Temperature Dependence of Low Cloud Optical Thickness in the GISS GCM: Contributing Mechanisms and Climate Implications

Abstract

A current-climate simulation of the Goddard Institute for Space Studies (GISS) 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 the warm subtropical and tropical latitudes and increases with temperature in the cold midlatitude regions. This behavior is in agreement with the results of two observational studies that analyzed satellite data from the International Satellite Cloud Climatology Project and Special Sensor Microwave/Imager datasets. The increase of low cloud optical thickness with temperature in the midlatitudes is due to vertical extent and cloud water increases, whereas the decrease with temperature in the warm latitudes is due to decreases 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-humidity-induced increases of cloud vertical extent with temperature dominate, whereas in the Tropics increases in cloud-top entrainment and precipitation with temperature produce decreases of cloud water content, whose effect on optical thickness outweighs the effect of entrainment-induced increases of cloud vertical extent with temperature. Doubled-CO2 simulations with the GISS GCM suggest that even though low cloud optical thickness changes have little effect on the global climate sensitivity of the model, they redistribute the temperature change and reduce the high-latitude amplification of the greenhouse warming. It is also found that the current-climate variations of low cloud optical thickness with temperature reproduce qualitatively but overestimate quantitatively the changes in optical thickness with climate warming.