous island Corsica. KITcube combines various in-situ and remote-sensing systems that allows to measure the process chain from energy exchange at the surface via turbulent transport to the formation of clouds and precipitation.

In contrast to days with weak synoptic winds, gravity waves formed in the lee of a high mountain ridge when strong synoptic winds prevailed. This waves were associated with a downward transport of warm, dry and aerosol-free air and strong momentum from the free atmosphere into the lower parts of valley atmosphere. They interacted with the boundary-layer structure and suppressed convection in the valley center.
Regularly, moist convection formed above the mountain ridges due to moisture transport up the slopes and mountain venting. Depending on the large-scale conditions (e.g., stability, humidity content) shallow convection also persisted above the valley center. Finally, we analyzed the differences between days with shallow and deep convection.
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Wind patterns associated with the development of daytime thunderstorms over Istria
Maja Telišman Prtenjak
Andrija Mohorovičić Geophysical Institute, Department of Geophysics, Faculty of Science, University of Zagreb, Zagreb, Croatia

Gabrijela Poljak
Andrija Mohorovičić Geophysical Institute, Department of Geophysics, Faculty of Science, University of Zagreb, Zagreb, Croatia

Marko Kvakić
CNRM-GAME, Meteo-France/CNRS URA 1357, 42 Avenue Gustave Coriolis, 31057, Toulouse Cedex, France

 

Corresponding author: Maja Telišman Prtenjak
Section: Convection

This study investigates the impact of the combined large-scale wind and thermally induced local wind on the moist convection development (cumulonimbus clouds) over the northeastern (NE) Adriatic. Former analyses revealed that the NE Adriatic, particularly the Istrian peninsula, is the area with (i) the highest frequency of thunderstorms in Croatia, typically during three wind regimes on a large scale (from the southwest (SW), northeast (NE) and northwest (NW)), and (ii) frequent appearances of sea breeze along the coast (every other summer day on average). The highest density of lightning strikes was observed in the NE mountainous part of the peninsula. Therefore, the three selected cases (one for each type of dominant large-scale wind) were analyzed using the available near-surface and remote measurements. They were simulated also by WRF high-resolution numerical model and examined by the sensitivity tests.
In all cases, the near-surface wind patterns consisted of sea breezes along the coastline that generated a narrow eastward-moving convergence zone along the Istria. When the large-scale SW wind (as an onshore wind) dominated in the upper troposphere, the thunderstorm event was the shortest and weakest with only a minor impact on the sea breeze. This was confirmed by the sensitivity tests without microphysics. The origins and locations of storm cells were completely controlled by the low-level convergence zone and the upward advection of low-level moisture at the sea breeze front. In the other two examined cases with offshore large-scale winds from the NE and NW, the mountain range hastened the beginning of convection and affected its intensity. Except for the low-level convergence zone, the advection of large-scale wind influenced the lifetimes and movements of the initial convective cells. Thus, while the local front collision with the NE wind advection caused the thunderstorm to propagate southward, the convergence zone and fronts interaction determined the afternoon northwestward storm movement against the NW large-scale wind.

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Influence of topography on the diurnal cycle of summertime moist convection over its near-and far-field regions
Hassanzadeh Hanieh, Institut for Atmospheric and Climate Science, ETHz University
Schmidli Jürg, Institut for Atmospheric and Climate Science, ETHz University
Langhans Wolfgang, Climate Sciences Department, Berkeley University
Schär Christoph, Institut for Atmospheric and Climate Science, ETHz University

 

Corresponding author: Hanieh Hassanzadeh
Section: Convection

The influence of the topography on the diurnal cycle of summertime moist convection is investigated in an idealized framework using a cloud-resolving model (CRM) with a horizontal grid spacing of 2km. In this framework, the atmosphere is continuously relaxed towards prescribed reference profiles of temperature, specific humidity and wind speed. This relaxation mimics the influence of a large-scale advection. The strength of the relaxation varies with height, with a relaxation time of 1 day in the upper troposphere and much weaker relaxation at lower levels. The simulations are run for 30 days. During the last 20 days a quasi-steady diurnal cycle is obtained, the diurnal equilibrium. Here, we investigate the influence of topography on the diurnal equilibrium evolution of clouds, precipitation and the associated net vertical fluxes of energy and water. As expected the influence is large, in terms of cloud and precipitation amount as well as in terms of timing of the diurnal cycle. In comparison to flat terrain, clouds and heavy precipitation occur earlier in some regions over the topography, but other adjacent regions receive very little precipitation. In this work, a particular focus will be on the analysis of the mountain effects as a function of the distance from the mountain (e.g. near-field and far-field effects) and a detailed investigation on the evolution of moist convection including an analysis of the dominant precipitation mechanisms over each region.
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Orographic triggering and mesoscale organization of extreme storms in subtropical South America
Kristen L. Rasmussen - University of Washington, Seattle, WA
Robert A. Houze, Jr. - University of Washington, Seattle, WA
Anil Kumar - NASA Godard Space Flight Center, Earth System Science Interdisciplinary Center, Greenbelt, MD

 

Corresponding author: Kristen Lani Rasmussen
Section: Convection

Extreme convection tends to form in the vicinity of mountain ranges, and the Andes in subtropical South America help spawn some of the most intense convection in the world. An investigation of the most intense storms for 11 years of TRMM Precipitation Radar (PR) data shows a tendency for squall lines to initiate and develop in this region with the canonical leading convective line/trailing stratiform structure. On average, South American mesoscale convective complex cloud shields are 60% larger than those over the United States and they have larger precipitation areas than those over the United States or Africa. The synoptic environment and structures of the extreme convection and MCSs in subtropical South America are similar to those found in other regions of the world, especially the United States. In subtropical South America, however, the topographical influence on the convective initiation and maintenance of the MCSs is unique. The Andes and other mountainous terrain of Argentina focus deep convective initiation in a narrow region. Subsequent to initiation, the convection often evolves into propagating mesoscale convective systems similar to those seen over the Great Plains of the U. S. and produces damaging tornadoes, hail, and floods across a wide agricultural region.

Numerical simulations conducted with the NCAR Weather Research and Forecasting (WRF) Model extend the observational analysis and provide an objective dynamical evaluation of storm initiation, development mechanisms, dynamics and microphysics. A capping inversion in the lee of the Andes is important in preventing premature triggering. The South American Low Level Jet impinging on low mountains to the east of the main Andes ranges triggers extremely deep and intense convection. Mesoscale organization into leading line/trailing stratiform system occurs as the storms propagate eastward. The simulated mesoscale systems closely resemble the storm structures seen by the TRMM satellite as well as the overall shape and character of the storms shown in the GOES satellite data. A sensitivity study removing small-scale topographic features that are hypothesized to focus deep convective initiation determines the role of the topography in triggering and geographical focusing the extreme convection. Preliminary results of the numerical experiment in which smaller mountain features are removed will be presented at the conference.
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On measuring and modeling Boundary Layer turbulence in complex terrain
Mathias W. Rotach, Institute for Meteorology and Geophysics, University of Innsbruck, Austria
Ivana Stiperski, Institute for Meteorology and Geophysics, University of Innsbruck, Austria
Alexander Gohm, Institute for Meteorology and Geophysics, University of Innsbruck, Austria

 

Corresponding author: Mathias W Rotach
Section: Turbulence

Atmospheric flows near the surface are largely determined by turbulent exchange processes – also (or possibly even more so) in complex mountainous terrain. Methods have been established for its measurement and modelling, which stem from, and apply to horizontally homogeneous and flat (HHF) surfaces. As for the ‘homogeneity requirement’ conceptual approaches (internal boundary layers, effective fluxes, footprints, etc.) have been devised, which are based on simple ‘A-B transitions’ where both sides again conform to the HHF conditions. Relatively little is known, on the other hand, if it comes to (truly) complex terrain (i.e. ‘non-F’ conditions) - except that the boundary layer over such surfaces is intrinsically inhomogeneous. Essentially, when dealing with boundary layer turbulence and its interpretation even in complex terrain, we employ similarity theory (Monin-Obukhov similarity theory, MOST, near the surface or other scaling regimes like Mixed Layer scaling higher up).

Many of the applications requiring knowledge of turbulence characteristics – for example hydrological (catchment) modelling, determination of the energy balance on glaciers – are intrinsically related to complex terrain. ‘Traditional‘ atmospheric applications such as numerical weather prediction (NWP) or climate modelling get more and more into the regime of complex terrain due to ever increasing model resolution. It therefore seems necessary to revisit the underlying assumptions currently being made when using similarity theory, and to establish the degree to which they are (not) fulfilled in complex terrain. In this contribution we first address the critical issues, put together available (and missing) evidence and summarize the necessary research in order to overcome the most important obstacles. In particular, we will also present first results from the ‘Innsbruck Box’ (or i-Box) which is a currently running project at our institute that aims at addressing some of the research questions as outlined above. The i-Box combines high-resolution numerical modelling with high-resolution observations of turbulence (and other meteorological) characteristics in complex terrain.

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Observations of turbulence in the stable surface layer over inhomogeneous terrain
Karmen Babic, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Zvjezdana Bencetic Klaic, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Zeljko Vecenaj, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia

 

Corresponding author: Karmen Babic
Section: Turbulence

The fate of pollutants in the boundary layer is strongly affected by turbulence which is extremely complicated in the complex terrain and over heterogeneous landscapes. The stable boundary layer (SBL) is generally favorable for establishment of air pollution episodes. Therefore, investigations of its characteristics are of the great importance. In this work we investigate turbulence characteristics in the stable, wintertime nocturnal boundary layer over inhomogeneous surface.
We analyze data measured at five levels of 62 m high mast located near the industrial town of Kutina. Kutina is located at the southern slopes of the afforested Moslavačka Mountain in the hilly region of Moslavina, Croatia. The mast is located in the vicinity of a petrochemical plant, which is the main source of the pollution in the area. The measurement tower is placed over the heterogeneous surface, surrounded by approximately 20 m high trees. The closest trees are 20 to 30 m away from the tower. Wind data were collected by five WindMaster ultrasonic anemometers (Gill Instruments), which were installed at heights of 20, 32, 40, 55 and 62 m. Each of the sonic anemometers measured three wind components and sonic temperature at a sampling frequency of 20 Hz. Since our intention was to capture highly stable situations, we analyze the wintertime, nighttime data collected during December 2008 – February 2009 period.
Here we address the issue of selecting the averaging time scale used for separation of turbulent perturbation from the mean flow, which is appropriate for the nocturnal, stable conditions. This scale is determined by applying several methods widely used in the literature. Preliminary results based on the case study analysis, which were obtained previously for the same location, suggest turbulence averaging time scales between 7 and 11.5 minutes and local maximums of vertical momentum flux and kinematic heat flux at the height of 55 m. Compared to the previous study, here we inspect the larger dataset. In addition, apart from the vertical profiles of vertical momentum flux and kinematic heat flux, here we also investigate vertical profiles of some other turbulence variables.
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Terrain-induced Turbulence: Insights Gained from Airborne In Situ and Remotely Sensed Data
Lukas Strauss (1), Stefano Serafin (1), Vanda Grubišić (1),(2)
(1) Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
(2) Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

 

Corresponding author: Lukas Strauss
Section: Turbulence

This study focuses on the observational analysis of turbulence during events of boundary-layer separation and rotor formation. The objective is to quantify turbulence intensity in these events. To this end, the variance of vertical wind speed and the eddy-dissipation rate are estimated from airborne in situ as well as remotely sensed data.

The study is based on observations from two recent field campaigns: i) NASA06 over the Medicine Bow Mountains in SE Wyoming, and ii) T-REX (2006) in the Sierra Nevada in Southern California. During these two campaigns, the University of Wyoming King Air (UWKA) research aircraft flew straight-and-level legs aligned with the mean wind direction to document the variation of flow and turbulence over the mountain ridges. Aircraft in situ data of wind, pressure and temperature were recorded at a frequency of 25 Hz. The Wyoming Cloud Radar, carried aboard UWKA, measured Doppler wind velocities at multiple levels at a frequency of 30 Hz.

The analysis of two events from the NASA06 campaign reveals “severe” turbulence intensities with maximum eddy-dissipation rates in the range 0.25-0.30 m^2 s^-3. While the maximum turbulence intensities in these two events were found to be similar, the spatial distribution of turbulence was remarkably different. This is a consequence of different gravity-wave response: i) a hydraulic jump-like flow feature on Jan 26, below which a rotor of large vertical extent formed, and ii) a lee-wave pattern on Feb 5 that remained laminar aloft, while the boundary-layer flow was heavily perturbed with a rather diffuse spatial distribution of turbulence. The turbulent regions of “wave-induced” boundary-layer separation are contrasted with more localized patches of turbulence in the immediate lee of smaller but steep topographic features. It is hypothesized that the latter correspond to a “bluff-body” separation of the boundary layer.

The analysis of data collected during T-REX IOPs 3 and 11 paints a similar picture and contrast between a lee wave and a hydraulic jump event. In addition to separation processes, radar Doppler velocities from within the cap cloud over the Sierra Nevada provide a striking evidence of convection over orography as another important turbulence-generating process, leading to turbulence intensities comparable to those in atmospheric rotors.
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Exploring wave-turbulence interaction through LES modeling
M. Udina. Dept. Astronomia i Meteorologia. Universitat de Barcelona, Barcelona, Spain
MR. Soler. Dept. Astronomia i Meteorologia. Universitat de Barcelona, Barcelona, Spain
J. Sun. National Center for Atmospheric Research, Boulder, Colorado.
B. Kosovic. National Center for Atmospheric Research, Boulder, Colorado.

 

Corresponding author: Mireia Udina
Section: Turbulence

Analysis of measurements carried out in a stably-stratified atmospheric boundary layer during CASES99 field experiment have shown the existence of three distinct turbulent regimes (Sun et al. 2012) depending on how the turbulence is generated.

The first regime is characterized by weak wind conditions, below an observed threshold, and weak turbulence. Turbulence in this regime is generated by local shear instability and modulated by vertical temperature gradients. The length scale of turbulent eddies in this regime is defined by local shear and eddies do not directly interact with the ground. The second regime is characterized by strong wind, above the observed threshold, and continuous turbulence with turbulent eddies that extend to the ground. Third regime is the turbulence regime when the wind speed is below the threshold value and top-down turbulence sporadically bursts into the otherwise weak turbulence regime resulting in intermittent turbulence.

WRF-Large-eddy simulations (LES) are performed under a range of different stability and wind speed conditions in order to study the ability of the model to reproduce the different turbulent regimes. In addition, to study the wave-turbulence interaction and to force the third regime, we simulate a flow crossing a small elliptical hill. Results show that under stably-stratified conditions, gravity waves are generated in the lee of the hill interacting with the shear produced in the boundary layer. We also explore the wave field developed leeward of the hill in the different regimes through wavelet analysis comparing the properties of the upwind and downwind to see the effects of the hill in the main flow.
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Generation of a local turbulent katabatic flow along a slope in Antarctica
Hélène Barral, Laboratoire de Glaciologie et de Géophysique de Grenoble, France
Christophe Brun, Laboratoire des Ecoulements Géophysiques et Industriels, France
Christophe Genthon, Laboratoire de Glaciologie et de Géophysique de Grenoble, France

 

Corresponding author: Brun Christophe
Section: Turbulence

Atmospheric boundary layers (ABL) under stable temperature stratification induce katabatic downslope flows with non-trivial turbulent mixing properties. Such complex turbulent shear flow can be found in mountain slope regions where they interact with the atmospheric dynamic in the valley. They are also a specific trend for glacier in mountains or glacier in polar regions, where extreme temperature conditions contribute to enhance the turbulent properties related to the flow. The behaviour of the ABL in polar regions is still poorly represented in global climate models while much of the warming is predicted with twice as much uncertainties. The understanding of turbulence processes for ABL in such extreme latitudes is a keypoint to improve boundary layer parametrizations in general circulation models and thus climate change predictions.

The present study focuses on the generation of a local katabatic flow along a slope in antarctica region, in summer after relative sunset. Once the sun passes behind the relief, incoming solar radiation does not balance anymore long wave radiative cooling for air close to the ground surface. The cold air-surface starts to flow down the slope and feeds the so-called katabatic wind. This local phenomenon has been observed in Antarctica during January 2012 and January 2013 along a slope, in the Terre Adélie coastal region, about 6 km from the french Dumont-Durville scientific station.
The present study consists in numerically reproducing the generation of the local katabatic flow on a real slope using the Meso-NH mesoscale model (Meteo-France). The slope is covered by blue ice and snow, with a width 200 m * 400 m and 15% steep (the local topography has been measured by a GPS in the field ). The full tropospheric layer is modeled up to 8 km height. 1.2 million grid points are considered for discretisation of the present katabatic wall jet. Navier-Stokes equations are solved for mass, momentum and potential temperature conservation, with an anelastic approximation which consists of a linearisation for the continuity equation, only. Large Eddy Simulation (LES) method is considered to solve the turbulent scales of the flow, including an extra transport equation for the sub-grid scale kinetic energy. Initial and boundary conditions are set from in-field measurements available and katabatic process toogether with the turbulent mixing
involved are analysed to (un)validate existing parametrisations for stable atmospheric boundary layers.
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Verification of radiation and cloudiness forecasts in mountain areas
Thomas Haiden
European Centre for Medium-Range Weather Forecasts, Reading, UK

 

Corresponding author: Thomas Haiden
Section: Mountain weather forecasting

Accurate prediction of cloudiness is crucial in mountain weather forecasting. In the past, routine verification of cloudiness at ECMWF was largely based on SYNOP observations. Recently this has been extended to include satellite-derived products and data from a network of surface radiation stations. EUMETSAT Climate Monitoring Satellite Application Facility (CM-SAF) cloud and radiation products are used to analyse various aspects of cloud forecast skill in mountain areas covered by the Meteosat 2nd Generation (MSG) instruments. The verification focuses on daytime conditions since quantities primarily used here are downward solar radiation at the surface and top of the atmosphere reflected solar radiation. The analysis is supplemented by station data from the Baseline Surface Radiation Network (BSRN) and the Austrian Radiation network (ARAD). Two aspects of cloudiness which are difficult to correctly represent in numerical weather prediction models and which have particular relevance for mountain areas are wintertime low stratus / inversion fog, and the diurnal cycle of convection. It is shown how these specific processes affect forecast skill in mountain areas and over flat terrain. Three-way comparison between SYNOP observations of cloudiness, BSRN/ARAD solar radiation, and satellite derived downward solar radiation in the alpine area allows estimation of systematic and non-systematic error contributions. It also allows separation of errors under clear-sky conditions from those specifically due to cloudiness. With respect to downward longwave radiation it is shown to what extent a long-standing negative bias which is more pronounced in cold, dry atmospheres and which occurs both under clear-sky and cloudy conditions, has recently improved. An unexpected result with respect to practical forecasting applications is that use of the ECMWF ensemble forecast improves cloudiness forecast skill even in the short range.
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Objective forecast verification of WRF compared to ALARO and the derived INCA-FVG outputs
ARTURO PUCILLO OSMER - ARPA FVG
AGOSTINO MANZATO OSMER - ARPA FVG

 

Corresponding author: AGOSTINO MANZATO
Section: Mountain weather forecasting

INCA is a nowcasting software developed by ZAMG (Austria) that merges
extrapolated observations by stations and radar with model outputs,
in order to overcome the LAM (Local Area Model) spin-up problem with
a computationally "light" system. This software is particularlyinteresting for areas with a high density of surface stations and
possibly covered by meteorological radars, because the 3-Dvar or
4-Dvar assimilation of these kind of data (radar in particular) isstill difficult in modern LAM and computationally hard.

OSMER - ARPA FVG runs operationally INCA at 1 km of spatial resolution,
fed by the numerous surface stations available in Friuli Venezia
Giulia (NE Italy) and by the Fossalon di Grado doppler radar, using
the ZAMG ALARO 5 (at 4.4 km of spatial resolution) as background model.
For a period of about 75 days of summer 2011, OSMER asked CETEMPS
(L'Aquila University) to run twice a day a special version of Advanced
Research WRF (Weather Research and Forecasting) model at 3 km resolution,that assimilates (with a 3D var method) the same set of surface stations
assimilated by INCA, plus some high resolution soundings, on a larger
domain than that of INCA. On the other way, the WRF simulations do not
assimilate the radar SRI data, that are a very important contribute to
the INCA forecasts for the very first hours.

In this work the preliminary results of an objective verification of
the INCA, ALARO and WRF rain forecasts are presented, using different
methodologies, varying from an object-oriented verification to a nearestneighbor area verification and a punctual verification. The results showthat there is a bit better nowcast performance shown by INCA-FVG system
in the first 3 hours of model lead time, whereas in the farest lead times
the WRF model seems better performing.
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Severe wet-snow storms in the complex terrain of Iceland: Simulations and observations
Hálfdán Ágústsson; Institute for Meteorological Research, Iceland; University of Iceland; Icelandic Metorological Office

Haraldur Ólafsson; University of Iceland; Icelandic Metorological Office; Bergen School of Meteorology, University of Bergen, Norway,

Árni Jón Elíasson; Landsnet, National Transmission Line Operator, Iceland,

Egill Þorsteins; Efla Consulting engineers, Iceland.

 

Corresponding author: Hálfdán Ágústsson
Section: Mountain weather forecasting

Two severe wet-snow windstorms hit Northeast-Iceland on 10 September and Northwest-Iceland on 30 December 2012. Both events were associated with heavy snowfall and strong winds. There was extreme accumulation of wet snow on overhead electric conductors in the affected regions. Combined with strong winds, a large number of distribution line poles and transmission line towers broke due to the severe load.

The extreme snowfall so early in September was exceptional and poorly forecasted by most operational models. There was accumulation of wet snow in a relatively wide altitude interval, with rainfall below approx. 150 metres. Most of the farmers in the region had not gathered their sheep from summer grazing in the mountains, and thousands of sheep were consequently killed after being buried by the wet, heavy snow.

In contrast to the event in September, the heaviest wet snow accumulation in December was more localized and mainly observed in the immediate lee of the very complex orography characterizing the region. Presumably the heaviest accumulations was combined with drifting snow from upstream hills, mountains and other landscape features where the flow was locally enhanced.

Detailed data were collected on the accumulated wet snow load on the affected transmission and distribution lines. Furthermore, accurate measurements of the wet snow accumulation were obtained from several operational load cells. The collected load data are unique in the sense that they describe in detail both the exact timing and the magnitude of the mechanical load resulting from the extreme wet snow accumulation. The events are furthermore well described by meteorological observations from a dense network of weather stations.

The atmospheric flow during the events is further analyzed based on high resolution atmospheric simulations. The simulated data are used as input to a cylindrical wet snow accretion model, producing both icing maps of the accreted wet snow diameter as well as time series at chosen locations, which then are compared to observed or measured wet snow loading. The performance of the accretion model is analysed, highlighting strong and weak points of the accretion models, as well as some key aspects of the flow and accretion process that need further improvement.

The work presented forms a part of the Icewind-project and the COST-action WIRE.
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Which weather situations are favouring high air pollution levels of PM10 and ozone in Ljubljana?
Anton Planinšek, Slovenian environment agency
Tanja Cegnar, Slovenian environment agency

 

Corresponding author: Tanja Cegnar
Section: Air quality and Fire

In 2013, the Year of the air, clean air is the focus of EU environmental policy discussions. According to the Eurobarometer survey, the impact of air pollution is something that European citizens feel strongly about. Data collected by the European environment agency show that lives are being cut short by air pollution and chronic respiratory disease makes life miserable for many across the continent. In spite legislation has led to improvements in many cases, we still register days with exceeded daily limit from time to time. The purpose of our study was to find out the main cause for high air pollution levels in Ljubljana. The hypothesis that the particular weather situation is the main cause for high air pollution has been tested.

Air pollution levels of PM10 and ozone vary from year to year. Criterions for it are limit and target values, set for health protection. Long-range transport depends from direction where air mass comes from. Emissions do not vary very much; therefore the main factor determining air pollution level must be the local weather, especially in such basin where Ljubljana lies.
Air pollution level for PM10 is defined with number of days with exceeded daily limit value. Allowed are 35 cases in calendar year. Similar is at ozone, where 25 exceedances of target value is allowed. Because of daily limit and target values are set, it is necessary to find daily weather situation favourable for exceeding of limit and target value. We took into account some weather parameters in 5 years series and compared them with daily PM10 concentration in winter and with maximum 8-hourly ozone concentration. We got fairly good agreement between number of yearly exceedances of PM10 limit value and ozone target value respectively with number of days that are favourable for such events. The result could be used for prediction purpose and to explain variation in number of days with air pollution exceeding the air pollution threshold.

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Current and future fire weather risk in Tyrol.
David Leidinger, Institute for Meteorology, University of Natural Resources and Life Sciences, Vienna
Herbert Formayer, Institute for Meteorology, University of Natural Resources and Life Sciences, Vienna

 

Corresponding author: David Leidinger
Section: Air quality and Fire

Forests are an important economic factor in Austria and are a valuable ecosystem. They provide protection from natural hazards like avalanches and rockfalls. Wildfires are a serious threat for forests. The risk of wildfires is predicted to increase with climate change. Fire weather indices (FWI) are a proxy for the risk of wildfires and are calculated using a combination of quantities such as temperaturte, precipitation, relative humidity, wind, and phenological data. Various FWI were calculated for Tyrol on a 1x1 km grid using INCA-data and three bias-corrected, localized climate-models to determine the current and future hotspots for the climate-prone risk of wildfires, which is one of many factors driving wildfires. We show an increasing trend in the number of days with high fire weather risk, and that the risk reaches areas, which are not yet affected, such as at high elevations.
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Towards soundproof dynamical core for COSMO model - high resolution weather prediction for the Alps
Zbigniew P. Piotrowski (IMGW-PIB, Warsaw, Poland)
Marcin J. Kurowski (National Center for Atmospheric Research, Boulder, CO, USA)
Bogdan Rosa (IMGW-PIB, Warsaw, Poland)
Damian K. Wójcik (IMGW-PIB, Warsaw, Poland)
Michał Z. Ziemiański (IMGW-PIB, Warsaw, Poland)

 

Corresponding author: Zbigniew P. Piotrowski
Section: Novel modelling

Mathematical model and numerical approximation constitute key elements of a successful numerical experiment. In weather and climate prediction community, there is an ongoing debate on the choice of equation set for dynamical core and the strategy of numerical approximation. Here we present the unique intercomparision of high resolution (2 km and better) weather forecast over the Alps using Runge-Kutta finite difference compressible and nonoscillatory forward-in-time (NFT) anelastic finite volume dynamical cores, within the Consortium for Small-Scale Modeling (COSMO) model for operational regional numerical weather prediction (NWP). For a series of convective and stable weather scenarios, we demonstrate the ability of anelastic dynamical core to handle steep slopes of high resolution topography with minimal, implicit filtering and the resulting high level of detail in dynamical fields. Presented general agreement between the compressible and anelastic strategies, good quality of anelastic solutions and its robustness make the anelastic NFT approach a good candidate for high horizontal resolutions of operational regional NWP models in foreseeable future.
The efforts discussed are part of the 'CELO' priority project of COSMO consortium to implement new dynamical core for operational weather prediction in second half of the decade.
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Wind resource assessment in complex terrain with a high-resolution numerical weather prediction model
Karin Gruber, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Stefano Serafin, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Manfred Dorninger, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Vanda Grubišić, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria and Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
Rudolf Zauner, VERBUND Renewable Power GmbH, Vienna, Austria

 

Corresponding author: Karin Gruber
Section: Novel modelling

A crucial step in planning new wind farms is the estimation of the amount of wind energy that can be harvested in possible target sites. Wind resource assessment traditionally entails deployment of masts equipped for wind speed measurements at several heights for a reasonably long period of time. Simplified linear models of atmospheric flow are then used for a spatial extrapolation of point measurements to a wide area.

While linear models have been successfully applied in the wind resource assessment in plains and offshore, their reliability in complex terrain is generally poor. This represents a major limitation to wind resource assessment in Austria, where high-altitude locations are being considered for new plant sites, given the higher frequency of sustained winds. The limitations of linear models descend from two key assumptions in their formulation, namely neutral stratification and attached boundary-layer flow, which often break down in complex terrain. Consequently, an accurate modeling of near-surface flow over mountains requires the adoption of a NWP model with high horizontal and vertical resolution.

This study explores the wind potential of a site in Styria in the North-Eastern Alps. The WRF model is used for simulations with a maximum horizontal resolution of 400 m. Three nested computational domains are defined, with the innermost one encompassing a stretch of the relatively broad Enns valley, flanked by the main crest of the Alps in the south and the Nördliche Kalkalpen of similar height in the north. In addition to the simulation results, we use data from eleven 10-m wind measurement sites (of which 6 in valleys and 5 near mountain tops) and from 2 masts (at hillside locations) in an area of 1600 square km around the target site.
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Perspectives from field experiments in Iceland in recent years
Haraldur Ólafsson et al.
University of Iceland, Icelandic Meteorological Office and Bergen School of Meteorology, Geophysical Institute, University of Bergen

 

Corresponding author: Haraldur Ólafsson
Section: Field campaigns and Measurement

Several observational campaigns have been carried out in Iceland and between Iceland and Greenland in recent years. The experiments were based on portable network of weather stations, manned and unmanned aircrafts, supported by radar and an ever-increasing amount of data from remote sensing and a permanent network of ground-based observations. In all these campaigns mountains played important, but different roles. A review of highlights of results from these campaigns will be given, including thermally driven mesoscale flows, wakes, gravity waves, drag, local jets and the interaction of some of the above features.
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METEOMET: METROLOGY FOR DATA QUALITY IN CLIMATE AND METEOROLOGICAL OBSERVATIONS
Domen Hudoklin, University of Ljubjana, Faculty of Electrical Engineering, Ljubljana, Slovenia
Janko Drnovšek, University of Ljubjana, Faculty of Electrical Engineering, Ljubljana, Slovenia
Gaber Begeš, University of Ljubjana, Faculty of Electrical Engineering, Ljubljana, Slovenia
Jovan Bojkovski, University of Ljubjana, Faculty of Electrical Engineering, Ljubljana, Slovenia
Fabio Bertiglia, Istituto Nazionale di ricerca Metrologica - INRiM, Torino, Italy
Giuseppina Lopardo, Istituto Nazionale di ricerca Metrologica - INRiM, Torino, Italy
Guido Roggero, Istituto Nazionale di ricerca Metrologica - INRiM, Torino, Italy
Francesa Sanna, Istituto per le macchine agricole e movimento terra IMAMOTER, Consiglio Nazionale delle ricerche - CNR, Ferrara, Italy
M. Carmen Garcia Izquierdo, Centro Espanol de Metrologia, Tres Cantos, Spain
Andrea Merlone, Istituto Nazionale di ricerca Metrologica - INRiM, Torino, Italy

 

Corresponding author: Domen Hudoklin
Section: Field campaigns and Measurement

In October 2011, a joint research project called “MeteoMet - Metrology for Meteorology” (www.meteomet.org) started, bringing together a wide consortium of partners: 18 national metrology institutes and 6 universities as partners plus 29 collaborators, including national meteorological organizations, research institutes associations and instruments companies. The project is aiming to respond to the needs for new stable and traceable measurement standards, protocols, sensors and calibration procedures, and uncertainty-evaluation methods, to enhance data reliability and reduce uncertainties in climate models. This project is part of the European Metrology Research Program (EMRP) coordinated by the European Association of National Metrology Institutes (EURAMET).

The activities that are on their full way now, comprise of investigation of new humidity sensors for upper air measurements with setting up a traceability chain based on tuneable diode laser absorption spectroscopy (TDLAS); development of novel methods and instruments for the measurement of temperature, humidity, and pressure in lower and upper atmosphere; development of traceable measurements methods and protocols for temperature, humidity, pressure and airspeed ground-based measurements needed for climate studies and meteorological long–term and wide scale observations; etc. Additionally, the project deals also with the construction of a facility for in-situ traceable calibration of weather stations. Three models of this facility have been developed for different uses, including extreme environmental conditions: one of those chambers will be transported and used at the Pyramid laboratory/observatory of Mount Everest, for the calibration of sensors positioned in the Himalayan area.

As the project will terminate in 2014, initial steps were already taken to start a follow-up project, which will place even more focus on alpine meteorological issues related to metrology. At present, several high mountain stations located mainly in alpine environment are run by different services or institutions without commonly agreed procedures for measurements, instruments involved and site characteristic, which hardly complies with the WMO directive. The particularity of high mountain environment requires new kind of site and instrument validation processes. A proposed reference station in high mountain alpine area, combined with other kind of monitoring (permafrost, water lakes/springs, etc.), is necessary in order to analyse the best way the relationship among atmosphere, geosphere, cryosphere, hydrosphere and biosphere in alpine region, particularly sensitive to climate change and where their effects are more pronounced. A pilot surface site in support of high mountain programs and in conjunction with the permafrost monitoring network would improve the quality of weather and climate parameters recorded in high alpine. The evaluation of mutual relations at seasonal, annual, long-term level would be possible.

For the measurement of the high mountains lakes temperature of the water and ice mantles it is proposed the definition of specific systems, best practice, calibration and measurement uncertainty evaluation, for a practical reference example to be implemented through instruments and calibration campaigns. Dedicated instruments calibration for temperature and soil moisture probes and assessment of calibration and measurement uncertainty are proposed too. A methodology to achieve calibration target uncertainty in the order of 0.01 °C is proposed for permafrost monitoring studies.

The adoption of standard equipment and procedures for alpine stations, would benefit the work of the European nations belonging to the Alps Convention in their common mission of climate and weather observations.
These ambitious objectives are planned to be discussed also in the scope of a first International workshop on traceability and uncertainty evaluation for meteorology, climatology and earth observations which is scheduled for late spring 2014.

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Thermal comfort analysis in the Alpine region
Reinhold Steinacker and Dieter Mayer, Department of Meteorology and Geophysics, University of Vienna, Austria

 

Corresponding author: Reinhold Steinacker
Section: Climate change

Besides the basic state variables of the atmosphere, complex quantities which need several variables for its computation are of special interest for applications in applied climatology. One of these specific quantity is the thermal comfort index for men, which is determined for both, cold and warm conditions. For its full determination the air temperature, humidity, wind speed and solar radiation is required. As the diurnal range of the relevant quantities may vary dramatically, it does not make sense to compute such indices on a daily or even monthly basis. At least three hourly - or even better hourly observations have to be taken to decribe the thermal comfort for men. With the aid of the hourly VERA (Vienna Enhanced Resolution Analysis) fields, we have started to determine the hourly fields and statistics of the comfort index - at present without considering the solar radiation - for the whole Alpine region with a very high resolution. To avoid a more or less pure topographic field distribution due the extreme hight differences, we focus in one display on the low lands and Alpine valley floors, where the vast majority of people live. The fields for 2012 show interesting and partly unexpected details in the occurrence and frequency of different levels of discomfort due to cold and hot weather conditions. We intend to recompute fields of thermal comfort for the last 30 years to see the corresponding trends and impacts of the climate change.
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Climate change effects on temperature and ice of the lake Bohinj, Slovenia
dr. Peter Frantar

 

Corresponding author: dr. Peter Frantar
Section: Climate change

Lake Bohinj is the biggest non-karstic natural lake in Slovenia. It is located in the Julian Alps 525 m asl. with maximum depth of 45 m and area of 3,2 km2. State monitoring of basic lake characteristics started near 1900 with water levels and ice monitoring and in 1939 with additional lake water temperature measurements. The analysis showed that climate change is having effects also on the lake ice duration and on the rise of lake water temperatures. Especially after the 1980 the ice is diminishing and the temperatures are growing. These 2 indicators can therefore be set also as climate change indicators for alpine space. We present the trend of lake ice duration, trend of yearly water temperature and the seasonal changes in water temperature regime.
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Evaluation of a 10-year Cloud-resolving climate simulation for the greater Alpine region
Nikolina Ban, ETH Zürich
Jürg Schmidli, ETH Zürich
Christoph Schär, ETH Zürich

 

Corresponding author: Nikolina Ban
Section: Climate change

Large uncertainties in current regional climate model integrations are partly related to the representation of clouds, moist convection, and complex topography. Experience with numerical weather prediction generally shows that increased spatial resolution, and switching from parametrized convection to cloud-resolving models, leads to a better forecast. For climate simulations, however, the potential of cloud-resolving models is not yet sufficiently investigated. Recent studies using cloud-resolving models in climate mode have shown highly promising results. Here we present a cloud-resolving simulation for a 10-year long period (1998-2007) integrated with the COSMO-CLM (Consortium for Small-Scale Modeling in Climate Mode). Two one way nested grids are used with horizontal resolution of 2.2 km for a cloud-resolving simulation (CRM) on an extended Alpine domain (1100km x 1100km) and of 12 km for a regional climate simulation (CPM) which covers Europe. The CRM is driven by hourly lateral boundary conditions from the CPM run while the CPM run is driven by 6-hourly lateral boundary conditions from ERA-Interim reanalysis data. To eliminate spin-up issues, initial soil moisture is taken from the CPM simulation which has been integrated over the 1993-2007 period. We will present evaluation results in terms of differences between the cloud-resolving simulation CRM and the parametrized convection run CPM. Particular consideration will be given to the inter-comparison of the precipitation distributions from the two simulations over complex terrain, including an assessment of extreme precipitation events.
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Ensemble approach to the homogenisation of monthly temperature series in Slovenia
Gregor Vertačnik, Damjan Dvoršek, Renato Bertalanič, Mojca Dolinar, Mateja Nadbath (all Slovenian Environment Agency, Slovenia, Ljubljana), Matija Klančar (University of Ljubljana, Slovenia, Ljubljana)

 

Corresponding author: Gregor Vertačnik
Section: Climate change

Many studies in recent years have stressed the importance of homogenous data series for assessing climate variability and climate change. Data series obtained from measurements of climate variables are rarely homogenous, mostly due to the relocations of weather stations and instrumentation changes. Homogenisation procedures are then often applied to correct for this kind of inhomogenities in the data series. While many statistical methods have been developed for the purpose, recent COST project HOME tried to evaluate them on the benchmark dataset. Some of best methods and practices for homogenisation were then packaged in a semi-objective homogenisation tool HOMER. To our knowledge no one has tried to estimate the subjectivity factor arising from the expert performing homogenisation procedure with this tool. In order to estimate this factor in the homogenised series, several experts performed homogenisation on the same dataset of real monthly series of mean, daily maximum and daily minimum temperature series.

Additionally, three subsets of mean temperature dataset were homogenised to assess the impact of network density on the final results. Analysis of homogenised series revealed only few significant differences in derived climate features between different experts and datasets. On the other hand, the ensemble approach indicated the need for high-quality and continuous climate measurements as well as the importance of dense network grid.

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ANALYSIS OF RECENT SNOW COVER AND SKIABILITY CONDITIONS IN A FEW MOUNTAINOUS AREAS OF SLOVENIA
Massimiliano Fazzini - University of Ferrara - Department of physics and earth sciences - Via Saragat, 1 - 44100 Ferrara - Italy
Paolo Billi - University of Ferrara - Department of physics and earth sciences - Via Saragat, 1 - 44100 Ferrara - Italy
Tanja Cegnar - Environmental Agency of the Republic of Slovenia - Lubjiana
Gabriele Avancini - University of Ferrara - Department of physics and earth sciences - Via Saragat, 1 - 44100 Ferrara - Italy

 

Corresponding author: Massimiliano Fazzini
Section: Climate change

In eastern Alpine system, nivometric measurements, carried out during the last three decades, indicate a decreasing trend of seasonal accumulation; for the same time interval, a marked increase in temperatures (more than 0.7 °C) of the winter semester has caused a patent rise of the snow limit, a decrease of snow permanence on the ground, hence of natural skiability, at least for elevations less than 1500 m a.s.l. Aim of this study is to analyze these parameters also for a few mountainous areas of Slovenia by means of time series ranging from 1980 to 2010 and characterized by no or very limited data gaps. Nine meteo-nivometric stations were selected. They are located in all the main mountain sectors of Slovenia (Julian Alps, Karavanke, Pohorie and other isolated mountains) at elevations between 480 and 2520 m a.s.l. and a few of them (Ratece, krvavec, Voijsko, Vogel) are included in important skiing resort districts. For the meteo station of Ratece, located within the famous skiing district of Kranjska Gora, a longer time series is available, hence the data analysis regarded the 1950 – 2010 interval. The analysis of daily thermometric and igrometric data has confirmed the possibility to produce artificial snow whenever the natural snow cover is not enough for alpine skiing sports. In fact, it has been observed that the amount of snow is decreasing significantly and constantly, at between two to four centimeters per season whereas the number of skiing days is decreasing from 0.7 to 1.4 per season, with the exception of Krvavec station where no negative trend is observed. We confirm that the skiing threshold of 100 days per years is reached only in the skiing sites located at an elevation higher than 1500 m a.s.l., whereas below such elevation the need of artificial snow is becoming more and more impelling. At the Ratece station, the number of natural skiing days equals 70 but, with the support of artificial snow, a total number of 100 skiing days is easily reached. Such a threshold is considered as the minimum condition for the economic developent of the skiing district.
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Dust storms in the Icelandic highlands

Pavla Dagsson Waldhauserova 1,2, Haraldur Ólafsson 1,3,4, and Ólafur Arnalds 2
1 University of Iceland, Department of Physics, Reykjavik, Iceland. 2 Agricultural University of Iceland, Faculty of Environment, Hvanneyri, Iceland. 3Meteorological Office of Iceland, Reykjavik, Iceland. 4 Bergen School of Meteorology, Geophysical Institute, University of Bergen, Norway.

 

Corresponding author: Haraldur Ólafsson
Section: Air quality and Fire

Manned weather observations in NE-Iceland have been explored in order to investigate the intensity and frequency of dust storms emanating from the Icelandic highlands in strong southerly winds.

A total of 2201 dust days were reported in Iceland in 1949-2011. The annual mean of 35 dust days pr. year in Iceland is similar to that found in dust-active parts of China, Mongolia or Iran.

As glaciers retreat in a warming climate, new dust sources emerge and dust storms may become an increasing problem.

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Understanding and forecasting air pollution episodes in Slovenia
Rahela Žabkar, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia and Center of Excellence SPACE‐SI, Ljubljana, Slovenia
Luka Honzak, Center of Excellence SPACE‐SI, Ljubljana, Slovenia
Marko Rus, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia

 

Corresponding author: Rahela Žabkar
Section: Air quality and Fire

Slovenia is known for its large topographical and climate diversity, where over an area of only 20 thousand square kilometers Mediterranean, Continental and Alpine climate zones are present. It is well known, that modeling air quality over geographically complex areas can be especially challenging. In the present contribution three modeling approaches used for air quality modeling in Slovenia are presented and evaluated: statistical regression model, regional scale modeling with in-line coupled meteorological-photochemistry WRF-Chem model and with off-line coupled ALADIN/CAMx modeling system. With the use of different approaches we investigate the characteristics of the temporal and spatial dynamics of the high O3 and PM10 episodes with the purpose to understand and forecast air quality in Slovenia. We also present the first results of the operational WRF/Chem and ALADIN/CAMx air quality forecasts running experimentally since January 2013.
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Time scales of the atmospheric boundary layer evolution in an idealized valley
Johannes Wagner
Alexander Gohm
Mathias Rotach

 

Corresponding author: Daniel Leukauf
Section: Boundary layer

In recent years, the mechanisms of thermally-driven wind systems over complex terrain have
been investigated through a series of idealized numerical simulations. The numerical models
used in these studies were typically initialized with an atmosphere at rest and a simplified
thermal stratification. In some studies, constant forcing was applied at the surface which does
not allow for the evolution of a full daily cycle and these simulations were typically integrated
only over a few hours. On the other hand, there are studies which use radiative forcing and
capture the day and night cycle but in most cases these models were integrated over less than
two days. The question remains whether the simulation period is long enough to capture a
quasi-steady state (QSS) or, in other words, how strong the conclusions drawn from these
studies are affected by the chosen analysis time. This question is fundamental if one aims
at developing a parametrization of exchange processes based on bulk fluxes of heat, moisture
and other properties from the valley to the free atmosphere evaluated from such idealized
simulations.

In this study we want to determine the time scales for reaching a QSS in the evolution
of the atmospheric boundary layer (ABL) as a function of various forcing parameters. A
QSS is characterized by a daily cycle of the ABL that is essentially identical from one to
the next day, e.g, the same maximum ABL height in the afternoon. For this purposewe
conduct large eddy simulations with the Weather Research and Forecasting (WRF) model of
thermally-driven flows in an idealized valley and compare the results to the ABL evolution
over flat terrain. An important aspect of the study is the impact of the soil moisture content
on the QSS time scale (QSSTS).

The chosen valley geometry consists of two sine-shaped mountain ridges which form a 20-
km wide and 40-km long valley with a flat valley floor. As the terrain is homogeneous in the
along-valley direction and periodic boundary conditions are used, only slope winds but no
valley winds evolve. We use full physical parametrizations including radiation and surface atmosphere exchange processes to represent realistic forcing. The initial vertical profiles are
characterized by a constant buoyancy frequency, a constant relative humidity and zero winds.
In addition to the valley simulation the model is run with flat terrain using a three times
smaller domain and all simulations are carried out over five full days.

It can be shown that the ABL reaches a QSS typically after three days for relatively dry soil
(semi desert). Using the QSS ABL profile of the flat-terrain simulation as initial conditions
for the valley simulation shortens the spin-up time by about one days. On the other hand, for
a moist soil the ABL does not reach a QSS within the five days of simulation and boundary
layer heights are much smaller.

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A remotely piloted aircraft for investigation of mountain flows
Joachim Reuder 1, Marius O. Jonassen 1 and Haraldur Ólafsson 1,2,3
1 Bergen School of Meteorology, Geophysical Institute, University of Bergen, 2 University of Iceland, 3 Icelandic Meteorological Office

 

Corresponding author: Haraldur Ólafsson
Section: Boundary layer

In recent years, the Small Unmanned Meteorological
Observer SUMO has been developed as a flexible tool for atmospheric
boundary layer (ABL) research to be operated as sounding system for the
lowest 4 km of the atmosphere. Recently two main technical improvements
have been accomplished. The integration of an inertial measurement
unit (IMU) into the Paparazzi autopilot system has expanded the
environmental conditions for SUMO operation. The implementation of
a 5-hole probe for determining the 3D flow vector with 100 Hz resolution
and a faster temperature sensor has enhanced the measurement
capabilities.
Results from two recent field campaigns are presented. During the
first one, in Denmark, the potential of the system to study the effects of
wind turbines on ABL turbulence was shown. During the second one, the
BLLAST field campaign at the foothills of the Pyrenees, SUMO data
proved to be highly valuable for studying the processes of the afternoon
transition of the convective boundary layer.
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Inside the Reykjavík shelter
Marius Opsanger Jonasson, Bergen School of Meteorology, Geophysical Institute, University of Bergen, Haraldur Ólafsson, University of Iceland, Icelandic Meteorological Office and Bergen School of Meteorology, Geophysical Institute, University of Bergen, Hálfdán Ágústsson, University of Iceland, Icelandic Meteorological Office and Institute for Meteorological Research and Ólafur Rögnvaldsson, Institute for Meteorological Research

 

Corresponding author: Haraldur Ólafsson
Section: Boundary layer

The Reykjavik shelter
The city of Reykjavík is located at the west coast of Iceland, with mountain ranges some 10 km to the south, southeast and northeast. The wind climate of Reykjavík is analyzed with ground-based observations, reanalysis data and by observations from a remotely piloted aircraft.
The mountain range to the northeast provides a remarkable shelter, while the mountains to the southeast do not. In northeasterly flows, Reykjavík is inside a wake and the mean surface wind speed is only about 35% of the wind speed at 850 hPa and this ratio is sensitive to the static stability. In southeasterly flow which also is downstream of a mountain range, the mean surface wind speed is about 50% of the 850 hPa winds and this ratio is independent of the static stability. A case of northerly flow is explored with a remotely piloted aircraft and a complex stratified wake structure is revealed.

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Near-surface wind shear over the complex terrain: observational analysis
Kristian Horvath, Meteorological and Hydrological Service, Gric 3, 10000 Zagreb, Croatia
Darko Koracin, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512
Craig Smith, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512

 

Corresponding author: Kristian Horvath
Section: Boundary layer

Although analysis of winds and wind regimes in complex terrain has received considerable attention, near-surface wind shear has been considerably less studied primarily due to scarcity of the observed data. Nevertheless, the importance of near-surface wind shear for air pollution, wind energy, transport and infrastructural design, as well as the increasing number of wind towers and ground-based remote sensing measurements promotes a more detailed focus on the analysis of near-surface wind shear.

Four meteorological towers in west-central Nevada, equipped with cup and vane anemometers at 10 m, 20 m, 30 m 40 m and 50 m AGL, provided an excellent dataset for analysis of the near-surface wind shear. Mean wind speed on average increased with height at all analyzed wind towers. The average wind shear decreased with height, however, the decrease was not necessarily monotonous. The temporal variability of wind shear was much higher during the night than during the day, while maximal wind shear values (both positive and negative) were found during episodes of nocturnal winds. Furthermore, relationship between wind shear and wind speed was qualitatively different during daytime and nighttime. Finally, a considerable portion of nighttime events with a negative wind shear suggests the presence of shallow nocturnal circulations, but negative wind shear can likewise occur during daytime and for the entire range of the wind speed distribution.

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An investigation of the Ora del Garda wind in the Alps by means of Kriging of airborne and surface measurements
Lavinia Laiti - University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy
Dino Zardi - University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy
Massimiliano de Franceschi - Major Seminary, Diocese of Bolzano-Bressanone, Bressanone, Italy
Gabriele Rampanelli - Depuration Agency, Autonomous Province of Trento, Trento, Italy

 

Corresponding author: Lavinia Laiti
Section: Boundary layer

The present study investigates a coupled lake-valley wind, known as Ora del Garda, which typically arises in the late morning on clear-sky days along the northern shorelines of Lake Garda and then channels northward into the Sarca and Lakes valleys, until breaking out into the Adige Valley north of Trento city (southeastern Italian Alps), where it interacts with the local up-valley wind in a complex fashion. This thermally-driven circulation displays great regularity, marking the local climate with a strongly mildening influence. A series of targeted measurement flights, performed by means of an instrumented motorglider, allowed to explore the valley atmosphere thermal structure at selected vertical sections, namely over the lake’s shore, at half valley, at the end of the valley, and at the junction with the Adige Valley. Dominant vertical profiles of potential temperature were inferred from airborne data, while 3D potential temperature fields were mapped over high-resolution regular grids for each explored section, through the application of a Residual Kriging (RK) technique both to airborne and to surface data from weather stations disseminated along the valley floor. These procedures allowed to identify atmospheric boundary layer (ABL) features typical of most diurnal valley winds. In particular, rather shallow convective mixed layers, surmounted by deep stable layers, occur up-valley. On the other hand, closer to the lake the advection of colder air tends to stabilize the atmosphere throughout the whole valley ABL depth. Small-scale features of the thermally-driven wind field produced by the coupling between the lake breeze and the up-valley circulation into a unified mesoscale flow were revealed by RK-interpolated potential temperature 3D fields; the development of a lake-breeze front structure was captured in the shoreline area, while up-valley sections displayed cross-valley thermal asymmetries, mostly amenable to the different irradiation of the valley sidewalls and to inhomogeneities in the surface coverage, but also to the curvature of the valley in its final part. Lastly, RK-interpolated fields suggested the occurrence of a hydraulic jump structure at the end of the Lakes Valley, where the Ora del Garda potentially cooler air overflows from an elevated saddle down to the underlying Adige Valley floor, producing there an anomalous and gusty cross-valley flow.
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Numerical simulations of the Ora del Garda wind in the Alps: a comparison with surface and airborne measurements
Lavinia Laiti - University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy
Lorenzo Giovannini - University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy
Dino Zardi - University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy

 

Corresponding author: Lavinia Laiti
Section: Boundary layer

In order to investigate the atmospheric boundary layer (ABL) structures associated with the development of a lake-breeze and valley-wind coupled system developing in the Lakes Valley (southeastern Italian Alps), the so-called “Ora del Garda” wind, a series of measurement flights were performed by means of an instrumented motorglider on four warm-season days. The flights explored specific valley sections at key locations in the study area, namely over the lake’s shore, at half valley and at the end of the valley where the breeze blows. Air pressure, temperature and relative humidity measurements were recorded. At the same time, surface observations from a number of local automated weather stations were collected. In order to get a complementary view of the phenomenon, high-resolution numerical simulations of the flights days were carried out using the model WRF. Five nested grids were used, achieving a final horizontal spacing of 0.5 km. High-resolution orography and land use datasets were adopted for the domain initialization, while NCEP reanalysis provided initial and boundary conditions for the meteorological fields. The simulations were initiated at 1800 UTC of the day preceding the flight. The Unified Noah land-surface model and the Bougeault-Lacarrere scheme for PBL physics were used. The preliminary results display a rather good agreement with the experimental dataset. In particular, the surface daily cycles of radiation, wind and air temperature are satisfactorily reproduced, despite some discrepancies in the timing of thermally-driven circulation onset and offset, probably due to initialization inaccuracies. The typical structure of the valley ABL, characterized by shallow or even absent mixed layers surmounted by slightly stable layers extending up to the lateral crest level, is also qualitatively well reproduced in the simulated fields. Moreover, the simulations confirmed characteristic local-scale features of the thermally-driven wind field that were suggested by the analysis of the airborne dataset. For example, the model showed the development of a well-defined lake breeze front in the lake’s shoreline area, as well as the formation of a hydraulic jump structure in the area north of Trento city, i.e. at the end of the Lakes Valley, where the Ora del Garda denser current flows down into the nearby Adige Valley from an elevated saddle (elevation difference: 400 m).

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Impact of mesoscale meteorological processes on the anomalous propagation conditions
Maja Telišman Prtenjak;
Andrija Mohorovičić Geophysical Institute, Department of Geophysics, Faculty of Science, University of Zagreb, Zagreb, Croatia

Igor Horvat
Meteorological and Hydrological Service of Croatia, Zagreb, Croatia

Mladen Viher
Croatian Air Force HQ,Croatia

 

Corresponding author: Maja Telišman Prtenjak
Section: Boundary layer

The occurrence of the anomalous propagation (so called anaprop) of radio waves: radioducts, superrefractions and subrefractions is created by the different properties of the atmosphere through the altitude difference in temperature, humidity and air pressure. Atmospheric variability is a result of large scale conditions, frequently modified by mesoscale structures that influence the height of boundary layer and produce local fronts with abrupt spatial changes in temperature and humidity.
Here we study the impact of 3 mesoscale phenomena (over the wider area of the northern Adriatic) on the occurrence of anaprop conditions: sea/land breeze, the bora wind and cumulonimbus clouds. The selected period that unites them is 13 - 20 August 2000. For the purpose of the analysis we used radiosoundings in Udine, with a sampling period of every 6 hours and numerical WRF-ARW model at the horizontal resolution of 1.5 km every hour. Results show that the model can successfully simulate the occurrence of anaprops in Udine, although their intensity is sometimes underestimated. The model has certain difficulties in reproducing the correct height and intensity of anaprops since the faithful vertical profile of the modified refractive index is the most dependent on the accuracy of the modeled vertical changes of relative humidity.
Spatial anaprop distributions show that the sea surface (between 30 and 100 m a.s.l.) is mainly covered by superrefractions and radioducts through the entire studied period. Sea breezes (SB) are continuously connected with the anaprop formations: (i) in the first 100 m above the ground within SB body where superrefractions and radioducts form due to advection of colder and moist air, (ii) in upper region of the SB front which are usually connected with elevated radioducts and superrefractions, (iii) inside transition layer between the SB body and anti-SB current with subrefractions. When a deep convection over land appears, we observe the elevated superrefractions and subrefractions between 0.5 and 1 km above ground. Subrefractions are caused by downdraft beneath the cumulonimbus cloud base in its mature phase that creates smaller pools of cold and dry air. Below subrefractions in the lowermost 200 m, the type of anaprop is changed from superrefractions to radioducts. The bora wind through by the advection of colder and drier air, in the shallow surface layer usually creates radioducts and superrefractions: over land and coast, inside the hydraulic jump, and over the sea and islands, in somewhat deeper layer. Bora is also associated with subrefractions: (i) over the sea surface along the edges of bora jets where a lateral exchange of air with various moisture content occur due to the convergence of flow and increased vorticity, and (ii) on the windward side of Dinaric Alps where the formation of the local vortices below the mountain top affect the moisture profile.

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Calculation of Snowline Climatology over the Alpine Region from ECMWF ERA-Interim reanalysis
Imran Nadeem, Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna Herbert Formayer, Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna

 

Corresponding author: Imran Nadeem
Section: Climate change

Snow is a key element within the Alpine region, which not only affects the ecosystem, but has an important economical factor in the Alps. For many adaptation measures in several economic sectors, information about the snow conditions is needed. In most climate impact assessments, snow conditions are simply a function of the elevation. But within the Alpine region, this elevation dependency is only correct for snow melt. The elevation of the snow level (elevation where falling snow melts to rain) highly depends on the air mass causing the precipitation. Air masses in the Alps coming from the Mediterranean or the Atlantic are several degrees warmer during winter than air masses arriving from the North or Baltic Sea. Correspondingly the elevation of the snow level is much higher in regions, where precipitation is caused from air masses from the Mediterranean or the Atlantic.

An algorithm has been developed to calculate snow line from pressure level data of temperature, relative humidity and geopotential. The algorithm calculates wet-bulb temperature (WB) at each pressure level during the precipitation event and then looks for the elevation where WB is zero. The algorithm also takes into account the melting of the snow in the atmosphere and evaporational cooling when melting starts. For the present study we used 3-hourly ECMWF ERA-Interim re-analyses at pressure levels of 700, 850, 925 and 1000 hPa. The snowline has been calculated for the period 1979-2012 over the whole Europe domain. For analysis we will present results only for the Alpine domain. A comparison of the last three decades will be presented which allows a quantification of the climate change impact on snow level within the Alpine region. A comparison of the effect on the snow level with the general warming within this period gives some findings, if dynamical effects have been relevant for the Alpine domain. The same method can be applied to regional climate model data. This will give a tool for comprehensive analyses of the climate change impact on the elevation of the snow level during the 21st century.
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Climate change scenarios of precipitation extremes in mountain regions of Europe based on ENSEMBLES regional climate models
Jan Kyselý, Institute of Atmospheric Physics & Technical University of Liberec, Czech Republic
Ladislav Gaál, Institute of Atmospheric Physics & Slovak University of Technology, Slovakia
Romana Beranová, Institute of Atmospheric Physics & Technical University of Liberec, Czech Republic

 

Corresponding author: Jan Kysely
Section: Climate change

Mountain regions and areas with complex orography are particularly prone to heavy precipitation and floods, and uncertainties in possible changes associated with climate change may be enhanced due to complexity of interactions between orography, atmospheric circulation, and local processes. The study examines possible changes in high quantiles of precipitation amounts in scenarios of late 21st century climate in an ensemble of regional climate model (RCM) simulations over Europe from the EU-FP6 ENSEMBLES project. The RCMs have a resolution around 25 km and are driven by several global climate models. The projected changes are evaluated in winter and summer seasons, and on a wide range of time scales from hourly to multi-day amounts. The region-of-influence method is applied as a pooling scheme when estimating distributions of seasonal maxima of precipitation, which leads to spatial patterns of high quantiles that are smoothed compared to local analysis. We focus on a comparison of results (scenarios) for several mountain and lowland regions of Europe, and evaluate altitudinal dependence of the projected changes of precipitation extremes. Specific attention is paid to the performance of the pooling scheme, particularly how the regional homogeneity criterion is met in areas with complex topography. Uncertainty of the projected changes due to inter-model variability is also assessed and compared for the selected regions.
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Daily air temperature range analysis for the Slovak Tatra Mountains part and scenarios up to the year 2100
Marian Melo (Comenius University in Bratislava, Slovakia)
Ivan Bastak-Duran (Czech Hydrometeorological Institute, Prague, Czech Republic)
Martin Gera (Comenius University in Bratislava, Slovakia)
Milan Lapin (Comenius University in Bratislava, Slovakia)

 

Corresponding author: Marian Melo
Section: Climate change

An analysis of daily maximum and minimum temperatures as well as of daily temperature range at selected meteorological stations in the Slovak part of the Tatra mountains region in 1951-2010 are studied. Mean daily temperature range is defined as a difference between the daily temperature maximum and minimum. Time series of temperature range means have been analyzed by trend analysis and other statistical tools. In the second part of contribution daily outputs of two regional climate change models (Dutch KNMI and German MPI, both with ECHAM5 boundary conditions) have been used for design of climate change scenarios (daily maximum and minimum of air temperature and daily range of air temperature) for selected stations in this region and the period 1951-2100. Comparisons of measured and modeled temperature characteristics (daily extremes and daily range) are included.
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Winter 2012-2013, exceptional snowfall in Trentino Dolomites: the example of Fiemme Valley
Massimiliano Fazzini - University of Ferrara, Department of physics and earth sciences - Via Saragat 1 44100 Ferrara - Italy
Sergio Zeni - ITAP Pampeago, Tesero (TN) Italy
Gérard Beltrando - *** Université Paris Diderot, UMR 8586 (PRODIG) du CNRS (cc 7001) 75205 Paris cedex 05

 

Corresponding author: Massimiliano fazzini
Section: Climate change

Notwithstanding the winter season is not over yet, it has shown uncommon meteoclimatic features with often perturbed and cold weather and very abundant snowfall also at low elevations. The seasonal values of the last ten years are characterized by a wide nivometric variability with the 2008-2009 season as the most snowy on average since 1951. A first analysis of nivometric parameters, carried out on data of a several stations located in the skyiing district “Ski Center Latemar” (Fiemme Valley, eastern Trentino), which includes the snow fields of Pampeago, 1760 m a.s.l., Monsorno, 2000 m a.s.l., Naturale Agnello 2080, m a.s.l., Tresca, 2180 m a.s.l., Gardoné, 1655 m a.s.l. and the snow fields of Alpe Cermis, 2100 m a.s.l., Passo Rolle, 2000 m a.s.l. and Passo Valles. 2040 m a.s.l., shows that the highest values of 2008-2009 season have been locally exceeded in terms of snowy days and fresh seasonal snow accumulation. In particular, by comparing the nivologic data of 2008-2009 and 2012-2013 a marked difference in temporal distribution and intensity daily results. The 2008-2009 season was characterized by a first trimester, spanning from the end of November to the end of February, with extreme abundant snowfalls, whereas during the second part of the season enough dryer and colder conditions prevailed. In the current 2012-2013 season, the snow precipitation spectrum has been cyclic and uninterrupted since the last days of October to the end of March. These conditions have been favored by the constant occurrence of the zonal flow, with an undulatory polar jet stream at latitudes lower than expected, whereas the extremely abundant snowfall event that characterized the beginning of the 2008-2009 winter season, determined by prefrontal massive advections of Mediterranean air, was absent.
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RECENT SNOW AND SYNOPTIC SITUATIONS BEARERS OF HEAVY SNOWFALL ON THE MOUNTAINS OF SICILY ISLAND
Massimiliano Fazzini - University of Ferrara, Department of Physics and earth sciences, Via Saragat, 1 - 44100 Ferrara;
Antonino Paladino - Corpo Forestale della Regione Siciliana
Vincenzo Romeo - Corpo Forestale dello Stato - Servizio Meteomont - Rome - Italy
Antonio Cardillo - Regione Molise - ARPC
Carmela Vaccaro - University of Ferrara, Department of Physics and earth sciences, Ferrara

 

Corresponding author: Massimiliano Fazzini*,Antonino Paladino, Vincenzo Romeo, Antonio Cardillo, Carmela Vaccaro
Section: Climate change

Sicily covers over 25,000 square kilometers in the Mediterranean basin, which appears to be the largest Island; it has a population of over 5 million inhabitants. The mountain domains stretch for 24% of the total area, and they appear to be numerous and diversified elevation, morphology and geology. In the northern part of the region, overlooking the Tyrrhenian Sea, stands the mountainous alignment Nebrodis-Peloritani-Madonie that rises up to 1980 m of Pizzo Carbonara. Along the east coast there is the active volcano Mount Etna - 3,343 meters high. Given a typically Mediterranean rainfall regime, with cool and very rainy winters - despite the latitude between 37 and 39 degrees north - in these domains mountains, snow is quite generous and above 1800 meters it is possible the practice of ski. On the top of Mount Etna, the snowpack remains generally from late October to mid-June. In the period 1960-1992, the Regional Hydrographic Service has monitored the condition of the snowpack in the winter resort of Floresta - located on Nebrodi Mountain at over 1200 meters above sea level. There are measures the height of the snow on the 10th, 20th and 30th of the month and the duration of snow while you do not have measures relating to the cumulative of fresh snow. First analyzes of the data shows that the snow is conspicuous at least above 1500 meters above sea level; the sum of snow varies between 150 and 250 cm while the snow remains on the ground for an average period of 55 -100 days. In the last two seasons, the snowfall the snowfalls were very abundant due at the frequent advection of continental or intermediate polar air which affected also the central and southern areas of the Mediterranean basin. In the mid-March 2012, the height of the snow on the ground has exceeded 250 cm. The study of the synoptic conditions bearers of heavy snowfall occurred in the last two seasons, completed those relating to historical snowfall which also affected the plains and hilly areas of the region in the last fifty years, completed the study.
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Climate change in Slovenia in the period of 1961–2011
Mojca Dolinar, Renato Bertalanič, Gregor Vertačnik, Damjan Dvoršek, Mateja Nadbath (all Slovenian Environment Agency, Slovenia, Ljubljana), Matija Klančar (University of Ljubljana, Slovenia, Ljubljana)

 

Corresponding author: Mojca Dolinar
Section: Climate change

Climate records have become increasingly important in the last 20 years, mostly due to the impacts of anthropogenic climate change. Analysing of historical data sets for the purpose of climate change monitoring is an immense task. Slovenian Environment Agency launched a project in 2008 to study climate variability since 1961 in Slovenia. It began with a thorough quality control and metadata collection. Bad quality data series were discarded from the further analysis, while the rest were homogenised and analysed. The most striking feature is a strong positive temperature trend of about 3.3 °C per century in the analysed 51-year period. The trend is slightly higher in eastern Slovenia. All seasons, except autumn, have warmed significantly. Precipitation sum is more variable, with the linear trend on seasonal scale being mostly insignificant. Snow cover depth has been the most variable and challenging climate element studied. Sunshine duration shows mostly increasing trend.
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The relevance of cut-off low systems to manifestation of large scale extreme precipitation events in the Alpine region
Nauman K. AWAN and H. Formayer
Institute of Meteorology, Department of Water, Atmosphere and Environment, University of Natural Resources and Applied Life Sciences (BOKU), Vienna

 

Corresponding author: Nauman Khurshid AWAN
Section: Climate change

In this study we attempt to highlight the relevance of cut-off Low systems (CoLs) to large scale extreme precipitation events. The CoLs are detected with the aid of a numerical algorithm which is based on the physical characteristics of CoLs. The European Center for Medium-Range Weather Forecasts (ECMWF) ERA-40 re-analysis dataset is used as an input. The output of the algorithm contains the co-ordinates of centers of CoLs. The main outcomes of this study are: 1) a detailed climatology (1971-1999) of CoLs for the European region, 2) contribution of CoLs to large scale extreme precipitation events in the Alpine region, 3) identification of regions mostly affected by large scale extreme precipitation, 4) identification of regions of CoLs recurrence, 5) identification of regions where presence of CoLs is related to extreme precipitation in Alps. For evaluating precipitation Swiss Federal Institute of Technology Zürich dataset is used. The findings of this study suggest that CoLs occur quite often in the Alpine region. They also have a significant influence on the climate of this region. However, they are more often in Summer then in any other season (more than 80 % of the events occur in summer season). The area around the Alps and West of Spain (over the Atlantic Ocean) is the most favored region. The most affected regions are in the East and North of the Alps.
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LONG-TERM VARIATION OF LARGE-SCALE CIRCULATION AND ITS RELATION TO CLIMATE CHANGES AT THE KOLA PENINSULA
Demin V.I., Polar Geophysical Institute of RAS, 14, Fersmana St., Apatity, 184209 Russia
Chernous P.A., Center for Avalanche Safety, "Apatit" JSC, 33a, 50-years of October St., Kirovsk,184250 Russia
Moroz N.V., Center for Avalanche Safety, "Apatit" JSC, 33a, 50-years of October St., Kirovsk,184250 Russia

 

Corresponding author: Pavel Chernous
Section: Climate change

All variety of atmospheric processes can be organized into several classes. G. Vangengeim generalized all types of atmospheric processes into three so-called circulation forms – zonal (W) and two meridianal (E and C). These circulation forms are represented as specific patterns of global baric and thermal fields in the troposphere. It is important that there are long-term changes in the frequency of circulation forms which are named as a circulation epochs. Their periods have a length of about ten and more years. During circulation epochs the frequency of one (or two) circulation form(s) exceeds the long-term mean value. Changes of the circulation epochs cause the long-term trends of regional weather conditions. Thus this factor should be taken into account for the detection of regional climate variations.
In this research long-term changes of air temperatures at the Kola Peninsula were analyzed separately in different homogeneous groups of the circulation processes (W, E and C). It is shown that mean anomalies of winter air temperature on the top of the Lovchorr Mountain (the Khibiny Mountains, 1091 m.a.s.l.) for periods of 1991-2011 are more 1.7, 07 and 0.9 °C than ones for period of 1961-1990 for the forms W, E, C correspondingly. Increases of mean anomalies of summer temperatures for same periods are 1.6, 0.4 and 1.3°C. Analogue results were obtained for other stations of the Kola Peninsula. Thus the modern warming in the region is very difficult to interpret as a result of changes atmosphere circulation since the increase of air temperatures is observed for all circulation forms.

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AN INCREASE IN THE UPPER TREE LIMIT IN THE KHIBINY MOUNTAINS (KOLA PENINSULA, RUSSIA) AND CLIMATE CHANGE
Demin V.I., Polar Geophysical Institute of RAS, 14, Fersmana St., Apatity, 184209 Russia
Chernous P.A., Center for Avalanche Safety, "Apatit" JSC, 33a, 50-years of October St., Kirovsk,184250 Russia
Moroz N.V., Center for Avalanche Safety, "Apatit" JSC, 33a, 50-years of October St., Kirovsk,184250 Russia

 

Corresponding author: Pavel Chernous
Section: Climate change

Changes of vertical temperature distribution in the Khibiny Mountains (Kola Peninsula, Russia) were analyzed on the basis of measurements at the mountain meteorological stations. It is shown that mean summer temperature in the Khibiny Mountains for period of 1961-2011 is 0.3-0.4°С higher than one for period of 1881-1960. This change is corresponds to movement of summer isotherms by 30-50 m upward. At the same time a comparison of the old and contemporary photos of mountain slopes shows that since the end of 19th century the upper tree limit in the Khibiny Mountains has risen up by 100-150 m. It has been found that the forest expansion continued also during the periods of cooling of 1940-1970. Most likely upper tree limit was located lower than possible thermal boundary, i.e. movement of tree limit has the time lag relatively to the temperature changes.
It is suggested that the observed rise of upper tree limit in the Khibiny Mountains is resulted of long-term (centuries) climate changes mainly and in a lesser degree is a response to short-term (decades) climatic variations including the present-day warmng in the region which is started from the second half of 1980’s.

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High resolution simulations of cold air pooling in small-scale valleys
John Hughes, Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, UK
Andrew N Ross, Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, UK
Simon Vosper, Met Office, Exeter, UK
Adrian Lock, Met Office, Exeter, UK

 

Corresponding author:
Section: Cold air pools

COLPEX (COLd-air Pooling EXperiment) was a large field campaign studying cold air pooling and fog formation in small-scale UK valleys with a width of ~1-2km and a depth of ~200m depth. A series of 100m resolution simulations were performed with the MetUM (the UK Met Office unified model). In addition to running IOP case studies, a 2 month long continuous run was performed to allow a more systematic assessment of the performance of the model at this high resolution over a range of different conditions. The data has been used to understanding the processes leading to cold air pool formation and development in these valleys. The results are also being used to understand the impact of these small scale processes on the parametrised fluxes in lower resolution models. Results from the high resolution simulations are coarse grained to a lower resolution and compared with results from simulations run at lower resolutions. The aim is to improve the parametrisation of these sub-grid cold air pooling processes in lower resolution weather and climate models. This paper will present results from the cold air pool heat budget study along with initial results from the coarse graining.
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Idealized simulations of canyon exit jets in Utah
Allison Charland, University of Utah
C. David Whiteman, University of Utah
Manuela Lehner, University of Utah

 

Corresponding author: Allison Charland
Section: Cold air pools

Nocturnal wind jets have been observed at the exit of Weber Canyon to the east of the Great Salt Lake Basin in Utah during late summer and early fall on clear, undisturbed nights. These canyon exit jets develop 1-3 h after sunset and can reach maximum wind speeds of 15-20 m s-1 overnight, then weaken after sunrise. Idealized simulations using CM1, a 3D, time dependent, non-hydrostatic numerical model, have been analyzed to determine the causes of the formation and dissipation of the canyon exit jets.

Simulations show the formation of a valley-exit jet around 1900 MST, about 1 h after sunset, which weakens in the late morning hours the next day. Cold, stable air builds up in Morgan Basin, upstream of Weber Canyon, overnight due to a down valley flow. The air drains through the canyon and accelerates as it is compressed downward at the canyon exit. Simulations show the development of a hydraulic jump at the canyon exit with high wind speeds present beneath the region of the hydraulic jump. The jet maximum, downstream of the hydraulic jump, occurs between 50-100 m above the ground.

High wind speeds extend nearly 10 km horizontally into the Great Salt Lake Basin from the canyon exit and are partially deflected northward due to a plateau located south of the exit region. There is also a second, weaker exit jet that develops out of Ogden Canyon, north of Weber Canyon.

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The Interaction of Convective Storms with Complex Terrain: A Case Study of an Alpine Supercell
Phillip Scheffknecht, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Stefano Serafin, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Vanda Grubišić, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria and Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

 

Corresponding author: Phillip Scheffknecht
Section: Convection

In this study we examine the interaction of a convective storm with complex terrain. On 2 August 2007 a supercell storm moved north of and along the Alpine main ridge for over 8 hours, passing over several individual peaks in excess of 2000 m. The event is very well suited to identify the ways in which topography influences the development of a convective system.

WRF simulations with a maximum horizontal resolution of 833 m are performed. Model output is verified with surface weather station data and radar imagery. We analyze how the intensity of the supercell's updraft is affected by the distribution of CAPE and moisture in the valleys and over mountains along the storm track.

The real case simulation produces a convective system whose development is very similar to the observed one. The storm intensity variations correlate with the altitude variations of the underlying terrain and with CAPE availability along the storm track. The depth of the moist layer tends to be higher over mountaintops than over valleys. In fact, thermal circulations push the warm and moist air out of valleys ahead of the storm and concentrate it along convergence lines. The storm updraft intensifies every time it crosses one of these areas.

In addition to the real case (control) simulation, two sensitivity tests are made to examine the effect of the terrain on the supercell development. Results from these semi-idealized simulations suggest that the Alpine ridge as a whole influences the wind field in such a way that the wind shear is favorable for the formation of a supercell. On the other hand, smaller-scale valleys and ridges influence the storm by supporting thermally driven circulations, which redistribute moisture in the boundary layer favoring an exceptionally long-lasting event.
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Extended intensive rainfall in Rijeka due to local orography
Martina Tudor, Kristian Horvath, Stjepan Ivatek-Šahdan
Croatian Meteorological and Hydrological Service

 

Corresponding author: Stjepan Ivatek-Šahdan
Section: Convection

A heavy precipitation event occurred over the north-eastern Italy, Slovenia and north-west Croatia in the afternoon and evening of 12th September 2012. The event was associated to a warm and moist low level air-mass from the Adriatic Sea and a cold front. Rain-gauges recorded 24 hourly precipitation exceeding 100 mm, up to 160 mm over north-eastern Italy, but reaching 220 mm in Rijeka, Croatia. That area received the most intensive rainfall during this event according to the data from the rain-gauges and TRMM.
In the morning, a cold front, associated to a pressure through moving eastward, interacted with warm and moist low level air and initiated severe deep convection first over northern Italy. The system moved eastward, according to the lightning maps.
Operational forecasts using ALADIN at CMHS did forecast rainfall amounts exceeding 100 mm in 24 hours, but most forecast runs put the area of maximum intensity over Slovenia, and only one operational forecast run moved the high intensity precipitation towards Rijeka. According to ombrograph, precipitation intensity was the highest from 21 to 22 UTC (85.3 mm/h), with 20.6 and 51.7 mm/h in the previous and the next hour.
The band of intensive rainfall moved eastward over flat plain of Po valley, more intensive above land than above water of northern Adriatic. But when it approached more complex terrain, secondary bands of intensive rainfall form in front of it, according to radar data. At the same time satellite figures show formation of cumulonimbus clouds. This intensive rainfall band reaches Trieste and Slovenia according to radar figures at 19:30 UTC and merges with the rainfall band that formed above Trieste at 18 UTC. Another rainfall band forms above Istrian peninsula at 19:30 UTC. Intensive rainfall spreads to Rijeka and remains there for several hours. During that time other rainfall bands form and arrive to Rijeka intensifying the precipitation and prolonging the period of high precipitation intensity.
The high resolution non-hydrostatic operational forecast shows upward motions all along the coastal mountains of Croatia and an approaching band associated to the rain band.

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The impact of data assimilation of different observation types on forecast of severe precipitation in the northern Adriatic during Hymex IOP 16
Antonio Stanesic (Croatian Meteorological and Hydrological Service)
Kristian Horvath (Croatian Meteorological and Hydrological Service)
Stjepan Ivatek-Sahdan (Croatian Meteorological and Hydrological Service)
Tomislav Kovacic (Croatian Meteorological and Hydrological Service)

 

Corresponding author: Antonio Stanesic
Section: Field campaigns and Measurement

During IOP 16 of the HYdrological cycle in the Mediterranean Experiment – Hymex, a heavy precipitation event occurred over the northern Adriatic region and surrounding mountains and lasted from 26 Oct – 28 Oct 2012. The areas most affected by the event were the municipality of Rijeka in Croatia and regions near Italian-Slovenian border, where the accumulated precipitation in those two days reached more than 200mm. This event was used to perform the initial analysis of the impact of the mesoscale data assimilation of different observational data types, including targeted observations, on the forecast accuracy using the Observing System Experiments (OSE) approach.

Simulations were performed using the operational hydrostatic model implemented at DHMZ, which is ALARO with 8km horizontal grid spacing and 37 vertical levels. The data assimilation system used in this study consists of the surface assimilation (OI) and the upper-air assimilation (3DVar) within the ALADIN model framework. The observation dataset used in this study includes SYNOP, SYNOP-SHIP, radiosoundings, MSG AMW winds and satellite radiances from NOAA (AMSU-A, AMSU-B) and MSG (SEVIRI). Furthermore, additional Data Targeting System extra-soundings performed during the IOP16 were used to study the potential for targeting heavy precipitation events in the northern Adriatic area. Forecasts were validated against rain-gauges and remote sensing instruments.
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Comparison of NWP-model chains by using novel verification methods
Manfred Dorninger, Departement of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Theresa Gorgas, Central Institute for Meteorology and Geodynamics, Vienna, Austria
Reinhold Steinacker, Departement of Meteorology and Geophysics, University of Vienna, Vienna, Austria

 

Corresponding author: Manfred Dorninger
Section: Field campaigns and Measurement

Forecasts of a set of three model chains characterising a variety of model versions and types are evaluated. Each model chain consists of three models with increasing resolution nested into one another. Rules for a fair model inter-comparison have been defined. Inter alia, they refer to the use of NWP-model independent analyses as reference data which, in this study, are provided by the VERA (Vienna Enhanced Resolution Analysis) system. Observational data and model data have been collected in a combined effort of COPS (Convective and Orographically induced Precipitation Study) and D-PHASE (Demonstration of the Probabilistic Hydrological and Atmospheric Simulation of flood Events in the alpine region). Verification parameters are precipitation and the gradient of equivalent potential temperature as front indicator. The verification domain covers Central Europe. Verification periods range from half a year to single case studies. A choice of novel and traditional verification metrics has been implemented to examine multiple aspects of the model chains.The results only partly confirm previous findings that the models with the highest resolution usually outperform their counterparts of lower resolution. We find a rather different behaviour from model chain to model chain. Additional forecast skill is not stringently added by the nested models with the highest resolution. In the case of frontal propagation it is the coarsest model, which shows the best results. Wavelet transforms are used to study phase and modulus coherence of forecast and analysis on different scales.
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Slope- and Valley Winds and their interaction – Observations with Doppler Wind LiDARs during MATERHORN
Sebastian W. Hoch, University of Utah, UT
Christopher Hocut, University of Notre Dame, IN
Amélie A. Klein, Ecole Normale Supérieure de Cachan, FR / University of Utah, UT
Yansen Wang, US Army Research Laboratory, MD
C. David Whiteman, University of Utah, UT
Harindra J. Fernando, University of Notre Dame, IN

 

Corresponding author: Sebastian W. Hoch
Section: Field campaigns and Measurement

The characterization of local drainage flows on an alluvial fan on the eastern slope of Granite Peak, and their interactions with the diurnal up- and down-valley circulation was a goal of Doppler Wind LiDAR measurements during the first experimental phase of MATERHORN (Mountain Terrain Modeling and Observation Program) conducted at Dugway Proving Ground, Utah, in Fall 2012.

LiDAR observations on the synoptically quiescent night of 1-2 October 2013 are presented. They show the formation and development of a shallow drainage flow on the eastern slope of Granite Peak, undercutting the previously dominating up-valley circulation by ~1800 MST. A down-valley flow becomes established by ~2100 MST bringing much colder air to the study area and undercutting the local drainage/slope flow. The interaction between the two flows includes the development of Kelvin-Helmholtz waves at their interface. During the course of the night, the shallow local drainage/slope flow becomes reestablished several times, mainly on the upper part of the slope seen by the LiDARs. The drainage flow appears to be separated from the surface and rides up onto the colder air put in place by the down-valley flow. The down slope flow can be followed for 1.5 km after separating from the slope and turning into the down-valley direction.
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Surface Energy Balance Observations during MATERHORN
Sebastian W. Hoch, University of Utah
Derek Jensen, University of Utah
Eric Pardyjak, University of Utah
C. David Whiteman, University of Utah
Harindra J. Fernando, University of Notre Dame

 

Corresponding author: Sebastian W. Hoch
Section: Field campaigns and Measurement

Surface Energy Balance Observations during MATERHORN

Observations of the earth's surface energy balance have long been a focus of atmospheric research. The partitioning of the available energy from the sun varies widely by geographic location, land surface type, exposure, soil properties, and available moisture. The energy balance near the surface ultimately controls boundary layer development and evolution, and spatial energy balance differences lead to the formation of thermally driven circulations, such as sea-, lake- and playa breezes, slope- and valley wind circulations. Most of the time, when all components of the energy balance (net radiation, ground heat flux, turbulent sensible and latent heat fluxes) are measured directly, the energy balance is not closed and a residual term remains.
All components of the surface energy balance were directly measured at three different sites using some of the best available instrumentation during the experimental phase of MATERHORN (Mountain Terrain Modeling and Observation Program) conducted at Dugway Proving Ground, Utah, in Fall 2012. One site is located in a large sparsely vegetated arid basin, a second on a playa (dry alkali flats which fills with water seasonally to form shallow lakes). The third site is located on a sparsely vegetated slope of an alluvial fan of Granite Peak.
Besides the measurements of the individual short- and longwave components of the radiation balance and eddy-covariance measurements of the turbulent fluxes, our special focus was directed to the soil heat flux. Pairs of self-calibrating heat flux plates were uses at all sites, and the heat storage term above the flux plates was calculated from soil temperature measurements at three levels along with direct observations of the soil's volumetric heat capacity.
Albedo differences between the sites are the main cause of variations in energy input, but we also show how variations in soil properties lead to large differences in the energy balance. Differences in the energy balances among the three sites and their implications for boundary layer evolution will be discussed, along with measurement uncertainties and the residual term.
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What type of föhn event causes the highest melt rates on the Larsen C Ice Shelf, Antarctica?
Andrew Elvidge - UEA
Ian Renfrew - UEA
Thomas Lachlan-Cope - BAS
John King - BAS
Andrew Orr - BAS
Mark Weeks - UK Met Office

 

Corresponding author: Andrew Elvidge
Section: Foehn, Bora and windstorms

Two contrasting cases of westerly flow across the Antarctic Peninsula are investigated with regard to the distribution of leeside warming and the consequent effect on Larsen C Ice Shelf melt rates. Data is derived from aircraft observations, AWS measurements and the UK Met Office Unified model run down to a horizontal resolution 1.5 km. In Case A relatively weak southwesterly cross-Peninsula flow determines a non-linear föhn event. The consequent strongly accelerated flow above, and hydraulic jump immediately downwind, of the lee slopes lead to high amplitude warming in the immediate lee of the AP, downwind of which föhn warming diminishes greatly due to the upward ascent of the turbulent föhn flow. Melt rates are insignificant on the Larsen C away from the AP. Case B defines a relatively linear (‘deep’) case associated with strong northwesterly winds. There is no laterally extensive hydraulic jump and föhn flow is able to flow at low level right across the ice shelf. Melt rates are high due to a combination of large solar flux (providing the largest surface flux) in association with a dry cloudless lee, and sensible heat flux due to the warm relatively well-mixed near-surface föhn flow. The implication is that whilst non-linear föhn events cause intense warming at the base of the lee slopes, linear föhn events cause more extensive near-surface warming in the downwind direction and consequently greater ice melt rates on the Larsen Ice Shelf.
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Longitudinal vortices in the bora wind
Danijel Belušić, School of Mathematical Sciences, Monash University, Melbourne, Victoria, Australia
Željko Večenaj, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Margaret A. LeMone, National Center for Atmospheric Research, Mesoscale and Microscale Meteorology Division, Boulder, Colorado, USA

 

Corresponding author: Danijel Belušić
Section: Foehn, Bora and windstorms

The bora downslope windstorm dynamics has been studied at several different scales, ranging from the synoptic scale, through various mesoscales, where the interaction with mountains occurs that generates jets and wakes and the related PV banners, wave breaking and rotors, down to turbulence scales and wind gusts. However, a rather narrow range of scales has been overlooked despite a large body of previous studies. Our results indicate that bora generates longitudinal roll vortices above the Adriatic Sea with the wavelength of about 1 km. The bora flow setup is ideal for the formation of rolls - cold air outbreak over the relatively warm sea, with moderate to strong winds and wind shear. It thus comes as a surprise that rolls have not been assumed nor reported in previous bora literature. The most likely cause of this omission is the absence of the distinctive clouds streets that are usually associated with the roll vortices. This study examines the characteristics of the bora rolls observed by the Electra aircraft on 07 November 1999 during the MAP experiment.
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A new windgust record in a downslope windstorm in Iceland
Hálfdán Ágústsson (1,2,3) and Haraldur Ólafsson (1,2,4)
(1) Háskóli Íslands (Univ. of Iceland), Reykjavík, Iceland, (2) Veðurstofa Íslands (Icelandic
Meteorol. Office), (3) Bergen School of Meteorology, Geophysical Institute, University of Bergen, Norway, (4) Reiknistofa í
veðurfræði (Institute for Meteorological Research), Reykjavík, Iceland

 

Corresponding author: Haraldur Ólafsson
Section: Foehn, Bora and windstorms

On 1 November 2012, 10 min mean winds of 38 m/s and a wind gust of 71 m/s were recorded at the
coast of Hamarsfjörður, Southeast-Iceland. This is the strongest windgust ever recorded by a reliable instrument
close to sea level in Iceland. The atmospheric conditions leading to the strong windgust are explored and described.
The flow is statically stable and there is strong gravity wave activity on scales ranging from the scale of Iceland
(>100 km) to the scale of the Hamarsfjörður fjord (<10 km). At mid-tropospheric levels, there is a critical level in
terms of wave energy trapping and on the 1 km horizontal scale, there is a lowering in topography. Both these
features appear to contribute to the acceleration of the flow. The mean wind speeds are reasonably well reproduced
in a high-resolution simulation, but the observed gusts are greater than predicted by the Brasseur method. Guided
by the low level wind speed, the wind direction and the vertical profile of temperature, the frequency of similar
events in future climate scenarios is explored.
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A nice orographically generated wintertime temperature record
Haraldur Ólafsson1,3,4 & Hálfdán Ágústsson1,2,3,
1 Háskóli Íslands (University of Iceland), 2 Reiknistofa í veðurfræði (Institute for Meteorological Research), Iceland, 3Veðurstofa Íslands (Icelandic Meteorological Office), 4Geophysical Institute, University of Bergen

 

Corresponding author: Haraldur Ólafsson
Section: Foehn, Bora and windstorms

On 29 March 2012, a temperature recording of 20.5 C was made at Kvísker in Southeast-Iceland.
This extreme event is analysed and described by a numerical simulation. The analysis of the event leads to
identification of the following important characteristics:

a) A warm airmass

b) A low level inversion and strong winds at mountain top level leading to an upstream blocking and yet
strong downslope flow

c) Weak winds (<2 m/s) leading to a superadiabatic surface layer

d) Snow-free and dry soil for a low albedo and a high Bowen ratio

Some of the above situations are infrequent, such as dry soil in March and warm and strong foehn winds
over the slopes in SE-Iceland, but calm at the same time at the weather station in question.

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The Reykjavík 2012 downslope windstorm
Einar Magnús Einarsson (1,2), Haraldur Ólafsson (1,2,3), Hálfdán Ágústsson (1,2,4), Ólafur Rögnvaldsson (3,4)
(1) Háskóli Íslands (Univ. Iceland), (2) Veðurstofa Íslands (Icelandic Meteorol. Office), (3) Bergen
School of Meteorology, Geophysical Institute, University of Bergen, Norway, (4) Reiknistofa í veðurfræði (Institute for
Meteorological Research), Reykjavík, Iceland

 

Corresponding author: Haraldur Ólafsson
Section: Foehn, Bora and windstorms

An extreme downslope windstorm occurred in November 2012 in the area commonly known as the Reykjavik wake in northerly flow . Unlike in most northerly flows, the downslope windstorm which is usually confined to
the foothills of the Esja mountain, propagated over the city of Reykjavik, more than 10 km downstream of Mt. Esja.
There was great temporal and spatial variability in the winds and this variability is to some extent reproduced in
high-resolution numerical simulations. The fluctuations in the wind pattern downstream of Mt. Esja coincide with
variations in the vertical structure of the flow, particularly the elevation of an inversion located close to mountain top level. The variations correspond to the flow pattern moving between Type E and Type S windstorm described in Agustsson & Olafsson (Meteorol. Atm. Phys., 2010) which makes this windstorm the first documented case of a single windstorm moving from one category to another.

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On the downslope nature of wind extremes in Iceland and their variability
Birta Líf Kristinsdóttir 1,2 & Haraldur Ólafsson 1,2,3
1 Univ. Iceland, 2 Icelandic Meteorol. Office, 3. Bergen School of Meteorology, Geophysical Institute, University of Bergen, Norway

 

Corresponding author: Haraldur Ólafsson
Section: Foehn, Bora and windstorms

Extreme winds are studied from observations of approximately 200 automatic weather stations in the complex
terrain of Iceland. In spite of the detrimental effect of surface roughness on low-level winds, hardly any of the
extreme windstorms are in winds blowing from the sea. On the contrary, about four out of five extreme windstorms
are when winds are blowing downwards from a nearby mountain, indicating that gravity waves are a dominating
factor in extreme winds in the complex terrain of Iceland. A second result is that extreme winds are significantly
more frequent in the night than during the day. This confirms the importance of gravity waves, as the static
stability is greater during the night than during the day.
The pattern agrees with dynamic numerical downscaling of winds over Iceland.
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Very high resolution idealized simulations of foehn wind conditions in Tatra mountains
Katarzyna Nurowska (Institute of Geophysics, University of Warsaw, Poland)
Szymon P. Malinowski (Institute of Geophysics, University of Warsaw, Poland)
Zbigniew P. Piotrowski (IMGW-PIB, Warsaw, Poland)

 

Corresponding author: Katarzyna Nurowska
Section: Foehn, Bora and windstorms

We present preliminary results of very high resolution (up to 60 m) modelling of foehn wind ("halny") conditions over Tatra mountain range in southern Poland and Slovakia. We set up EULAG model for geopfysical flows on a domain of 86 by 57 km with topography based on ASTER dataset. Simulations are initiated with spatially uniform atmospheric conditions derived from atmospheric sounding in Poprad. We choose situations representing reported cases of extremly high wind on the lee side of Tatras. We analize flow response, model robustness and computational performance for a series of horizontal resolutions ranging from 300 m to 60 m.
This research serves as a reconnaissance of a gray zone are between cloud resolving numerical weather prediction and LES, It is complementary to the implementation of EULAG model as a new soundproof dynamical core of COSMO framework, reported in the presentation of Piotrowski et al. and is planned to be later concluded with experimental simulations of full COSMO framework at such resolutions.
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On the turbulence integral scales for the bora flows at the NE Adriatic coast
Željko Večenaj, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Danijel Belušić, School of Mathematical Sciences, Monash University, Melbourne, Victoria, Australia
Branko Grisogono, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia

 

Corresponding author: Željko Večenaj
Section: Foehn, Bora and windstorms

Single point 3D high frequency wind measurements were conducted at the north-eastern Adriatic coast in the town of Senj (44.99°N, 14.90°E, 2 m above MSL, 13 m above the ground, March 2004 – June 2006), and Vratnik Pass (44.98°N, 14.98°E, 700 m above MSL, 10 m above the ground, Oct. 2004 – Sep. 2005), using WindMaster ultrasonic anemometers (Gill Instruments). These instruments recorded the data with a sampling frequency of 4 Hz. They were continuously operational for more than two years in Senj and almost a year in Vratnik Pass, recording all kinds of airflows. Using these data sets, we have a great opportunity to investigate turbulence characteristics of the local wind in this region.

We estimate turbulent kinetic energy and its dissipation rate for bora events in both Senj and Vratnik Pass between the coastal mountains. Using these estimations, we estimate integral turbulence length scale and integral turbulence time scale (a total eddy lifetime) through the turbulence closure techniques. The relationships between these scales and turbulence dissipation scales are investigated and compared with the relationships already reported in the literature. Among other reasons, these quantities are important for numerical model evaluations and turbulence parameterization scheme improvements.

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Changes in surface water amounts considering climate change in Slovenian mountain region
Florjana Ulaga M.Sc., Slovenian Environment Agency
Mira Kobold Ph.D., Slovenian Environment Agency

 

Corresponding author: Florjana Ulaga M.Sc.
Section: Hydrology and Snow

Monitoring and studying the hydrological conditions and events have become in recent years, when the frequency of flood and drought events due to climate change is more noticeable, more relevant. Temporal changes in water quantities in Slovenian rivers were analyzed with trends of river discharge as an important indicator of changes in the hydrological regime. The analysis was performed for small, medium and large flows measurement with a set of data over 30 years. Results show a general reduction of water, but trends throughout Slovenia are not statistically significant, nor are all declining. Statistical classification of mean annual flow of Slovenian rivers suggests five balanced groups of stations, each of which represent the largest group of stations in the basins of northwestern Slovenia. This group merging gauging station the Soča catchment, Sava catchment and upper part of the catchment Savinja. They are characterized by strong, statistically significant decrease in mean annual flows, the decline of small flows and large flows increase. The trend is most significant at the gauging stations in upper Sava and upper Soča catchment, where the impact of the Alpine mountain hinterland is most pronounced.
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Can a point measurement represent the snow depth in its vicinity? A comparison of areal snow depth measurements with selected index sites.
Grünewald Thomas; WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 35, 7260 Davos, Switzerland and Cryos, School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne, GRAO 402 - Station 2, 1015 Lausanne, Switzerland
Lehning Michael; WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 35, 7260 Davos, Switzerland and Cryos, School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne, GRAO 402 - Station 2, 1015 Lausanne, Switzerland

 

Corresponding author: Grünewald Thomas
Section: Hydrology and Snow

Information on the amount of snow stored in a catchment or on a mountain is an important issue for hydrology, climatology, natural hazards, winter tourism or mountain ecology. The snow cover is shaped through the processes of deposition, redistribution and ablation. These processes strongly interact with the local terrain and result in a strong spatial variability of the snow depth distribution. Typical patterns are deeper snow packs in sheltered locations or an increase of snow depth with elevation. Area-wide measurements of the snow depth in an appropriate spatial and temporal resolution are usually not available as they are costly and difficult to obtain for larger areas. In practice one needs to rely on few, selected point measurements, very often connected to meteorological stations, which give snow depth at a specific location. One then assumes that these index sites represent the snow cover in their vicinity. Such index sites are usually located in flat and sheltered terrain and it has been questioned if such places are really capable to represent the snow cover of their larger surrounding. In this study we use a large data set of areal snow depth measurements obtained by airborne LiDAR in six different mountain regions. With a spatial resolution of 1 m and an accuracy of few decimetres, the data sets are well representing the real snow distribution at the time of the survey, which was close to the peak of the accumulation season for most sites. By applying moving window techniques on digital elevation models and snow depth maps for each study area, we automatically identify flat places with homogeneous snow cover. We then define these locations as virtual index stations and analyse, if the snow depths at the sites do represent the real snow amount in the direct vicinity and of the entire catchment. We show that single stations are not able to represent the snow cover and that most index stations tend to clearly overestimate snow depth. There are, however, also index sites which measure much smaller snow depths than the mean in the surrounding area. Such stations are typically wind-exposed. We also show that elevation gradients of snow depth are qualitatively captured by index sites, even though there often occur large differences in comparison to the real gradients.
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Characterisation of snowpack for better understanding of water balance in Julian Alps (Slovenia)
Iztok Sinjur, Slovenian Forestry Institute, Ljubljana, Slovenia
Gregor Vertačnik, Slovenian Environment Agency, Ljubljana, Slovenia
Jaka Ortar, Anton Melik Geographical Institute, Ljubljana, Slovenia
Mihael Brenčič, University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of geology, Ljubljana, Slovenia
Polona Vreča, Jožef Stefan Institute, Department of Environmental Sciences, Ljubljana, Slovenia

 

Corresponding author: Iztok Sinjur
Section: Hydrology and Snow

The area of Julian Alps (NW Slovenia) represents the upper catchment area of river Sava, a tributary to Danube. The area is locally characterised as one with the highest annual precipitation amount in Europe, rapid runoffs, low evaporation and regular snow cover. The storage of precipitation in snowpack in alpine areas, and the subsequent melting, substantially impacts the water cycle and the important part of the annual runoff takes place in connection with the spring melt. Recent climate warming and changes in atmospheric circulation patterns have resulted, also in Slovenia, in reductions in the duration of the snow cover season, the amount of water stored in the snowpack, a widespread trend toward earlier melt as well as in changes in discharge regime. However, these processes, spatial and temporal variability of snow, as well as snow cover contribution to the water balance in Julian Alps remain poorly investigated. Therefore, the main objectives of the on-going research are 1) to better characterise the snowpack properties, 2) to determine the isotopic composition of snow and other components of water cycle (precipitation, groundwater and surface water) and finally 3) to improve understanding of influence of snow and snowmelt on infiltration and recharge of important Slovene karstic aquifers and the dynamics of springs recharged from these aquifers.

Preliminary snow investigations started in 2011 at different locations at altitudes of 1300 – 1600 m asl but with different climatic conditions (Komna and Pokljuka) in the recharge areas of rivers Savica and Radovna, the two important tributaries to river Sava. We focused on sampling at frost hollow at Komna and at two locations with different canopy structures at Pokljuka (i. e. clearing and forest stand). Selected sampling locations were in vicinity of automated weather stations which are on sites for many years. First results showed spatial and temporal variability of snow characteristics, including isotopic composition, that were related to different precipitation events and to processes that change the initial isotopic signal. Results showed that more detailed investigations are needed for appropriate evaluation of snow and snowmelt influence on runoff. In winter 2011/12 we continued with investigations but they were limited due to extremely dry winter. In winter 2012/13 sampling was expanded to more locations and due to high snowpack (150 – 300 cm) investigations are still in progress.
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An Icelandic wind atlas
Nikolai Nawri, Icelandic Met Office, Reykjavik, Iceland
Guðrún Nína Petersen, Icelandic Met Office, Reykjavik, Iceland
Halldór Bjornsson, Icelandic Met Office, Reykjavik, Iceland
Kristján Jónasson, Department of Computer Science, School of Engineering and Natural Sciences, University of Iceland, Reykjavík, Iceland

 

Corresponding author: Guðrún Nína Petersen
Section: Mountain weather forecasting

Although there is ample wind in Iceland its use for energy production has been limited. However, recently there has been increasing interest in the potential of using wind power to supplement hydro- and geothermal power. A wind atlas has been developed for Iceland, to obtain estimate of the wind energy potential.
The atlas is based on the wind obervations, supplemented with mesoscale model runs produced with the Weather Research and Forecasting (WRF) Model and high-resolution regional analyses obtained through the Wind Atlas Analysis and Application Program (WAsP). The wind atlas shows that the wind energy potential is considerable, but, due to the orography of Iceland, the variations in space are large. The regions with the strongest average winds are though impractical for wind farms, due to lack of infrastructure and often harsh climate. The orography of Iceland and its location in the middle of the North Atlantic storm track makes the identification of the most suitable areas demanding, but at the same time these two factors also result in the estimated annual power densities of many areas that are among the highest in Western Europe.

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Applications of cross-sections from operational high-resolution simulations for weather forecasting in mountainous areas
Bolli Pálmason, Icelandic Meteorological Office, Iceland
Guðrún Nína Petersen, Icelandic Meteorological Office, Iceland
Haraldur Ólafsson, University of Iceland, Iceland, Icelandic Meteorological Office, Iceland, and Bergen School of Meteorology, Geophysical Institute, University of Bergen, Norway

 

Corresponding author: Guðrún Nína Petersen
Section: Mountain weather forecasting

Systematic validation of high-resolution numerical weather forecasts made by several models for Iceland has shown that strong winds are often underestimated, particularly in the highlands. In order to improve the availability of information needed to forecast surface winds, cross-section of winds and temperature over Iceland are now a part of the visualisation of operational high-resolution numerical forecasts. These sections often show high winds aloft that the model is unable to bring to the surface. Currently this is being investigated further, especially the role of the surface roughness on high wind speeds. The cross-sections thus provide a guidance for the wind forecasts and increase the awareness of low level jets. They also reveal details in the patterns of important meteorological features such as orographic disturbances and atmospheric fronts.

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Visualization of real-time simulations of the atmosphere for education
Haraldur Ólafsson (1,2,3), Bolli Pálmason (2), Hrafn Guðmundsson (2), and Trausti Jónsson ()
(1) Háskóli Íslands (Univ. Iceland), (2) Veðurstofa Íslands (Icelandic Meteorol. Office),
(3) Bergen School of Meteorology, Geophysical Institute, University of Bergen, Norway

 

Corresponding author: Haraldur Ólafsson
Section: Mountain weather forecasting

Following the three-dimensional evolution of the atmosphere in real time is motivating for students and a
challenging for all people interested in weather and climate. Recently, the Icelandic Meteorological Office has
started to provide a large collection of output from operational numerical models on the web in real-time. The
products consist of plots of basic parameters such as pressure, temperature, winds and humidity as well as a series
of functions of these such as vorticity, potential vorticity, thermal winds, turbulent fluxes and turbulence kinetic
energy. The plots are suitable as a tool to deal with a wide range of tasks in dynamic meteorology, including
elements such as barotropic and baroclinic flows, planetary to short-scale waves, orographic disturbances, quasigeostropic theory and atmospheric fronts. The plots illustrate processes ranging from mesoscale to hemispheric
scale and are used increasingly for educational purposes. The plotting tools are VisualWeather, Magics++ and
GrADS and the web address is brunnur.vedur.is/kort/spakort.
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Background error matrix in the Adriatic region: Characteristics and seasonal variability
Kristian Horvath, Meteorological and Hydrological Service, Gric 3, Croatia
Antonio Stanesic, Meteorological and Hydrological Service, Gric 3, Croatia
Tomislav Kovacic, Meteorological and Hydrological Service, Gric 3, Croatia

 

Corresponding author: Kristian Horvath
Section: Mountain weather forecasting

Numerical weather prediction of heavy precipitation events is typically very sensitive to the initial and lateral boundary conditions. Therefore, the initial conditions for mesoscale predictions are often improved by mesoscale data assimilation. In 3D variational data assimilation, information about the state of atmosphere introduced by the observations needs to be spread in space. The key component that determines the horizontal and vertical propagation of information gained from observations is the background error covariance matrix.

Six different formulations of multivariate background error covariance matrix were calculated and analyzed using limited area model ALADIN (Aire Limiteé Adaptation Dynamique dévelopement InterNational). First, a standard NMC and an ensemble type of background error statistics were compared for the same period using vertical profiles of standard deviations and correlation length-scales of the four control variables: vorticity, divergence, temperature and specific humidity. Mean vertical correlations and cross-covariances among control variables were further studied to assess the changes in the covariance matrix structure. Second, four different ensemble matrices were calculated to study the seasonal variability of the background error matrix. Conclusions were drawn on the properties of different formulations of the background error statistics and the underlying dynamical properties and physical aspects of their seasonal variability.

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Lagrangian Perspective of Orographic Blocking
Michael Sprenger, ETH Zurich
Erica Madonna, ETH Zurich
Alexandre Roch, ETH Zurich

 

Corresponding author: Michael Sprenger
Section: Mountain weather forecasting

In Alpine mountain meteorology, orographic blocking is important because of its impact on: a) leecyclogenesis; b) the passage and modification of warm and cold fronts; c) the geographical distribution and intensity of (heavy) precipitation events. Traditionally, orographic blocking is studied with Eulerian methods based on the estimation of the inverse Froude number or dimensionless mountain height F=N/HU, where N is the stratification, H the mountain height and U the usptream velocity. Here we present the Lagrangian perspective of orographic blocking.

Meteorological fields are taken from the high-resolution NWP model COSMO, which is operationally run at the Swiss weather service. Winds are taken from a three-year (2000-2002) reanalysis simulation with COSMO. Based on these winds, kinematic forward trajectories are started at a distance of 300 km all around the Alps and at two height levels (750 and 1500 m). The 24-h trajectories are then investigated in their capability to surmount the Alpine barrier.

The blocking climatology is separated into three weather classes: westerly flow, northerly flow and southerly flow, the latter being restricted to south Foehn cases. For each class the percentage of trajectories surmounting the Alps and the percentage of air parcels flowing around the Alps is determined. Furthermore, trajectory densities are calculated to show the different air streams which start from selected upstream positions. The blocking frequencies are compared for air streams starting at 750 m and at 1500 m.

Finally, the Lagrangian method to identify orographic blocking is compared to the Euerlian one. The advantages of the Lagrangian method are discussed, as well as its limitations.
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Balanced and unbalanced kinetic energy spectra in the limited area model ALADIN
Vanja Blažica, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia
Nedjeljka Žagar, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia and Centre of Excellence SPACE-SI, Ljubljana, Slovenia

 

Corresponding author: Vanja Blažica
Section: Mountain weather forecasting

In spite of a major progress in NWP and climate modeling in high resolution and complex terrain, some fundamental questions remain puzzling. In the present study, we address the problem of the quantification of divergent energy at mesoscale in a typical NWP mesoscale model. We ask how the importance of the divergent energy changes as a function of the horizontal scale and altitude. The results are presented in terms of the kinetic energy spectra split into rotational and divergent components. The mesoscale NWP model ALADIN (4.4 km horizontal resolution) is used.
The comparison between the rotational and divergent energy contributions is made at various horizontal scales and vertical levels. The results show that about 50% of kinetic energy in the free troposphere in ALADIN is divergent energy. The percentage increases to nearly 70% at the surface and similarly in the upper troposphere towards 100 hPa. The maximum percentage of divergent energy is located at stratospheric levels around 100 hPa and at scales below 100 km, which are not represented by the global models.
Due to existence of several methods for obtaining periodic fields over a limited area domain, the results are presented for two different methods. One uses the standard ALADIN approach for the spectra computation based on the extension zone while the other, commonly used in mesoscale models, is based on removing the large scale trend from the data. For both sets of results, a very similar vertical distribution of divergent energy is obtained.
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A very high resolution real time diagnostic temperature model over the Alps
Dieter Mayer and Reinhold Steinacker, Department of Meteorology and Geophysics, University of Vienna, Austria

 

Corresponding author: Dieter Mayer
Section: Novel modelling

Today´s high resolution prognostic models are able to predict atmospheric fields with a considerable quality. In areas of steep orography, deep valleys and compex mountain massifs, however, these models still have some problems with correctly describing the thermal condition of the boundary layer. To follow the discrepancies between forecasts and observations in real time, we have developed a sophisticated three dimensional analysis system, which takes into account the 3D temperature and pressure observations. By applying a complex quality control routine and utilizing the hydrostatic relation, we can nicely resolve the vertical temperature profiles in and around the Alpine massif. shallow and deep inversions may be encountered as well as deeply mixed boundary layers, even with superadiabatic lapse rates close to the surface. This model is ideally suited for monitoring and real time model validation purposes. The setup of the model and some examples will be shown and discussed.
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Bridging the Meso-gamma and Micro-scales in complex orography using a modified version of the AR-WRF atmospheric model
Ólafur Rögnvaldsson; Institute for Meteorological Research, Reykjavík, Iceland
John Michalakes; National Renewable Energy Laboratory, Golden, Colorado, USA
Hálfdán Ágústsson; Institute for Meteorological Research, Iceland; University of Iceland; Icelandic Metorological Office

 

Corresponding author: Hálfdán Ágústsson
Section: Novel modelling

As horizontal model resolution of atmospheric models approaches the scale, l, of energy containing turbulent eddies, current one-dimensional planetary boundary layer (PBL) schemes may fail. This numerical region in which 1-D PBL schemes fail is termed “Terra Incognita” by Wyngaard (2004). It is this region we now fast approach when modeling weather in complex orography as the need for ever increasing horizontal resolutions and more detailed model results is emerging.

The AR-WRF model includes a number of formulations for parameterizing sub-grid turbulent mixing. Among these is the so-called 1.5 order turbulence kinetic energy scheme where the horizontal and vertical eddy viscosities are computed using

K = Ck * l * e^0.5,

where e is the turbulence kinetic energy, Ck is an empirical parameter and l is the length scale of energy containing turbulent eddies (Skamarock et al., 2008). This Large Eddy Simulation (LES) scheme for sub-grid mixing is however numerically unstable when used at horizontal resolutions on the Meso-gamma scale. As the standard version of AR-WRF does not support mixing PBL schemes (intended for use on the Meso-gamma scale) with LES schemes (intended for use on the Micro-scale) it has been necessary to use intermediate steps to bridge the gap between the two length scales (see e.g. Rögnvaldsson et al., 2011), e.g. when going below 1 km horizontal resolution atmospheric simulations in very complex terrain.

A novel solution to this problem has been developed by offering the potential of adjusting the length scales l in order to gain numerical stability of the 1.5 order Turbulent Kinetic Energy scheme at horizontal resolutions between 1 and 10 km. Results show that the AR-WRF model, when run using this solution, produces results that are comparable to simulations using the MYJ and YSU PBL schemes, when the mixing length is scaled appropriately.

References

Rögnvaldsson, Ó., Ágústsson, H. and Ólafsson, H., 2011. WRFLES – A system for high-resolution limited area numerical simulations. Tech.Rep. 1, Institute for Meteorological Research. Available on-line: ftp://betravedur.is/pub/publications/UniRes_WRFLES.pdf.

Skamarock, W. C., Klemp, J. B., Dudhia, J., Gill, D. O., Barker, D. M., Duda, M. B., Huang, X.-Y., Wang, W. and Powers, J. G., 2008. A Description of the Advanced Research WRF Version 3. NCAR Technical Note, NCAR/TN–475+(STR). National Center for Atmospheric Research, Boulder, CO, 113 pp.

Wyngaard, J. C., 2004. Toward Numerical Modeling in the "Terra Incognita". J. Atmos. Sci., 61: 1816–1826.

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Boundary-layer phenomena in the vicinity of an isolated mountain: A climatography based on an operational high-resolution forecast system
Stefano Serafin, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Stephan F.J. De Wekker, Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA
Jason Knievel, Research Applications Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

 

Corresponding author: Stefano Serafin
Section: Novel modelling

Granite Peak, located in the Dugway Proving Ground (DPG) in western Utah, is an isolated mountain rising ~800 m above the surrounding terrain. It has an approximately ellipsoidal shape oriented in the NNW-SSE direction and its main axes are respectively ~10- and ~6-km long. A flat dry lake (playa) lies west and northwest of the peak, while a NW-sloping plain covered by herbaceous vegetation extends to the eastern part of DPG.

Because of these topography and land-use features, a variety of different flow phenomena are expected to occur over and around Granite Peak. These include upslope and drainage winds, local breeze systems, gap flows, dynamically accelerated downslope winds and potentially boundary layer separation and the formation of wakes. Consequently, the area is an ideal location for studying the interaction between mountain flows and the atmospheric boundary layer.

During fall 2012, DPG was the target area of the first field campaign of the MATERHORN project (http://www3.nd.edu/~dynamics/materhorn/). Since 2007, DPG has used a continuously operating meso-gamma-scale analysis and forecast system (4DWX) developed by the NCAR Research Applications Laboratory (RAL). The system is based on WRF, runs with a grid spacing of 1.1-km in its innermost domain, applies observational nudging in a three-hour cycle, and provides weather analyses and forecasts at hourly intervals.

In this study, model output from the 4DWX system is used to build a short-term climatography of the prevailing boundary layer flow regimes in DPG. The aim is to demonstrate how results from numerical modeling can provide useful information for the preparation of a field measurement campaign, including the planning of research aircraft missions and the choice of optimal locations of ground-based sensors.

As an example, estimates of the boundary layer depth are used as guidance in developing flight plans for airborne lidar measurements. Furthermore, two-dimensional near-surface fields of wind speed and direction, temperature and pressure are analyzed to identify the most favorable regions for the occurrence of flow separation phenomena. Finally, measurements from the MATERHORN field experiment are used for a preliminary verification of the quality of the 4DWX simulations.
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Simulations of precipitation at a 2D mountain with the COSMO model - dependence on microphysical schemes and initial conditions
Annette K. Miltenberger, Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstr. 16, 8092 Zurich, Switzerland
Hanna Joos, Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstr. 16, 8092 Zurich, Switzerland
Heini Wernli, Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstr. 16, 8092 Zurich, Switzerland

 

Corresponding author: Annette Miltenberger
Section: Precipitation

Orographic precipitation provides an ideal testbed to improve our understanding of precipitation formation, its interaction with the flow dynamics and to assess different microphysical parameterizations. We therefore used the COSMO model to simulate the moist flow over a two-dimensional bell-shaped hill with different initial conditions and different microphysical parameterizations. The simulations cover a broad range of dry Froude numbers from approximately 0.4 to 2.3 and different temperature regimes from completely non-iced to completely iced clouds. In addition each simulation is done with a 1-moment and a 2-moment microphysical scheme. By analyzing the surface rain as well as the hydrometeor distribution above the hill, insights can be gained into the behavior of different microphysical schemes. The variation due to different microphysical schemes can be directly compared to the changes in hydrometeor fields and surface precipitation related to shifts in the physical space (temperature, velocity, ...). This gives useful insights in the uncertainties related to microphysical parameterizations and their dependence on the dynamical setting.
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EXCEPTIONAL SNOWFALLS IN THE REGION OF MOLISE (CENTRAL ITALY) IN A CONTEXT OF EXTREMES OF CLIMATE
DR. DI PILLA SERGIO - AGENZIA REGIONALE DI PROTEZIONE CIVILE
PROF. MASSIMILIANO FAZZINI - UNIVERSITA' FERRARA

 

Corresponding author: DR. ANTONIO CARDILLO
Section: Precipitation

After a first part of winter characterized by a almost total absence of snowfalls, between the end of January and mid February 2012, the Italian peninsula has been affected by the influence of many advections of arctic-continental air. These, that have generate a series of front; this brought to diffuse mainly snowy precipitation, mostly affecting in particular the central-southern regions. Main aim of this study is to analyze the cumulative height of snow measured in 43 stations of the different monitoring networks in the region of Molise and their comparisons to data for other major events observed during the winters of 1929,1956 and 1985 in twelve stations. Compared to these earlier events, accumulations of fresh snow recorded during the events of 2012 can be characterized as "exceptional" in the mid-altitude areas of Molise region, in ordinary contexts thermal and anemometer. After this period extremely cold and snowy, occurs a phase meteorological characterized by near total absence of new snow and temperature much more elevated to climatic averages. An analysis of MODIS images show that at the end of the month, the snowpack has melted completely at odds medium-low; the complementary study of nivometric data evidence that, only above 1200-1400 m a.s.l. the snow remained at the ground until the first day of the month of April. This signal, although limited to a single winter season, can be considered a significant indicator in the context of climate extremes, particularly intense in the mountains of the Mediterranean basin.
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Application of limited area numerical model for regional drought monitoring
Gregor Gregorič, Slovenian Environmental Agency
Jožef Roškar, Slovenian Environmental Agency

 

Corresponding author: Gregor Gregorič
Section: Precipitation

Seasonal and climatological runs of limited area numerical models (LAMs) are nowadays performed routinely in order to improve regional details in spatial distribution of precipitation statistics, mainly in mountainous regions. Application of LAMs for drought monitoring is being performed in Slovenia for the area of SE Europe. In order to obtain model climatology, ECMWF’s ERA Interim analyses for the period 1979-2011 were used as initial and boundary conditions. High resolution regional analyses were computed by Non-hydrostatical Meso-scale Model (NMM). NMM is suitable for use in a broad range of applications across scales ranging from meters to continental scale. Some results of reanalysis as well as operational runs and their possible application for drought monitoring will be presented.
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Analysis of movement of hail events and analysis of model indicators of hail in Slovenia
Gregor Skok, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia and Center of Excellence SPACE‐SI, Ljubljana, Slovenia
Jože Rakovec, Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia and Center of Excellence SPACE‐SI, Ljubljana, Slovenia
Benedikt Strajnar, Slovenian Environment Agency

 

Corresponding author: Gregor Skok
Section: Precipitation

An analysis of movement of hail events was performed using data from the only meteorological radar in Slovenia. The analysis showed that most of the hail events only last a short period but some events can exists longer (even hours) – these events can travel great distances in their life cycle, even a few hundred kilometers. The hail events can move in any direction but eastward movement is the most frequent. An analysis of model indicators of hail in Slovenia was performed with the aim to determine if model indicators of strong convection are different for convective events with or without hail. Model ALADIN-SI with resolution 4.4 km was used to make a 24h forecast for 28 cases when strong convection occurred (23 cases with hail and 5 cases without). The results showed there are no statistically significant differences in terms of model indicators of strong convection for cases with and without hail.
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The Öræfajökull precipitation experiment - ÖREX
Hálfdán Ágústsson; Institute for Meteorological Research, Iceland; University of Iceland; Icelandic Metorological Office
Haraldur Ólafsson; University of Iceland; Icelandic Metorological Office, Bergen School of Meteorology, University of Bergen, Norway
Marius O. Jonassen; Bergen School of Meteorology, University of Bergen, Norway
Ólafur Örn Haraldsson;
Hugo Mencoboni; Ecole des mines d'Albi, France
Sylvain Laporte; Ecole des mines d'Albi, France
Margot Pouget; Ecole des mines d'Albi, France
Léon Jourdain; Ecole des mines d'Albi, France

 

Corresponding author: Hálfdán Ágústsson
Section: Precipitation

We present the main findings of ÖREX, the Öræfajökull precipitation experiment. In July 2010, 27 automatic rain gauges (HOBOs), were placed near Hnappavellir in the south of the glacier covered Mt. Öræfajökull, Iceland’s highest mountain of 2111 m. The rain gauges were placed in a grid with an average spacing of 1 km, running from the coast to an altitude of 1100 m, at the maximum glacial extent on the mountain, over a distance of about 10 km. The gauges were operational during the summer and autumn of 2010 and recorded large orographic precipitation events related to synoptic weather systems as well as smaller events related to small scale convective systems. The gauges were successfully retrieved again in the summer of 2011, except for a few which had been attacked by the notorious killer sheep of Öræfi and one presumably lost during an severe windstorm. The high spatio-temporal observational data is compared to atmospheric analysis and other available observations of weather from both synoptic as well as automatic weather stations. Chosen events of interest are analyzed further, based on the results from high-resolution atmospheric simulations.
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Comparison of observed and simulated precipitation with the mass balance of Mýrdalsjökull ice cap, South-Iceland
Hálfdán Ágústsson; Institute for Meteorological Research, Iceland; University of Iceland; Icelandic Metorological Office
Hrafnhildur Hannesdóttir; Instute of Earth Sciences, Iceland
Þorsteinn Þorsteinsson; Icelandic Metorological Office
Finnur Pálsson; Instute of Earth Sciences, Iceland
Björn Oddsson; Icelandic Civil Protection Department
Haraldur Ólafsson; University of Iceland; Icelandic Metorological Office, Bergen School of Meteorology, University of Bergen, Norway

 

Corresponding author: Hálfdán Ágústsson
Section: Precipitation

Mass balance measurements on the caldera plateau of Mýrdalsjökull ice cap in South Iceland have been carried out regularly since 2007. The ice-cap covers the active Katla central volcano, and the glacier plateau ranges from 1350-1500 m a.s.l. The measured winter balance lies in the range 4375-6331 mm (water equivalent), with a maximum winter layer snow thickness of nearly 12 m. The annual mass balance is 2442-4742 mm (w. eq.), and summer ablation is highly variable (924-2690 mm, w.eq.). Comparison of the measured winter balance with observations of precipitation at sea level, indicates that the plateau receives on average 1-1.7 m of precipitation during summer. The mean estimated winter precipitation falling on the glacier furthermore compares well with results from high-resolution numerical simulations made with a state of the art atmospheric model. The simulations indicate a slight decrease in the mean precipitation falling on the plateau since 1994. The estimated annual precipitation amounts are of a similar magnitude as the highest values reported from the caldera plateau of Öræfajökull ice cap (2111 m), Iceland’s highest mountain.

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Numerical simulations of sheared conditionally unstable flows over a mountain ridge
Mario Marcello Miglietta (ISAC-CNR, Lecce, Italy)
Richard Rotunno (NCAR, Boulder, Colorado)

 

Corresponding author: Mario Marcello Miglietta
Section: Precipitation

In two recent papers, the authors reported on numerical simulations of conditionally unstable flows past an idealized mesoscale mountain ridge. These idealized simulations, which were performed with a three-dimensional, explicitly cloud-resolving model, allowed the investigation of simulated-precipitation characteristics as a function of the prescribed environment. The numerical solutions were carried out for a uniform wind flowing past a bell-shaped ridge and using an idealized unstable (Weisman-Klemp) sounding with prescribed values of the relevant parameters. More recently, the application of these theoretical results to some observed cases of orographically forced wind profiles, showed that, in order to reproduce larger rainfall rates, in closer agreement with observations, it was necessary to initialize the sounding with a wind profile characterized by low-level flow towards the mountain with weak flow aloft (as observed).
In order to generalize the results obtained in these case studies, additional experiments using the Weisman-Klemp sounding, but with non-uniform wind profiles, were performed to identify the conditions in which the presence of a strong low-level jet together with calm aloft may increase the rainrates in conditionally unstable flow. The sensitivity of the solutions to the wind speed at the bottom and the top of the shear layer was systematically analyzed.
Preliminary results show that very large rainfall rates are obtained when a deep, quasi-stationary cold pool is confined just upstream of the mountain; this solution has no correspondence with the numerical solutions previously obtained for uniform wind profiles. The heavy-rain quasi-stationary solution occurs within a narrow region of the parameter space explored, separating two main categories of solutions: orographic rainfall and upstream propagating cold pool. This large rainfall is due to a subtle balance between the environmental wind and the cold pool: when they approximately balance each other, the cold pool remains confined near the mountain and can cooperate to produce a deeper uplift, behaving as an extension of the orographic obstacle.

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A new field experiment to observe orographic rain enhancement in the UK.
Jeremy Price, UK Met Office
Amanda Kerr-Munslow, UK Met Office
Samantha Smith, UK Met Office

 

Corresponding author: Samantha Smith
Section: Precipitation

A new field and modelling trial is described to examine the performance of precipitation forecasts from a high resolution (1.5km) forecast model over Bodmin Moor on the South-West Penninsula of the UK. The trial will last for one year and observational data will be collected from raingauges placed approximately 3km apart in a line some 35km long which intersects the hill and is aligned into the prevailing wind. Analysis will target periods when orographic enhancement of precipitation occurs over the hill, and will compare the distribution and rainfall amounts predicted by a high resolution model with the observations. Results will also be used to test the performance of a scheme to represent the orographic rain enhancement due to subgrid scale orography. The trial started during March 2013, and some preliminary results are presented.
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Evaluation of vertical eddy fluxes divergence for multiple bora events
Nevio Babić, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Željko Večenaj, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Kristian Horvath, Meteorological and Hydrological Service, Zagreb, Croatia
Branko Grisogono, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia

 

Corresponding author: Nevio Babić
Section: Turbulence

The eastern Adriatic coast is well known for the downslope windstorm called bora. This windstorm often causes difficulties in traffic, agriculture and generally in everyday life. Hence, it is a subject of intensive and continuous scientific research. In the past few decades various statistical, synoptic and mesoscale characteristics of the bora flow have been investigated intensively; however, certain aspects of bora dynamics still remain unexplored, especially its turbulence. The main reason for such a situation was the lack of appropriate measurements needed to address bora turbulence. This has been especially the case for the middle and southern Adriatic coast where, until recently, no systematic turbulence measurements were conducted.

From April 2010 to June 2011, a three-level tower (10, 20 and 40 m) was equipped with WindMaster Pro ultrasonic anemometers (Gill Instruments), measuring 3D wind speed and sonic temperature at 5 Hz sampling rate. The tower was mounted at the top of Pometeno brdo (≈ 600 m ASL) in Dugopolje, bora upwind site from the city of Split. During that period, the anemometers captured multiple bora events in all annual seasons. This work concentrates on 17 registered bora events during a three month winter period, ranging from 10 up to 120 hours in bora duration. Upon determining the appropriate Reynolds averaging scale in order to define various turbulent perturbations, a thorough analysis of all kinematic vertical eddy fluxes (both momentum and heat) has been made. In particular, divergence of the aforementioned fluxes was also estimated, in order to determine their role in the very simplified equations of motion, as well as to determine the justification of assumptions that are used, such as horizontal homogeneity. Considering the measuring site and the nearby orography, interesting results were obtained. It is also worth noting that vertical eddy fluxes and the corresponding divergences, like those assessed here, have never been provided for a bora case.
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On the turbulence local isotropy during MAP IOP 15 bora event
Željko Večenaj, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia
Danijel Belušić, School of Mathematical Sciences, Monash University, Melbourne, Victoria, Australia
Branko Grisogono, University of Zagreb, Faculty of Science, Department of Geophysics, Zagreb, Croatia

 

Corresponding author: Željko Večenaj
Section: Turbulence

In the lee of the complex terrain of the Dinaric Alps, a well-known strong and gusty downslope airflow, called Bora, extends from the north-eastern quadrant perpendicular to the coastal mountains. Bora may be induced by different synoptic conditions and occurs most frequently during the winter season with duration of several hours to several days. It possesses a wide spectrum of average wind speeds and due to its gustiness the wind-speed maxima may surpass 60 ms−1. The corresponding turbulence is well developed during such vigorous events but, so far, it is addressed inadequately.

It is often suitable to use spectral theory and spectral analysis in turbulence research. Such deployment, for example, leads to estimation of the turbulent kinetic energy (TKE) and its dissipation rate, ε. There are several approaches for determination of ε that are based on the spectral analysis and all of them require that in the Fourier spectra of the velocity components the inertial subrange exists; a part of the wavenumber/frequency domain in which the turbulence is locally isotropic and where the turbulent eddies perform the cascade of TKE from the larger energy containing toward much smaller dissipating eddies. A strong statement of the local isotropy (and therefore, the existence of the inertial subrange) in space is the 4/3 ratio of the lateral to longitudinal spectra Sv(k)/Su(k) and vertical to longitudinal spectra Sw(k)/Su(k). This 4/3 ratio is tested on the MAP IOP 15 bora event which occurred on 07 November 1999 and was documented by the Electra and P-3 aircrafts. According to the results obtained, there are indications that bora turbulence is actually locally anisotropic.

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Topographically induced waves in a pycnocline: internal solitary waves and trapped orographic waves in the Toulouse stratified water flume
Alexandre Paci (CNRM-GAME, UMR3589 Meteo-France/CNRS, Toulouse, France)
Yvan Dossmann (CNRM-GAME; LA; Research School of Earth Sciences, The Australian National University, Canberra, Australia)
Francis Auclair (Laboratoire d'Aérologie, UMR5560 CNRS/UPS, Toulouse, France)
Jeanne Colin (CNRM-GAME)
Fabien Stoop (CNRM-GAME)
E.R. Johnson (Department of Mathematics, University College London, London, U.K.)
Laurent Lacaze (Institut de Mécanique des Fluides de Toulouse, UMR5502 CNRS/INPT/UPS, Toulouse, France)
Emmanuel Cid (Laboratoire de Génie Chimique, UMR5503 CNRS/INPT/UPS, Toulouse, France)

 

Corresponding author: Alexandre Paci
Section: Waves and rotors

The Meteo-France and CNRS geophysical fluid dynamics laboratory located in Toulouse (French meteorological service research center CNRM-GAME, UMR3589) provides facilities for fundamental and applied study of homogeneous, stratified and/or rotating flows. The research activities of the team focus on atmospheric boundary layers and internal gravity waves. Two recent experiments related to topographically induced internal gravity waves are presented here.

The Toulouse stratified water flume is a unique facility to study neutral or stratified flows. It has been specially designed to generate accurate and exhaustive datasets on flows similar to the atmospheric or oceanic ones under perfectly controlled conditions. It is thus a good extension of field experiments which are limited by the fact that data are scattered and conditions are not well controlled. This 30 m long, 3 m wide and 1.6 m deep density-stratified water flume can also be operated as a towing tank filled with water or with density-stratified brines.

Experiments have been recently carried out in order to investigate internal solitary waves generated over a ridge in a configuration close to the one used by Dossmann et al. 2011, but in a much larger tank. An extensive dataset has been collected on waves generated in a pycnocline by two different processes. Various flow regimes have been observed, including soliton and train of solitons. Similar waves could occur in the atmosphere when internal gravity waves are trapped in a stable layer formed by a temperature
inversion, leading to potentially hazardous wind shear for aircrafts.

Another set of experiments deals with trapped orographic waves generated over an isolated mountain. These experiments have been inspired by a theoretical model which predicts the structure of internal waves and the drag exerted by a mountain on the atmosphere from a small set of parameters. The latter is of particular importance to climate modelers and researchers involved in the development of numerical weather prediction models, because of the need to parameterize the drag exerted by orography. Boundary layer and internal waves interactions are explored from complementary numerical simulations in order to investigate the cause of some discrepancies between predictions of the theory and results of the experiments.
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Large eddy simulations of lee-wave interference over double mountain ridges
Brigitta Goger, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Stefano Serafin, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Ivana Stiperski, Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria
Vanda Grubišić, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria & Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

 

Corresponding author: Brigitta Goger
Section: Waves and rotors

This study presents large-eddy simulations of lee waves generated by flow over double mountain ridges. The results are compared with those from previously conducted two-dimensional idealized simulations with a mesoscale NWP model.

Two-layer flow over 2D topography with two Gaussian-shaped obstacles and variable valley widths is considered. The lower layer is neutrally stratified and is capped by an inversion, separating it from a stably stratified upper layer with a constant Brunt-Väisälä frequency. The background wind speed is constant with height. Dependent on the inversion strength, lee waves, rotors or hydraulic jumps develop. Simulations generally display extremely unsteady flow, especially between the two mountain ridges.

Under favorable conditions, both ridges generate their own train of resonant waves and the two sets interact with each other, causing either constructive or destructive lee wave interference. Destructive interference may lead to the complete cancellation of waves, whereas constructive interference enhances the wave amplitude in the lee of the second mountain. As in the previous study, the interference pattern depends primarily on the ratio of heights of the two obstacles and on their separation distance. An analysis of second-order turbulent moments from LES output allows identification of the flow regions with the most intense turbulence.

The significance of these results towards the planning of water-tank experiments of flow over double topography is briefly discussed.
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Mountain waves encountered on 4 March 1974
Birta Líf Kristinsdóttir, University of Iceland and Icelandic Meteorological Office, Iceland
Haraldur Ólafsson, University of Iceland, Iceland, Icelandic Meteorological Office, Iceland, and Bergen School of Meteorology, Geophysical Institute, University of Bergen, Norway
Guðrún Nína Petersen, Icelandic Meteorological Office, Iceland
Hálfdán Ágústsson, University of Iceland, Iceland, Icelandic Meteorological Office, Iceland, and Institute for Meteorological Research, Iceland

 

Corresponding author: Guðrún Nína Petersen
Section: Waves and rotors

On 4 March 1974 a commercial aircraft encountered extreme up- and downdrafts above the mountains north of Reykjavik, Iceland. The records from the aircraft and the pilots have been investigated and compared to a high-resolution simulation of the atmospheric conditions during the event. High horizontal resolution is crucial for the reproduction of the very strong vertical velocities that have been related to gravity waves. The evolution of the wave activity is described in time and space and related to the upstream structure of the incoming flow.
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Dependence of boundary-layer separation regimes on stability, wind speed and surface friction: An analysis based on large-eddy-simulations
Johannes Sachsperger, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Stefano Serafin, Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Vanda Grubišić, Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA & Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria

 

Corresponding author: Johannes Sachsperger
Section: Waves and rotors

Boundary layer separation (BLS) may occur when a strong adverse pressure gradient force is imposed on boundary layer flow. The strong deceleration caused by the pressure gradient results, through mass continuity, in the detachment of streamlines from the surface. The boundary layer commonly separates at the salient edge of very sharp obstacles. However, in the case of stratified flow, pressure perturbations strong enough to cause BLS can also be induced by internal gravity waves. A well-known phenomenon related to wave-induced BLS is that of atmospheric rotors.

Laboratory experiments in a stratified water tank show that the key governing parameters for BLS over a two-dimensional obstacle are the airflow speed U, the stratification N, the height H and half-width L of the mountain. These quantities can be combined to form the non-dimensional numbers NH/U (non-dimensional mountain height) and H/L (vertical aspect ratio), which can be used for a concise characterization of the flow regime. Wave-induced BLS was shown to occur preferentially for relatively high non-dimensional mountain height and for relatively low aspect ratio. Linear theory suggests similar results.

The behavior of BLS in strongly nonlinear flow regimes and its response to variable surface friction is less known. In this study, the CM1 model is used to perform large eddy simulations (LES) under free-slip and quasi-no-slip lower boundary conditions. Conditions favorable for BLS are investigated under different combinations of NH/U, H/L and the surface momentum flux.

Results confirm the importance of NH/U and H/L as key parameters in defining the flow regime in the free-slip case, and show the expected transition from subcritical to supercritical flow associated with wave breaking and downslope windstorms. Preliminary findings from quasi-no-slip simulations with variable bulk momentum transfer coefficients are also discussed.

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A spectral analysis of gravity waves above the Antarctic Peninsula
Victoria Smith, University of Leeds / National Centre for Atmospheric Science, UK
Stephen Mobbs, University of Leeds / National Centre for Atmospheric Science, UK

 

Corresponding author: Victoria Smith
Section: Waves and rotors

As part of the Orographic Flows and the Climate of the Antarctic Peninsula (OFCAP) field project, vertical profiles from radiosondes launched from a site east of the Antarctic Peninsula have been analysed to determine the power spectra of orographically and none orographically-generated gravity waves in different regions of the atmosphere. The Antarctic Peninsula forms an approximately 700m long mountainous barrier to fast flowing circumpolar westerly winds in the southern hemisphere. With peaks of approximately 3000m, and a consistent plateau of 1500-2000m, the region is known to generate high amplitude gravity waves as the zonal flow is forced to ascend over the mountains. A spectral analysis will attempt to determine the relative contribution of different wavelengths in both the troposphere and stratosphere to the power spectrum. Wavelengths that are found to have strong peaks will be investigated further assess their contribution to atmospheric drag.
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