COMPARISON OF THE SEASONAL CHANGE IN CLOUD-RADIATIVE FORCING FROM ATMOSPHERIC GENERAL-CIRCULATION MODELS AND SATELLITE-OBSERVATIONS

We compare seasonal changes in cloud-radiative forcing (CRF) at the to p of the atmosphere from 18 atmospheric general circulation models, an d observations from the Earth Radiation Budget Experiment (ERBE). To e nhance the CRF signal and suppress interannual variability, we conside r only zonal mean quantities for which the extreme months (January and July), as well as the northern and southern hemispheres, have been di fferenced. Since seasonal variations of the shortwave component of CRF are caused by seasonal changes in both cloudiness and solar irradianc e, the latter was removed. In the ERBE data, seasonal changes in CRF a re driven primarily by changes in cloud amount. The same conclusion ap plies to the models. The shortwave component of seasonal CRF is a meas ure of changes in cloud amount at all altitudes, while the longwave co mponent is more a measure of upper level clouds. Thus important insigh ts into seasonal cloud amount variations of the models have been obtai ned by comparing both components, as generated by the models, with the satellite data. For example, in 10 of the 18 models the seasonal osci llations of zonal cloud patterns extend too far poleward by one latitu dinal grid.