The influence of cirrus cloud-radiative forcing on climate and climate sensitivity in a general circulation model

Six numerical experiments have been performed with a general circulation model (GCM) to study the influence of high-level cirrus clouds and global sea surface temperature (SST) perturbations on climate and climate sensitivity. The CCM used in this investigation is the third-generation ECHAM3 model developed jointly by the Max Planck Institute for Meteorology and the University of Hamburg. It is shown that the model is able to reproduce many features of the observed cloud radiative forcing with considerable skill, such as the annual mean distribution, the response to seasonal forcing, and the response to observed SST variations in the equatorial Pacific. In addition to a reference experiment where the cirrus emissivity is computed as a function of the cloud water content, two sensitivity experiments have been performed in which the cirrus emissivity is either set to zero everywhere above 400 hPa (''transparent cirrus'') or set to 1 (''black cirrus''). These three experiments are repeated identically, except for prescribing a globally uniform SST warming of 4 K. Similar to earlier GCM studies, the changed cloud radiative heating within the troposphere has a profound impact on the model climate. Since the initial radiative forcing introduced by the changed cirrus emissivity is much smaller than the convective or dynamical response, we conclude that the tropical circulation, in particular, is maintained through a positive feedback loop involving cirrus radiative heating, deep cumulus convection, and moisture supply through the large-scale dynamics. Since this interaction has been identified in at least two other GCMs employing different cumulus parameterizations, it does not crucially depend on the respective closure assumption. Moreover, the radiative-convective-dynamical coupling in the tropics is relevant also in the global warming experiment through the increase of cloud water and hence cirrus radiative heating in the warmer atmosphere. It is shown that the spin-up of the Walker circulation in both the global warming and the increased cirrus emissivity experiments is a result of a selection process which enhances the diabatic heat source through asymmetries of the circulation itself, and the extra differential heating feeds back positively on the circulation. It is also shown that cirrus clouds have a significant influence on the global climate sensitivity of the model. In the climate change experiment with the standard model, the climate sensitivity is 20% higher than in a clear-sky reference atmosphere because the increase of cirrus emissivity in the warmer atmosphere contributes substantially to the overall positive cloud feedback. In the transparent cirrus model the cloud feedback is negative, and the global sensitivity is reduced by 20% as compared to a clear-sky reference atmosphere