RESPONSE OF AN ATMOSPHERIC GENERAL-CIRCULATION MODEL TO RADIATIVE FORCING OF TROPICAL CLOUDS

The effects of upper tropospheric cloud radiative forcing (CRF) on the atmosphere have been examined using a recent version of the atmospher ic general circulation model (AGCM) developed by the Max Planck Instit ute of Meteorology and the University of Hamburg. This model reproduce s satellite-observed radiative forcing of clouds well overall, except that model maxima somewhat exceed those of observations. Three simulat ions have been performed where the clouds above 600 mbar have been ren dered transparent to all radiation: first, throughout the tropics in t he ''NC'' experiment; then only over oceans warmer than 25 degrees C i n the ''NCW'' experiment; and finally, only over the western Pacific w arm oceans in the ''NCWP'' experiment. The local radiative effects of these clouds when they are present in the model are radiative heating of the middle to upper troposphere due to convergence of longwave and solar radiation; radiative cooling of the tropical atmosphere near and above the tropopause; a large reduction of solar radiation (50 to 100 W/m(2)) reaching the surface; and a slight increase (5 to 20 W/m(2)) in the downward longwave radiation at the surface. The removal of clou d radiative forcing significantly alters the circulation of the model atmosphere, as in previous AGCM studies, showing that a seemingly mode rate heat source such as CRF is nonetheless capable of widespread infl uence over the global circulation and precipitation. The experiment re sponses include a significant weakening (in NCW) or rearrangement (in NCWP) of the Walker circulation. Zonal mean cloud cover, rainfall, and low-level convergence change modestly in the experiments, while zonal departures of these from their tropical means shift considerably. Reg ions over the warmest oceans which lose CRF become much less cloudy, i ndicating a positive local feedback to convection. The experiment circ ulation changes are diagnosed in terms of simple energy budget argumen ts, which suggest that the importance of CRF is enabled by the small m agnitude of the atmospheric moist energy transport in the tropics. The y also suggest that the response of the zonal mean atmosphere may be s trongly dependent on the response of zonal eddies and on interactions between surface fluxes and tropospheric lapse rates. The response of t he zonal eddies itself should be relatively independent of these inter actions.