Comparison of mechanisms of cloud-climate feedbacks in GCMs

International model comparisons of cloud-climate feedbacks have typically been restricted to assessing only the radiative effect of changes in clouds and have not attempted to explain the mechanisms for differences in cloud feedbacks. This paper uses different versions of the U.K. Meteorological Office GCM run at the Hadley Centre to illustrate the usefulness of a derailed comparison of microphysical cloud properties in understanding cloud feedback mechanisms and their effect on the regional distribution of the predicted warming in simulations of climate change. The inclusion of interactive cloud radiative properties explains much of the difference in the spatial patterns of cloud feedback and leads to a marked difference in the response of the large-scale circulation and in the resulting meridional gradient of surface temperature changes. In the model versions that include interactive radiative properties, the strength of the related feedback is determined by the water path of the cloud in the control experiment. Difficulties in performing such a detailed comparison on a wider range of models may arise from the lack of diagnostics in a common format being available from different models and because of the range of assumptions about how clouds are treated by different radiation schemes. A suggestion is put forward for a possible common format that would enable comparison of such diagnostics.