Sensitivity of the LMD General Circulation Model to Greenhouse Forcing Associated with Two Different Cloud Water Parameterizations

Abstract

The atmospheric general circulation model of the Laboratoire de Météorologic Dynamique is coupled to a slab ocean model and is used to investigate the climatic impact of a C02 doubling. Two versions of the model are used with two different representations of the cloud-radiation interaction. Both of them contain a prognostic equation for the cloud liquid water content, but they differ in the treatment of the precipitation mechanism. The annual and global mean of the surface warming is similar in the two experiments in spite of regional differences. To understand the behavior of the model versions, the total climate change is split into a direct C02 forcing and different feedback effects (water vapor. cloud, and surface albedo). The results show that, in the second model version, the cloud feedback decreases significantly, especially at high latitudes, due to an increase of low-level clouds in the 2?C02 simulation. The modification of the cloud scheme influences also the water vapor variation and the associated feedback is reduced, in particular, over the subtropical regions. The surface albedo feedback is increased. This is due to the fact that the cloudiness is smaller over high latitudes and the surface snow is more directly exposed to incoming radiation. Although the results are qualitatively similar to the results obtained with other models, the occurrence of such compensations between different feedback mechanisms leads to a different evaluation of the overall climate sensitivity.