Absorption of solar energy in cloudless and cloudy atmospheres over Germany and in GCMs

Clear-sky and all-sky shortwave atmospheric absorption in several General Circulation Models (GCM) is assessed using observational data from selected sites in Germany. Estimates of shortwave absorption in the atmosphere are obtained by combining surface clear-sky and all-sky climatologies derived from the ground observations with collocated top of atmosphere (TOA) clear-sky and all-sky fluxes from the Earth Radiation Budget Experiment (ERBE). It is shown that the atmospheric absorption in the cloud-free atmosphere is on average 12 Wm(-2) too low in the GCMs. GCMs which include state-of-the-art radiation schemes and aerosol effects may be able to eliminate a large part of this clear-sky bias. The model-calculated absorption in the cloudy (all-sky) atmosphere is generally not significantly enhanced compared to the cloud-free atmosphere. This is in line with the observational estimates which are similar under all-sky and clear-sky conditions, indicating that the presence of clouds does not substantially alter the absorption of solar radiation within the atmospheric column. The observations therefore do not support recent arguments that clouds in GCMs absorb insufficient solar radiation.