Cloudiness as a Climatic Feedback Mechanism: Effects on Cloud Amounts of Prescribed Global and Regional Surface Temperature Changes in the NCAR GCM

Abstract

In a series of January experiments, prescribed changes of 2 K to the ocean surface temperatures of the NCAR GCM are imposed either globally or in one of two zonal strips centered at 5°S and 15°N. The global perturbations cause a global response in 3 km cloudiness of opposite sign to the surface temperature change, whereas a warm surface temperature perturbation at 15°N, under the subsiding branch of the Hadley cell, causes a cloudiness increase over the strip and smaller cloudiness decreases in zones adjacent to the strip. On the other hand, a 2 K strip placed at 5°S under the ascending branch of the Hadley cell causes little cloudiness response over the strip, but a statistically significant cloudiness decrease in adjacent zones. It is concluded that, in addition to a term which accounts for the effects on global cloudiness of global temperature changes, a comparably important additional term is needed to compute the global cloudiness feedback problem. This additional term integrates the local change in surface temperatures with the local vertical velocity. Although these model results could have applicability to climate sensitivity studies, definitive verification with real data of these (or any) GCM cloudiness-surface temperature feedback effects will remain a serious deficiency for all such GCM sensitivity studies for the immediate future.