Response of Cloud Supersaturation to Radiative Forcing

Abstract

The diffusional growth or evaporation of cloud droplets due to net emission or absorption of radiation is studied. Time dependent solutions for droplet temperatures and supersaturation are obtained, taking into account the partitioning of the net radiation budget between the droplets and the ambient air. Radiative perturbations are shown to cause extremely high rates of change in droplet temperatures. Due to efficient exchange of thermal energy within the cloud, the time constant for reducing these rates to the ambient rate is typically less than a few milliseconds and is approximately proportional to the square of the droplet radius. As the droplets evaporate or grow due to radiative effects, the saturation ratio of the ambient air adjusts due to changes in the water vapor density and the temperature of the air. The time constant for adjustment is found to be a few seconds, and the steady state saturation ratio decreases linearly with increased net radiation absorbed by the cloud. Droplet growth caused by longwave emission occurs under slightly supersaturated conditions. The net radiation budgets of individual droplets and the supersaturation appear to affect the evolution of the droplet size distribution, but are not needed to assess radiatively induced changes in the cloud temperature and liquid water content, which depend only on the total radiation budget of the cloud.