Radar Based climatological studies of the influence of orography on thunderstorm activity in Central Europe

The knowledge of the influence of orography on initiation and development of thunderstorms is important for both, the design of convection schemes of numerical weather prediction models and the routine of operational weather forecasting. Nowcasting thunderstorms within an orographically structured region requires besides anticipation of the synoptical and mesoscale environment the assessment of possible orographical forcing on time and location of initiation as well as further movement and development of existing thunderstorms. Data from operational radar networks are crucial in monitoring these features.

The Deutscher Wetterdienst (DWD) operates a radar network of 16 radar stations which are monitoring Central Europe and measure reflectivity, with 11 radar stations also Doppler radial winds. 5 radar stations have provided nearly homogeneous data since the eighties. These data have been used here to investigate climatological aspects of the geographical distribution and temporal development of thunderstorms in summer. Reflectivity data of thunderstorm cases from 1992 to 2000 have been taken together with local warning markers for thunderstorms and hail, based on reflectivity thresholds in the vertical profile, first defined in the Swiss hail experiments. Focus is given on regions from the northern slopes of the Alps to the hilly regions of southern and middle Germany and low-lands of northern Germany. Part of the results has been provided during the European Nowcasting project COST78.

Following results may be inferred : Summer thunderstorm activity in Central Europe mostly develops in south-westerly flow in unstable subtropical air ahead and directly within convergence lines and cold-fronts. Earliest source regions of strong convection at a thunderstorm day tend to be the western and south-western hilly regions. Deep convection is triggered in many cases already from noon. In the early afternoon the thunderstorms spread out downstream from these hilly regions, though new development may further repeatedly be observed there. The peak of activity measured by reflectivity and warning markers is normally reached in middle afternoon. By influence of coastal wind convergences the temporal culmination of thunderstorm activity in the most northern low-lands of Germany is somewhat later. The northern slopes of the Alps and directly adjacing regions build up a third regime : Start of strong convection is often even after the noon and reaches peak only in early or middle evening. After that time thunderstorms at the flanks of the Alps are as a rule dying out quickly , with some activity still remaining in the neighbouring table-lands. On the whole the areas near and directly at the northern slopes of the Alps act as culmination regions of thunderstorm frequency in Central Europe. The inferred results have been confirmed by investigations of thunderstorm activity performed by the DLR Oberpfaffenhofen (Finke, Hauf) on basis of lightning data. Synoptic reasoning indicates that much of thunderstorm features north of the Alps is influenced by the thermally driven circulation at the Alpine slopes and the advection of thunderstorm systems from source-regions further to the southwest (Switzerland, France).