Global observations of aerosol impacts on precipitation occurrence in warm maritime clouds

The impact of aerosols on precipitation occurrence in warm clouds is assessed using a combination of multisensor satellite cloud and precipitation data sets and aerosol information from both satellite and a global transport model. Aerosols are found to suppress the formation of precipitation in polluted regions, evidenced by a trend toward higher liquid water path prior to the onset of light rainfall. Polluted clouds are also found to be more vertically developed than those in more pristine environments. Coupled with an apparent reduction in the size of the raindrops that subsequently form in these clouds, these findings indicate that pollution inhibits precipitation processes by redistributing water among a greater number of smaller cloud droplets. Evidence is also provided that sea-salt aerosols have the opposite effect on precipitation development. Maritime clouds that form in regions of enhanced sea-salt concentrations tend to precipitate more frequently, form larger raindrops, and be less vertically developed. This suggests that the nucleation of sea-salt particles may provide a source of embryonic raindrops in maritime clouds accelerating precipitation processes and ultimately reducing cloud lifetime. The net effect of aerosols on the onset of precipitation in any given region is, therefore, defined by the relative magnitudes of the competing effects of sulfate aerosols and sea-salt particles, the strengths of which depend strongly on both cloud liquid water path and the thermodynamic properties of the local environment.