Convective System Analysis and Nowcasting with VERA

Accurate forecasts of thunderstorms are both, for science and public, a major concern, as convective events are one of the most severe natural hazards during summer in the Alpine region. To fulfil the users requirements in the forecast (due to location, time of appearance and intensity of the event), investigations of convection in the meso- scale are necessary for understanding processes leading to thunderstorms. For this purpose, a field experiment located in southern Germany will be accomplished within two international projects, COPS and MAP D-PHASE, in the next year. The primary aim of the field measurement campaign is the creation of a high quality data set for investigations of convective processes. This will help to answer questions concerning convective initiation, building of clouds and precipitation as well as cloud physics and analysis of hydrometeors. With this knowledge, meteorological (and hydrological) forecast for thunderstorms shall be improved. The Department of Meteorology and Geophysics is involved in the preparing and developing phase of both, COPS (Convective and Orographically-induced Precipitation Study; a part of the Priority Program SSP 1167 of the "Deutsche Forschungsgemeinschaft") and MAP D-PHASE (Mesoscale Alpine Program Demonstration of Probabilistic Hydrological and Atmospheric Simulation of flood Events in the Alpine region, a project sponsored by the World Meteorological Organization). Within the proposed project, the field campaign shall be supported by the implementation of a meso-measurement network and the supply of people operating the stations. This will be an important contribution to the creation of this unique data set. Other groups of an international consortium will contribute by the implementation of different observing systems (ground measurements, Doppler radar, Lidar, satellites, aircraft, radio soundings ). Using the unique data set of the field campaign, the analysis system VERA, developed at the Department in the framework of two FWF-projects, will be advanced. Two major aspects will be nowcasting of thunderstorms and the improvement of convective precipitation analyses. For both tasks, the fingerprint-concept, used for the implementation of additional information for downscaling meteorological fields, will be enhanced. This will concern a 3D approach, the integration of several fingerprints and high resolution (on the order of 1 kilometre grid distance) in the VERA analyses. The data set will furthermore be used for the development of new fingerprints. By implementing them, the predictability of thunderstorms in real-time using routine data will be improved as well as the accuracy of precipitation analyses. The analysis tool will be also used for high resolution model validation, whereas a new approach for the comparison, based on the fingerprint-concept, will be tested for a better predictability of thunderstorms.

Within the FWF-Project CONSTANCE, embedded in the international framework of COPS (Convective Orographhic Precipitation Study), for the first time a very high resolution meteorological surface network (micronet) with more than 100 automatic stations was operated in Europe (in the Northern Black Forest region). The spatial resolution of 1km and the temporal resolution of 1 min allows a detailed detection of small scale meteorological processes like convection and orographically induced circulations. The evaluation of the data set furthermore allows a quantification of the representativity of single stations with respect to areal averages and hence a sound comparison of prognostic fields with observations. Probably the highest value of such micronet data lies in the possibility to validate remotely sensed data like Radar data with ground truth. The major part of the data evaluation within CONSTANCE focussed on the latter and allowed to quantify deficiencies of radar estimates of precipitation. Concrete results gave e.g. evidence of a small orientation (Northing) error of one of the surrounding radar stations. As radar estimates of precipitation are derived from the lowest elevation of radar beams, which are roughly 1 km above the surface due to the orographic shading and curvature of the earth`s surface, differences in timing related to the fall velocity of precipitation and in location related to the wind drift of the falling precipitation - could be quantified. For operational nowcasting purposes of radar derived precipitation such timing and location differences are vital. Furthermore inaccuracies of radar estimates of precipitation could be investigated by a simultainous observation by a vertical pointing Radar and a distrometer, also operated by the Department of Meteorology and Geophysics of the University of Vienna in the centre of the micronet. The vertical pointing radar allows the detection of the vertical precipitation profile and the droplet spectrum above the ground, the Distrometer the detection of the surface droplet spectrum. Both informations allow a critical insight into the validity of the radar calibration (e. g. the so called Z-R relation). It could be shown, that most of the errors of radar estimates of precipitation (up to 50%!) are caused by calibration errors.The lessons learned during CONSTANCE e. g. by the pioneering operation of a meteorological micronet, will help for designing and operating similar systems in future meteorological field experiments. It could be shown, how important it is, to operate surface micronets to complement and validate the detection of the atmospheric state by remote sensing devices. As the quality controlled data set collected during COPS is openly available to the scientific community, also future further evaluations and investigations seem to be promising.