EFFECT OF ICE-CLOUD ON GCM CLIMATE SIMULATIONS

The effect of the interaction of SW (shortwave, solar) radiation with ice clouds on climate simulations is studied using the GEOS (Goddard E arth Observing System) climate GCM (general circulation model) coupled with an ocean mixed-layer model. Due to much larger cloud particles, the single-scattering co-albedo of ice clouds is nearly ten times larg er than that of water clouds, but the asymmetry factor is generally sm aller. Offline calculations with a radiative transfer model show that inclusion of the ice cloud effect increases SW absorption in the upper troposphere and decreases the SW flux at the surface. For the earth-a tmosphere system, the change in the SW absorption is small due to comp ensation of the co-albedo and asymmetry factor effects. The effect of ice clouds tends to increase the atmospheric stability by enhancing SW heating in the upper troposphere and reducing SW heating in the lower troposphere and at the surface. The GCM experiments demonstrate that the inclusion of ice clouds produces colder surface temperature, and l eads to reduction in SW heating of the earth-atmosphere system associa ted with changes in surface albedo and in cloud distribution. The enha nced atmospheric stability leads to decreased high cloud amount by a m aximum of 80% and to increased middle cloud by a maximum of 60% over t he tropics.