Sensitivity of the Lmd General Circulation Model to Greenhouse Forcing Associated with Two Diierent Cloud Water Parameterizations

1 ABSTRACT The atmospheric General Circulation Model of the Laboratoire de M et eorologie Dynamique (LMD) is coupled to a slab ocean model and is used to investigate the climatic impact of a CO 2 doubling. Two versions of the model are used with two diierent representations of the cloud-radiation interaction. Both of them contain a prognostic equation for the cloud liquid water content, but they diier 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 diierences. To understand the behavior of the model versions, we split the total climate change into a direct CO 2 forcing and diierent feedback eeects (water vapor, cloud and surface albedo). The results show that, in the second model version, the cloud feedback decreases signiicantly, especially at high latitudes, due to an increase of low level clouds in the 2CO 2 simulation. The modiication of the cloud scheme innuences 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 diierent feedback mechanisms leads to a diierent evaluation of the overall climate sensitivity. 2