On dynamic and thermodynamic components of cloud changes
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Clouds are sensitive to changes in both the large-scale circulation and the thermodynamic structure of the atmosphere. In the tropics, temperature changes that occur on seasonal to decadal time scales are often associated with circulation changes. Therefore, it is difficult to determine the part of cloud variations that results from a change in the dynamics from the part that may result from the temperature change itself. This study proposes a simple framework to unravel the dynamic and non-dynamic (referred to as thermodynamic) components of the cloud response to climate variations. It is used to analyze the contrasted response, to a prescribed ocean warming, of the tropically-averaged cloud radiative forcing (CRF) simulated by the ECMWF, LMD and UKMO climate models. In each model, the dynamic component largely dominates the CRF response at the regional scale, but this is the thermodynamic component that explains most of the average CRF response to the imposed perturbation. It is shown that this component strongly depends on the behaviour of the low-level clouds that occur in regions of moderate subsidence (e.g. in the trade wind regions). These clouds exhibit a moderate sensitivity to temperature changes, but this is mostly their huge statistical weight that explains their large influence on the tropical radiation budget. Several propositions are made for assessing the sensitivity of clouds to changes in temperature and in large-scale motions using satellite observations and meteorological analyses on the one hand, and mesoscale models on the other hand.
KeywordsOutgoing Longwave Radiation Cloud Property Cloud Radiative Force Cloud Response Climate Perturbation
This work benefited from discussions with Kerry Emanuel, Jean-Yves Grandpeix, Christian Jakob, Laurent Li, Mark Webb and Yun-Ichi Yano, and from comments by anonymous reviewers. Part of this study was supported by the Environmental Program of the Commission of the European Communities (project ENV4-CT95-0126 entitled Cloud Feedbacks and Validation). French participants acknowledge the Programme National d’Etude Du Climat (PNEDC).
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