Collocated surface and satellite observations as constraints for Earth radiation budget simulations with global climate models

Satellite measurements show that the exchange of solar energy between the global climate system and outer space is well simulated by the current generation of General Circulation Models (GCM). However, this alone does not ensure that these models also reproduce the distribution of solar energy within the simulated climate system correctly. Thus, the present study uses in addition to the satellite data a collocated set of surface observations for a more vigorous assessment of the solar energy in the climate system than could ever be achieved using satellite data alone. It is shown that GCMs typically underestimate the absorption of solar energy in the atmosphere, by 10–20 Wm-2. In other words, the present study suggests that the global mean shortwave atmospheric absorption, a highly debated quantity, should rather be between 80–90 Wm-2, than around 70 Wm-2 as found in many current GCMs. This leads to excessive insolation at the GCM surface compared to more than 700 globally distributed observation sites. In a case study based on data from observation sites in Germany, the relative portion of solar energy absorbed in the cloud-free atmosphere and its cloudy counterpart is investigated. No indications are found that the absorption of solar radiation in the GCM atmospheres should be significantly enhanced when clouds are present, which has been postulated in other studies. Rather, the underestimation in the atmospheric absorption in many GCMs seems to be caused by a lack of absorption in the cloud-free atmosphere, related to an underestimated water-vapor and aerosol absorption.