Response of deep convective clouds to increases in aerosol burdens

A number of studies have found evidence for the reduction in ice particle size in deep convective clouds subjected to elevated concentrations of aerosol particles. In this study, the aerosol, cloud, and radiation budget data contained in the Cloud and Earth's Radiant Energy System (CERES) Single Satellite Footprint (SSF) data are used to re-examine the findings of the earlier studies. Three years of Terra CERES observations were used to survey the Earth for deep convective clouds and aerosols. As in earlier studies, deep convective clouds were identified as those for which the 11-micron brightness temperature within a CERES field of view (~20 km) fell below 210 K. To ensure that the cloud was in a region of active convection, only those fields of view for which the clouds were opaque at 11 microns were taken to be deep convective clouds. The three-year survey led to the selection of the Pacific Warm Pool, South America, Africa, and the Indian Ocean as regions frequented by deep convective clouds and high aerosol burdens. Within the separate regions and for each day, 2° x 2° latitude x longitude regions were selected in which deep convective clouds and aerosols were found together. If within a 10° x 10° region, two or more of the smaller 2° x 2° regions were found, then differences in the properties of the clouds coexisting with the higher and lower aerosol burdens were calculated. Differences in cloud properties were compared with the differences in aerosol burdens. This strategy was adopted to ensure that the clouds and aerosols existed together and that nearby clouds, presumably subject to similar large-scale meteorological environments, were compared. Regardless of the region and the seasons for which the analyses were performed, no differences in cloud properties were found to be linked to differences in aerosol burdens. Ice particle size, however, was found to decrease as the temperature of deep convective clouds decreased. Likewise, cloud optical depth and cloud albedo increased as the temperature decreased. Such relationships among the cloud properties and the inclusion of semitransparent clouds in the earlier studies might explain why the findings of this study differ from those of the earlier studies.