EFFECT OF PRECIPITATION ON THE ALBEDO SUSCEPTIBILITY OF CLOUDS IN THE MARINE BOUNDARY-LAYER

TROPOSPHERIC aerosols are thought to have three important effects on the Earth's radiation budget: the direct radiative effect(1) (perturbation of clear-sky reflectivity), the indirect radiative effect(2) (modification of cloud albedo) and the effect on the hydrological cycle(3) (modification of the vertical thickness and horizontal extent of clouds). The first two effects have been understood in principle for nearly 20 years, and quantitative estimates of their magnitudes have been provided by models and observations(4). The third phenomenon, and its relation to the other two, has received far less attention. Previous work(3) has shown, however, that increases in aerosol concentration may act to increase cloud albedo by increasing horizontal cloud fraction as well as cloud reflectivity. Here we use a simple model of the marine cloud-topped boundary layer to investigate the changes in cloud thickness and albedo that result from changes in precipitation as particle concentrations vary. We find that the sensitivity of layer cloud albedo to droplet number concentration (the albedo susceptibility) is increased by 50-200% when the dependence of cloud thickness on particle number is included. The results suggest that the response of cloud thickness to changes in aerosol particle concentration must be taken into account for accurate prediction of global albedo by climate models.