Semi-direct Effects of Aerosols on Low Clouds: Mechanisms for Climate Cooling by Black Carbon Aerosols (Invited Presentation)

Light-absorbing black carbon aerosols coincide with low clouds in many locations within the tropics and sub-tropics where extensive biomass burning or dirty transportation and industrial combustion occur. In many of these locations the aerosols have been documented to increase the drop concentration of the clouds through the microphysical Twomey effect. Furthermore, the absorption of sunlight changes the thermal structure of the lower atmosphere, which can further impact cloud cover. Absorption increases temperature, and in the absence of other effects reduces relative humidity, which should tend to reduce cloud cover. However, we find that light-absorbing aerosols also reduce boundary layer turbulent kinetic energy. In the case of both South Atlantic Ocean subtropical stratocumulus and North Indian Ocean trade cumulus, this reduction in turbulence acts to enhance relative humidity in the boundary layer and contribute to building clouds. In the North Indian Ocean this semi-direct enhancement of cloud development combines with the microphysical effects of aerosols to substantially increase cloud albedo. This talk will address both in-situ and satellite techniques for investigating the impacts of black carbon aerosols on boundary layer dynamics and low clouds.