Climate sensitivity to cloud optical properties

A radiative-convective model was developed to investigate the sensitivity o f climate to cloud optical properties and the related feedback processes. T his model demonstrates that the Earth's surface temperature increases with cloud optical depth when the clouds are very thin but decreases with cloud optical depth when the cloud shortwave (solar) radiative forcing is larger than the cloud longwave (terrestrial) radiative forcing. When clouds are in cluded in the model, the magnitude of the greenhouse effect due to a doubli ng of the CO2 concentration varies with the cloud optical depth: the thicke r the clouds, the weaker the greenhouse warming. In addition, a small varia tion in the cloud droplet size has a larger impact on the equilibrium state temperature in the lower atmosphere than the warming caused by a doubling of the CO2 concentration: a 2% increase in the average cloud droplet size p er degree increase in temperature doubles the warming caused by the doublin g of the CO2 concentration. These findings suggest that physically reliable correlations between the cloud droplet size and macrophysical meteorologic al variables such as temperature, wind and water vapor fields are needed on a global climate scale to assess the climate impact of increases in greenh ouse gases.