Low Cloud Responses to Solar Absorption By Black Carbon Aerosols

Light-absorbing black carbon aerosols coincide with low clouds in many locations within the tropics and sub-tropics where extensive biomass burning or 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 cloudiness. Absorption increases temperature in the lower troposphere, and in the absence of other effects reduces relative humidity, which should tend to reduce cloudiness. However, surface cooling due to aerosol attenuation of solar radiation reaching the surface or enhanced static stability can drive dynamic responses that suppress turbulent mixing and cause subsidence of the marine boundary layer top. Such responses could enhance cloudiness. Evidence of such responses is present in in-situ and satellite data from several black carbon aerosol hot-spots in the tropics and subtropics. Here we examine data from recent field campaigns and numerical model experiments for evidence of these responses. We also discuss prospects for evaluating low-cloud dynamical responses to light-absorbing aerosols in decade-scale time series of satellite observations from the NASA A-train and related missions.