Orographic Enhancement of Precipitation over Low Mountain Ranges. Part II: Simulations of Heavy Precipitation Events over Southwest Germany

Abstract

A diagnostic precipitation model that combines linear theory of hydrostatic flow with parameterized microphysics is applied to several stratiform heavy precipitation events over the low mountain ranges of southwestern Germany. Model-simulated rainfall is in good agreement with observations in both magnitude and location, yielding correlation coefficients against observational data between 0.74 and 0.90. Two events that caused local flooding over and near the Black Forest mountains, on 11?13 December 1997 and on 28?29 October 1998, are discussed in detail. Results show that, in addition to orographic features, wind speed U, moist static stability Nm, and melting level are important parameters to describe the amount and spatial distribution of orographic precipitation. The effect of hydrometeor drifting significantly reduces the precipitation peaks near the crests, and the inclusion of evaporation decreases precipitation mainly in descent regions downstream of the mountains. Using the upslope approach instead of linear theory, the precipitation intensities increase substantially and primarily over and downstream of the mountain peaks, whereas the maxima are shifted slightly downstream. The best simulation results relative to the observations were obtained on a 2.5-km grid, whereas areal rainfall is underestimated by about 10% on a 5-km grid and by about 35% on a 10-km grid.