REGIONAL CLIMATE SIMULATION WITH A HIGH-RESOLUTION GCM - SURFACE HYDROLOGY

Aspects of the surface hydrology of high resolution (T106) versions of the ECHAM3 and ECHAM4 general circulation models are analysed over th e European region and compared with available observations. The focus is on evaporation, and surface measurements are shown to be useful for the identification of systematic deficiencies in the regional-scale p erformance of climate models on an annual and seasonal basis, such as the excessive summer dryness over continents. The annual mean evaporat ion at the available European observation sites is overestimated by 4 mm/month by the ECHAM3 T106, quantitatively consistent with an overest imated surface net radiation of 4 Wm(-2) over Europe. In winter, ECHAM S shows an overestimated evaporation which compensates for an overesti mated downward sensible heat flux. This is primarily related to a too strong zonalisation of the large-scale flow and associated overestimat ed warm air advection and windspeed. Inaccurate local land surface par ameters (e.g. leaf area index, roughness length) are minor contributor s to the overestimation. In early summer, the excessive solar radiatio n at the surface calculated with the ECHAM3 radiation scheme generates a too large evaporation and an excessive depletion of the soil moistu re reservoirs. This favours the subsequent excessive summer dryness ov er Europe with too low values of evaporation, convective precipitation and soil moisture content, leading to a too high surface temperature. In the ECHAM4 T106 simulation, the problem of the European summer dry ness is largely reduced, and the simulated evaporation as well as conv ective precipitation, cloud amount and soil moisture content during su mmer are substantially improved. The new ECHAM4 radiation scheme appea rs to be an important factor for this improvement, since it calculates smaller insolation values in better agreement with observations and s ubsequently may avoid an excessive drying of the soil.