Radiative effects of aerosols on the evolution of the atmospheric boundary layer

This study investigates the impacts of tropospheric aerosols on the evolution of the atmospheric boundary layer (ABL) for dry subsiding regions by conducting simulations with a high-resolution ABL model. The scattering and absorption of aerosols diminish the surface radiation, inhibiting the sensible heat flux and evaporation and inducing feedbacks such as the enhanced stratification and change in relative humidity in the surface layer. The reduced sensible heat due to aerosol backscattering lowers the air temperature and suppresses the growth of the ABL. The resultant reduction of entrainment heating contributes to an additional cooling. The decreased entrainment drying competes with the reduced surface evaporation, so that the net effect can be either an increase or a decrease of the ABL moisture, depending on the soil moisture. Aerosol absorption decreases the turbulent heating but simultaneously increases the solar heating, increasing the air temperature and decreasing the strength of capping inversion. The resultant rise of the top of the ABL compensates the lowering due to the reduced buoyancy flux. With strong aerosol absorption, the increased entrainment heating enhances the ABL warming. Both the increased entrainment drying and the reduced evaporation decrease the ABL moisture. The increased warmth and dryness of the ABL imply that absorbing aerosols within the ABL decrease the probability of formation of boundary layer clouds, causing additional warming through cloud-feedbacks. The results are sensitive to the vertical distribution of absorbing aerosols. Absorbing aerosol above the ABL increases the strength of capping inversion and reduces the top of the ABL, hence decreasing the entrainment drying and moistening the ABL.