ALBERT

Description: In this paper, high-resolution numerical simulations of the 12-13 August 2002 flooding event in eastern Germany are presented. The simulations are performed with the Penn State/National Center for Atmospheric Research mesoscale model MM5 in a four-domain configuration with a finest horizontal resolution of 1 km. Sensitivity experiments are performed with coarser resolutions (3 and 9 kin), with different cloud microphysical parametrizations and with a different date of initialization. Moreover, tests with 1 km resolution but the smoothed topography of the 9 km runs are conducted in order to isolate the contribution of the model topography to the differences between the 1 km runs and the 9 km runs. The results show that the high-resolution runs reproduce the observed structure of the precipitation field very well. In particular, the location of the rainfall maximum is correct to within 15 km. The quantitative agreement between model results and observations is fairly good in regions with light to moderate rain, but large amounts of precipitation tend to be underpredicted. For observed 36-hour rainfall accumulations exceeding 200 mm, the negative bias typically ranges between 15 and 30% with fairly little dependence on the microphysical parametrization. In the coarser-resolved runs, the bias at high precipitation amounts becomes even more pronounced, and it is found that this behaviour is not only due to the fact that a coarser resolution is associated with a smoother topography. This indicates a systematic deficiency in the microphysical parametrizations, limiting their ability to simulate heavy precipitation.