Quantification of topographic effects on predicted precipitation for typical Elbe-catchments in the Erzgebirge

点击翻译按钮获取中文摘要

In the flood event of 2002 precipitation forecasts were rather accurate. The overall rains on the whole slope of the Erzgebirge were rather well represented. Looking at individual pixels, however, large local and temporal deviations between forecast and observed values occurred. The major flood damage occurred in the small rivers and streams. Many of these Elbe contributaries are regulated by dams. The sizes of these catchments ranges from about 5 km² to about 200 km². This means that the typical catchment area is covered by one single pixel of a meteorological forecast model. As these catchments are located on the slope of the mountain ranges their individual topography gradient will deviate from the one in the model. In particular these gradients are threedimensional. The precipitations of the August 2002 event were to a large extent topography induced. To find out which partition of catchment wide rains is topography induced the effects of the topography of 12 dam provided catchment areas will be quantified for selected precipitation events. This means to use all available catchment area related data comprising the area covering radar-data calibrated with rain gauge data, these data themselves, and furthermore, the information available from the water budget as measured by dam authorities. The deviations between the catchment precipitations and the ones of a references site will be used to find an empirical correction factor for topographic effects. The differences between the geometries of real catchment areas and forecast model geometries will be used to derive the differences in the vertical motion, a geometry factor. The empirical and the geometry factor are applied to forecast rains, compared to the observed rains and finally used to formulate a correction factor for forecast models.