Excessive transmission of solar radiation through the cloud-free atmosphere in GCMs

Large uncertainties exist in the distribution of solar radiation in the global climate system and its representation in General Circulation Models (GCMs). The present paper assesses the transmission of solar radiation through cloud-free atmosphere in three GCMs using information from direct observations at the surface. Clear-sky insolation climatologies have been constructed from hourly radiation data through composites of clear-sky episodes for seven sites in Germany with ten years of accurate measurements. The results suggest that GCMs overestimate clear-sky insolation at the surface, indicating that the cloud-free atmosphere is overly transparent to solar radiation in the models. Quantitatively, the annual mean overestimation for the German sites is on the order of 10 Wm(-2) in the models which include an aerosol climatology and 30 Wm(-2) in the models without aerosol. The GCM-predicted atmospheric water vapor content is abundant compared to the ECMWF Re-Analysis and therefore not considered responsible for the missing absorption in the model atmosphere. Rather, understimated water vapor absorption in the GCM radiation codes is implied.