The role of clouds and the cloud-free atmosphere in the problem of underestimated absorption of solar radiation in GCM atmospheres

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Large uncertainties exist to date in the distribution of solar radiation in the global climate system and its representation in General Circulation Models (GCMs). Based on a comprehensive dataset of collocated surface and satellite observations at more than 700 sites, it is shown that GCM atmospheres typically absorb too little solar radiation, in the range of 10–20 Wm −2 . While several studies have pointed to a possible underestimation of solar absorption in clouds which may explain this bias, the present study shows evidence that the absorption in the cloud-free atmosphere is too small. These findings are based on a direct comparison of the GCM fluxes with observed clear-sky climatologies at surface radiation sites, and on off-line validations of the GCM radiation schemes for clear-sky conditions. Quantitatively, the absorption in the cloud-free GCM atmospheres is estimated to be too small on the order of 10 Wm −2 , and even more in GCMs which neglect aerosol. Possible causes are an underestimation of solar absorption by water vapour and the neglect of aerosol effects. With respect to clouds, our observational data cannot exclude a possible lack of cloud absorption in GCMs at low latitudes, which is, however, unlikely to be as large as recently proposed. At higher latitudes, on the other hand, no evidence for a lack of cloud absorption in the GCMs is found when compared to the observational dataset. Overall, this study points out that in current GCMs not only the clouds, but also the cloud-free atmosphere can be responsible for the discrepancies between observational and simulated estimates of shortwave atmospheric absorption.