Aerosol Influence on Cloud Microphysics Examined by Satellite Measurements and Chemical Transport Modeling

Anthropogenic aerosols are hypothesized to decrease cloud drop radius and increase cloud droplet number concentration enhancing cloud optical depth and albedo. Here results have been used from a chemical transport model driven by the output of a numerical weather prediction model to identify an incursion of sulfate-laden air from the European continent over the mid–North Atlantic under the influence of a cutoff low pressure system during 2–8 April 1987. Advanced Very High Resolution Radiometer (AVHRR) measurements of visible and near-infrared radiance are used to infer microphysical properties of low-altitude (T 5 260–275 K) maritime clouds over the course of the event. Examination of the cloud optical depth, drop radius, and drop number concentration on the highand low-sulfate days has allowed identification of the increase in cloud droplet number concentration and decrease in cloud drop radius associated with the sulfate incursion. These observations are consistent with the Twomey mechanism of indirect radiative forcing of climate by aerosols.