Summary And Conclusions

A comprehensive dataset of collocated surface and satellite observations has been used to assess the distribution of solar radiation in GCMs. Data from 720 sites present evidence that the GCMs have no problems in simulating the total amount of solar energy absorbed in the climate system correctly, but that the relative fraction of absorption within the atmosphere and at the surface is often substantially biased. The GCM atmospheres are general ly too transparent for solar radiation: the global mean values of absorbed solar radiation within the atmosphere, typically around 70 Wm"2 in GCMs, have been estimated to be too low by 10 - 20 Wm"2. This puts the most likely value of global mean shortwave atmospheric absorption in the real world, a highly controversial number, to in line with the estimates derived in Wild et al. (1998).

The shortwave absorption was separately assessed for cloudy and cloud-free conditions in the atmospheric columns above selected observation sites in Germany. Based on collocated surface and satellite observations, estimates of shortwave atmospheric absorption for both clear-sky and all-sky conditions were obtained. The GCMs typically absorb too little solar radiation, not just in the cloudy, but also in the cloud-free atmosphere. Increased absorption in state-of-the-art radiation codes and the additional inclusion of absorbing aerosol help to narrow the gaps between simulated and observed estimates of clear-sky absorption. Under cloudy (all-sky) conditions, a significant increase of shortwave atmospheric absorption is not detectable in the observations. Therefore no direct evidence is found here that clouds in the GCMs absorb insufficient solar radiation as recently postulated in other studies. Rather, the present study points to deficiencies in the absorption in the cloud-free atmosphere which are responsible for the lack of shortwave absorption in the GCM atmospheres.

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