Research areas of the meteorology group

The research areas of the meteorology group at the Department of Physics include

Research methods involve analytical approaches, modelling and use of observations. Numerical modelling is based on both simplified, idealized models used also in the teaching process and full-scale, 3D models. The latter include the WRF and WRF-Chem models, the ALADIN model and the COAMPS model. We use mesoscale model simulations, global analysis and reanalysis data, satellite and conventional observations for model verification and for self-standing observation studies. The research is carried out in collaboration with national and international colleagues. We collaborate with the Environmental Agency of Slovenia (ARSO) and its partners within the ALADIN consortium on the diagnostic and application of the ALADIN model. With colleagues from the National Center for Atmospheric Research (NCAR) in Colorado, USA, we collaborate on several topics including the global energetics and use of satellite precipitation measurements, in particular the TRMM dataset. In data assimilation research we collaborate with the European Center for Medium-Range Weather Forecasts (ECMWF). An important research area in recent years has been the coupled chemical-dynamical modelling. The WRF-Chem model is applied for the complex region of Slovenia and surrounding areas to model pollution dispersion in relation to synoptic and mesoscale weather conditions and local and distant pollution sources. A current project in collaboration with ARSO aims at developing a real-time forecasting system for the pollution dispersion based on coupling the chemical CAMx model with the operational weather forecast model (ALADIN-SI). Recently our efforts have extended to the area of regional climate modelling. The goal is to provide estimates of the uncertainty of current regional climate projections over Slovenia and surrounding regions with respect to natural variability.


Dynamics research ranges from the large-scale circulations to small, orographically triggered processes. The latter includes research on bora, severe downslope winds along the eastern Adriatic coast. Another study associated with the flow dynamics in the complex terrain has been downscaling of reanalysis data for the purpose of wind energy estimates. On large scales we have studied global energetics and in particular the role of inertio-gravity waves. A recent study intercompared four analysis systems: operational analyses of ECMWF and NCEP, the NCEP/NCAR reanalyses and the DART/CAM, an ensemble analysis system developed at NCAR. The applied methodology for analyzing the large-scale circulation is based on the three-dimensional normal-mode function (NMF) expansion which represents motions in terms of the balanced and the inertio-gravity motions. Currently we study balance issues based on the global and mesocale models. Global fields primarily come from the global (re)analysis system of ECMWF while the mesoscale studies currently use the ALADIN model outputs.

Recent publications:
Rakovec, J., Žagar, M., Bertalanič, R., Cedilnik, J., Gregorič, G., Skok, G., Žagar, N., 2009: Vetrovnost v Sloveniji (Wind energy in Slovenia). Ljubljana: Založba ZRC, ZRC SAZU, 2009. 177 str., ilustr. ISBN 978-961-254-160-6.
Žagar, N. , J. Tribbia, J. Anderson and K. Raeder, 2009: Uncertainties of estimates of inertio-gravity energy in the atmosphere. Part I: Intercomparison of four analysis systems. Mon. Wea. Rev., 137, 3837-3857.
Žagar, N. , J. Tribbia, J. Anderson and K. Raeder, 2009: Uncertainties of estimates of inertio-gravity energy in the atmosphere. Part II: Large-scale equatorial waves. Mon. Wea. Rev., 137, 3858-3873. Coupled chemical-dynamical modelling in complex terrain.
Belušić, D., Žagar, M., Grisogono, B, 2007: Numerical simulation of pulsations in the bora wind. Q. J. R. Meteorol. Soc., 133, 1371-1388. Žagar, N., M. Žagar, J. Cedilnik. G. Gregorič and J. Rakovec, 2006: Validation of mesoscale low-level winds obtained by dynamical downscaling of ERA40 over complex terrain. Tellus, 58A, str. 445-455.

Coupled chemical-dynamical modelling in complex terrain

Air quality is of great interest to society, because it affects human health, crops, forests, and other ecosystems. Although measurements are the source of basic information, numerical models are nowadays the primary tools for studying and forecasting air quality. Our current research includes the large-scale and regional transport of polluted air masses as well as modeling of meso-scale processes in complex topography, which especially during the low-wind meteorological conditions govern the high pollution episodes. For example, we studied the mechanisms which lead to ozone pollution during the warm months over the Mediterranean Slovenia, known for the highest frequency of ozone threshold values exceedances in Slovenia. Our primary modeling tool is a coupled meteorological-photochemistry WRF/Chem model. For the purpose of operational ozone and PM forecasting at ARSO, we are also in the final phase of coupling the photochemical CAMx model with the operational weather forecast model (ALADIN-SI). Finally, our research also includes the dispersion of passive tracers from emission sources in local scale over complex terrain, where beside WRF/Chem a CALPUFF modeling system is used in collaboration with Electric Power Research Institute of Ljubljana (EIMV).

Recent publications:
Žabkar, R., Rakovec, J., Koračin, D.. The roles of regional accumulation and advection of ozone during high ozone episodes in Slovenia : a WRF/Chem modelling study. Atmospheric Environment, 2011, 45 (5), 1192-1202.
Žabkar, R., Rakovec, J., Gaberšek, S.. A trajectory analysis of summertime ozone pollution in Slovenia. Geofizika (Zagreb), 2008, vol. 25 (2), 179-202.
Žabkar, J., Žabkar, R., Vladušič, D., Čemas, D., Šuc, D., Bratko, I. Q2 prediction of ozone concentrations. Ecollogical Modelling, 2006, vol. 191 (1), 68-82.

Precipitation measurements by satellites and modelling

Precipitation is one of the most important weather related resource for human society. Therefore the analysis and forecast of precipitation is very important, weather for short range forecasts (a few days into the future) or for climate simulations. The abilities of models to correctly predict precipitation has to be verified. The weakness of standard verification approaches is that they often do not provide results that are consistent with subjective perceptions of the quality of a forecast. On the other hand, the standard verification approaches usually provide a statistically valid approach which is usually not possible in subjective analysis. An object-based verification method combines the two approaches; the analysis of objects has some statistical validity while at the same time, the method mimics some attributes of human capability in determining the “goodness” of the forecasts. The method objectively identifies “objects” in the forecast and observed fields that are relevant to a human observer. These objects can then be characterized by a few attributes (e.g. location, size, shape, orientation, lifespan) by which the forecast and observed objects can be compared. Object based diagnostic analysis method was applied on two satellite derived precipitation datasets – the TRMM 3B42 and the PERSIANN dataset – over the low- and mid-latitude Pacific Ocean. This method showed a good ability at identifying, classifying and tracking of precipitation systems in the two datasets. Another study verified precipitation systems in the NCAR WRF Tropical Channel Simulation over the tropical Pacific Ocean. Such an object-based analysis in the tropical Pacific contributed to the verification of WRF model ability of simulating of large scale tropical features such as the Madden-Julian oscillation, the Intertropical Convergence Zone (ITCZ) and the El Nino-Southern Oscillation.

Recent publications:
Skok, G., J. Tribbia, J. Rakovec and B. Brown, 2009: Object-Based Analysis of Satellite-Derived Precipitation Systems over the Low- and Midlatitude Pacific Ocean. Monthly Weather Review, 137(10), 3196–3218.
Skok, G., J. Tribbia and J. Rakovec, 2010: Object-Based Analysis and Verification of WRF Model Precipitation in the Low- and Midlatitude Pacific Ocean. Monthly Weather Review, 138(12), 4561–4575.

Data assimilation modelling

Data assimilation deals with the preparation of the initial conditions for the numerical weather prediction. In this process, observations and a priori information (a short-range model forecast) are combined to produce the best estimate of the 3D atmospheric state. Two information sources are combined according to their error characteristics. The estimate of forecast errors poses a real challenge since the whole truth will never be within our grasp. Forecast errors are therefore modelled by using different methods and known and assumed properties of the physical system. Our research deals with the properties of short-range forecast error properties, in particular the balance. New developments in this field include multivariate relationships for data assimilation in the tropics and its application to the potential impact of new Doppler Wind Lidar satellite.

Recent publications:
Žagar, N., J. Tribbia, J. Anderson, K. Raeder and D.T. Kleist, 2010: Diagnosis of systematic analysis increments by using normal modes. Q. J. R. Meteorol. Soc., 136, 61-76.
Žagar, N., A. Stoffelen, G.-J. Marseille, C. Accadia and P. Schluessel, 2008: Impact assessment of simulated Doppler wind lidars with a multivariate variational assimilation in the tropics. Mon. Wea. Rev., 136, 2443-2460.
Žagar, N., Andersson, E., Fisher, M. and A. Untch, 2007: Influence of the quasi-biennial oscillation on the ECMWF model short-range forecast errors in the tropical stratosphere. Q. J. R. Meteorol. Soc., 133, 1843-1853.
Žagar, N., Andersson, E. and M. Fisher, 2005: Balanced tropical data assimilation based on a study of equatorial waves in ECMWF short-range forecast errors. Q. J. R. Meteorol. Soc., 131, 987-1011.

Regional climate modelling

More recent research efforts include the regional climate modelling and modelling of uncertainties of the climate downscaling results. Current climate models, such as models that participated in the last report of the Intergovernmental panel on Climate Change have horizontal grid spacing larger than 100 km meaning that their results can hardly be directly used to discuss the climate variability over Slovenia. Similarly, existing regional climate simulations have insufficient spatial resolutions for resolving the orography impact on weather systems which define the climate of Slovenia, such as mesoscale cyclones and storms. Thus the models need to be adjusted to higher resolution by carrying out the downscaling. In this process a regional model with a horizontal resolution of the order of 10 km is run using the initial and lateral boundary conditions from a lower-resolution global model. Results of downscaling greatly depend on the forcing model implying that using different lateral boundary forcing leads to different distribution of the regional climate signals. We are studying these and other uncertainties of the downscaling process over the region of Slovenia and surrounding countries. This research is carried out within the recently established Center of Excellence Space-SI. Link to Space-SI web-page.