An Improved Soil Moisture Parametrization for Regional Climate Simulations in Europe

Abstract

AbstractA new depth‐dependent saturated soil hydraulic conductivity function is implemented in the Regional Climate Model COSMO‐CLM coupled to the Land Surface Model VEG3D (CCLM‐VEG3D), to improve the physical description of soils in regional climate simulations for the Coordinated Regional Climate Downscaling Experiment‐European Domain. This new function is characterized by conductivity values decreasing exponentially with depth, accounting for the reduced pore volume in deeper soil layers. As a result, the new function reduces the soil water amount in the upper soil layers. Thus, less water is available for evapotranspiration and the latent heat fluxes are reduced. In turn, soil heating and the sensible heat fluxes are increased, leading to higher near‐surface temperatures in summer. Because of that, an observed cold bias in CCLM‐VEG3D simulations with a depth‐constant saturated soil hydraulic conductivity function is reduced over large parts of Europe. However, the decreased evapotranspiration results in a reduced cloud cover, which enhances the upward longwave net radiation and the cold bias in winter is getting stronger. But a comparison with observations shows that the new function effectively reduces the mean annual cold bias over almost all of Europe systematically, since the soil moisture‐temperature coupling is strongest in summer. This approach is potentially also an improvement for other Land Surface Models.