The response of a general circulation model to cloud longwave radiative forcing. II: Further studies

Abstract

AbstractFollowing on from the first part of this study, the impact of cloud longwave forcing on the general circulation has been studied further using a series of 510‐day, constant January integrations with the NCAR Community Climate Model. The sensitivity of the global response to the vertical profile of the forcing has been assessed by replacing the cloud prediction scheme by that used in the ECMWF model. The results confirm those reported in Part I and emphasize the influence of tropical cirrus clouds on the local thermal structure and on the strength of the subtropical jets. The impact on the model's hydrological cycle of the cloud longwave forcing associated with boundary‐layer clouds is also shown to be important.Several integrations are described which assess the relative importance of the three major tropical forcing maxima over Indonesia, South Africa and South America in determining both the local and remote responses. For each region, the perturbation to the upper tropospheric diabatic heating by the radiative effects of the cirrus clouds excites an anticyclonic vorticity pair, located near the longitude of the forcing and almost symmetric about the equator. The influence of these anticyclones on the upper tropospheric circulation is substantial. The results also indicate that a perturbation to the diabatic heating over South America, provided in this case by the cloud longwave forcing, may have an important effect on the Walker circulation and on the extra‐tropical flow. The implications of these results for the problem of deforestation are discussed.