Cloud Feedback Processes in a General Circulation Model

The influence of the cloud feedback process upon the sensitivity of climate is investigated by comparing the behavior of two versions of a climate model with predicted and prescribed cloud cover. The model used for this study is a general circulation model of the atmosphere coupled with a mixed layer model of the oceans. The sensitivity of each version of the model is inferred from the equilibrium response of the model to a doubling of the atmospheric concentration of carbon dioxide. It is found that the cloud feedback process in the present model enhances the sensitivity of the model climate. In response to the increase of atmospheric carbon dioxide, cloudiness increases around the tropopause and is reduced in the upper troposphere, thereby raising the height of the cloud layer in the upper troposphere. This rise of the high cloud layer implies a reduction of the temperature of the cloud top and, accordingly, of the upward terrestrial radiation from the top of the model atmosphere. Thus, the heat loss from the atmosphere-earth system of the model is reduced. As the high cloud layer rises, the vertical distribution of cloudiness changes, thereby affecting the absorption of solar radiation by the model atmosphere. At most latitudes the effect of reduced cloud amount in the upper troposphere overshadows that of increased cloudiness around the tropopause, thereby lowering the global mean planetary albedo and enhancing the CO2 induced warming. On the other hand, the increase of low cloudiness in high latitudes raises the planetary albedo and thus decreases the CO2 induced warming of climate. However, the contribution of this negative feedback process is much smaller than the effect of the positive feedback process involving the change of high cloud. The model used here does not take into consideration the possible change in the optical properties of clouds due to the change of their liquid water content. In view of the extreme idealization in the formulation of the cloud feedback process in the model, this study should be regarded as a study of the mechanisms involved in this process rather than the quantitative assessment of its influence on the sensitivity of climate.