The Regional Atmospheric Water Budget over Southwestern Germany under Different Synoptic Conditions

Abstract

his study addresses the question of how complex topography in a low-mountain region affects the partitioning and the variability of the atmospheric water budget components (WBCs) as a function of synoptic-scale flow conditions. The WBCs are calculated for regions of different size and location in southwestern Germany and the summer months from 2005 to 2009 using the high-resolution regional climate model COSMO-CLM driven by Global Model (GME) analyses. Comparisons with observations from the Convective and Orographically-induced Precipitation Study (COPS) performed in summer 2007 show that the model is capable of simulating the atmospheric water budget reasonably (absolute mean error between 0.1 and 0.7 kg m?2 day?1). To investigate the influence of synoptic weather conditions, the daily WBCs are classified based on the inflow direction of the air masses and the cyclonality at 500 hPa. Using statistical tests, four groups out of the six synoptic conditions have significantly different distributions of the WBCs. This can be explained by differences in the air mass features and the influence of high/low pressure systems. The sensitivity of the modeled WBCs to topography and land cover is investigated by comparing a region in the flat upper Rhine Valley with one in the mountainous Black Forest/Swabian Jura. Compared to the Rhine Valley, increases of evapotranspiration (+5% to +16%), precipitation (+26% to +57%), and moisture convergence (+24% to +93%) are noticeable in the low-mountain region. Local modifications of the synoptic-scale flow, thermally induced winds, and land use cause this intensification of the atmospheric water budget, especially on the windward slopes of the mountains.