The December, January and February mean zonal average net cloud radiative forcing from 25 AMIP models and observations (ERBE) show generally negative values in the Southern Hemisphere tropics and extratropics. This corresponds primarily to the short-wave cooling effect of clouds. When compared to ERBE, the models appear to exaggerate this cooling in the tropics, and virtually all the models display a maximum systematic error between 30 degrees N and 30°S. Poleward of these latitudes it appears that a different error may be occurring unrelated to that in the tropics. For most models the apparent error is so large (tens of Wm-2) that there is little doubt that it would have a major impact on the general characteristics of surface and atmospheric heating.
Model output from a subset of the AMIP models shows that it is the tropical
short-wave cloud radiative forcing that is systematically too strong when
compared to satellite observations. The models also produce a negative
systematic error in cloud amount when compared to ISCCP in the extratropics.
Cloud amount error accounts for much of the error in short-wave cloud forcing
because of their linear relationship although errors in the parameterization
of cloud optical properties also contribute.