Aerosol Effects on Water Clouds and Warm Precipitation

Aerosol particles are hypothesized to lengthen the lifetime of clouds because increased concentrations of smaller droplets lead to decreased drizzle production and reduced precipitation efficiency (Albrecht 1989). It is difficult to devise observational studies that can separate the cloud lifetime from the cloud albedo effect. Thus, observational studies provide estimates of the combined effects. Similarly, climate models cannot easily separate the cloud lifetime indirect effect once the aerosol scheme is fully coupled to a cloud microphysics scheme. Instead, they calculate the combined cloud albedo, lifetime, and semi-direct effect.

GCM studies suggest that, in the absence of giant CCN and aerosol-induced changes in ice microphysics, anthropogenic aerosols suppress precipitation. It should be noted, however, that precipitation would also be suppressed in mixed-phase clouds in which the ice phase plays only a minor role. A decrease in the formation of precipitation leads to increased cloud processing of aerosols. CRM studies have shown that cloud processing can lead to either an increase or a decrease in precipitation in subsequent cloud cycles, depending on the size and concentration of activated CCN (e.g., Feingold and Kreidenweis 2002). When the actual cloud lifetime is analyzed in CRM simulations, an increase in aerosol concentration—from very clean to strongly anthropogeni-cally influenced situations—does not increase cloud lifetime, even though precipitation is suppressed (Jiang et al. 2006). This results from competition between precipitation suppression and enhanced evaporation of the more numerous smaller cloud droplets at high cloud droplet concentration. Giant sea-salt nuclei, on the other hand, may override the precipitation suppression effect of the large number of small CCN.

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