The aerosol semi-direct effect is generally explained as follows: aerosols, such as black carbon (BC) and mineral dust, absorb solar radiation, which warms and stabilizes the atmosphere, resulting in reduced cloudiness and cloud formation. However, the present study suggests that BC can intensify atmospheric instability and thus increase cloud water and precipitation if the BC is concentrated near the surface. Simulations using a global aerosol climate model, based on a general circulation model, show decreased cloud water over biomass-burning regions where BC is emitted to the free troposphere through the boundary layer. In contrast, increased cloud water is indicated over East and South Asia where BC from urban and industrial activities is concentrated near the surface. While the global mean change in the radiation budget at the top of the atmosphere due to the semi-direct effect of BC is estimated to be as small as +0.06 W m-2, regional changes in cloud water, precipitation, and shortwave radiation are suggested to be large enough to modify meteorological conditions in urban and biomass-burning regions.
2011 by the Meteorological Society of Japan