Multisensor satellite observations of aerosol effects on warm clouds

A global multisensor satellite examination of aerosol indirect effects on warm oceanic clouds is presented. The study centers on the water path response of cloud to aerosol burden. A unique element of the study is a rigorous rain screening methodology that is utilized to separate the responses of nonraining and raining clouds. It is demonstrated that high aerosol environments are associated with reduced liquid water path in nonprecipitating clouds and that the reduction in liquid water path reduces the albedo enhancement expected from decreasing effective radius. Furthermore the reduction in liquid water path is greater in thermodynamically unstable environments than in stable environments suggesting a greater sensitivity of cumulus cloud than stratiform cloud liquid water path to aerosol. In sharp contrast with nonprecipitating clouds, the cloud liquid water path of transitional and precipitating clouds increases dramatically with aerosol, which may be indicative of an inhibited coalescence process. The evidence further indicates that increasing aerosol requires greater amounts of cloud condensate before the onset of precipitation. Additional support for this hypothesis is found in a reduction in the probability of precipitation by as much as 10% depending on the thermodynamical state of the environment. Independent estimates of the broadband cloudy‐sky albedo are used to confirm that the cloud albedo responds to the trends that are identified in the liquid water path. In particular, it is found that the water path effect dominates the cloud albedo response for precipitating and transitional clouds. Finally, regional analysis demonstrates that the magnitude of the relationship between aerosol and cloud albedo is greatest in the extratropics and subtropical stratus regions primarily in the winter hemisphere. These relationships are used to estimate the magnitude of the first indirect effect as −0.42 W/m 2 .