Temperature dependence of low cloud optical thickness in the GISS GCM: Contributing mechanisms and climate implications

A current-climate simulation of the Goddard Institute for Space Studies (GI SS) GCM, which includes interactive cloud optical properties that depend on the predicted cloud water content, is analyzed to document the variations of low cloud optical thickness with temperature in the model atmosphere. It is found that low cloud optical thickness decreases with temperature in th e warm subtropical and tropical latitudes and increases with temperature in the cold midlatitude regions. This behavior is in agreement with the resul ts of two observational studies that analyzed satellite data from the Inter national Satellite Cloud Climatology Project and Special Sensor Microwave/I mager datasets. The increase of low cloud optical thickness with temperatur e in the midlatitudes is due to vertical extent and cloud water increases, whereas the decrease with temperature in the warm latitudes is due to decre ases in cloud water content and happens despite increases in cloud vertical extent. The cloud processes that produce the cloud property changes in the model also vary with latitude. In the midlatitude regions relative-humidit y-induced increases of cloud vertical extent with temperature dominate, whe reas in the Tropics increases in cloud-top entrainment and precipitation wi th temperature produce decreases of cloud water content, whose effect on op tical thickness outweighs the effect of entrainment-induced increases of cl oud vertical extent with temperature. Doubled-CO2 simulations with the GISS GCM suggest that even though low cloud optical thickness changes have litt le effect on the global climate sensitivity of the model, they redistribute the temperature change and reduce the high-latitude amplification of the g reenhouse warming. It is also found that the current climate variations of low cloud optical thickness with temperature reproduce qualitatively but ov erestimate quantitatively the changes in optical thickness with climate war ming.