The response of a general circulation model to cloud longwave radiative forcing. I: Introduction and initial experiments

A new version of the NCAR Community Climate Model (CCM1) is used to study the effect of cloud radiative forcing on model simulations. Previous attempts to determine the role played by clouds in climate and the general circulation of the atmosphere are reviewed first. The concept of cloud radiative forcing is discussed and the forcings in the shortwave and longwave spectral regions are contrasted. At low latitudes, the cloud longwave forcing is primarily within the atmosphere, rather than at the surface. It is thus appropriate to study its effect with a model which employs fixed sea surface temperatures (s.s.t.s). The impact of cloud longwave forcing is studied in 510-day integrations for constant January conditions. The experiments isolate the forcing by tropical and extra-tropical clouds. Tropical cloud forcing warms the tropical upper troposphere and accelerates the subtropical jets. There are subtle interactions between the forcing, the clear-sky longwave heating and the latent heating in the tropics. These additional diabatic heating terms oppose the forcing in some regions and enhance it in others. The tropical cloud forcing strengthens the precipitation maxima at low latitudes. There are also changes in the extra-tropical flow, including the excitation of a pattern in the 200 mb geopotential height differences which is similar to those found in previous studies of the effect of s.s.t. anomalies. This confirms that the forcing may be as important as latent heat release in determining the atmospheric response to such anomalies. The mode is excited with comparable strength by the extra-tropical cloud forcing. Finally, some concerns regarding the generality of these results and their applicability to other models and the real atmosphere are discussed. These concerns suggest the need for sensitivity studies, some of which are already in progress.

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