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

TROPOSPHERIC aerosols are thought to have three important effects on t he Earth's radiation budget: the direct radiative effect(1) (perturbat ion of clear-sky reflectivity), the indirect radiative effect(2) (modi fication of cloud albedo) and the effect on the hydrological cycle(3) (modification of the vertical thickness and horizontal extent of cloud s). The first two effects have been understood in principle for nearly 20 years, and quantitative estimates of their magnitudes have been pr ovided by models and observations(4). The third phenomenon, and its re lation to the other two, has received far less attention. Previous wor k(3) has shown, however, that increases in aerosol concentration may a ct 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 thickn ess and albedo that result from changes in precipitation as particle c oncentrations vary. We find that the sensitivity of layer cloud albedo to droplet number concentration (the albedo susceptibility) is increa sed by 50-200% when the dependence of cloud thickness on particle numb er is included. The results suggest that the response of cloud thickne ss to changes in aerosol particle concentration must be taken into acc ount for accurate prediction of global albedo by climate models.