Impact of Aerosol Shortwave Radiative Heating on Entrainment in the Atmospheric Boundary Layer

Haze pollution has frequently occurred during recent years and received great concern in China. Understanding the interactions between aerosols and the atmospheric boundary layer (ABL) is essential for predicting aerosol concentration and improving air quality forecast. However, only few studies addressed the effect of aerosol radiative heating on the entrainment in the ABL. Entrainment is critical to the development of the ABL as it controls exchange of heat, water vapor, CO₂, O₃, and air pollutants between the ABL and the free atmosphere. Parameterization of entrainment processes is one of the most challenging problems in the ABL research. In this study, a radiation transfer model is coupled with a large eddy simulation (LES) to investigate the impacts of aerosol shortwave radiative heating on the entrainment in the dry ABL. A series of LES runs are conducted with varying aerosol optical depth (AOD) and different geostrophic winds. Several findings are obtained from the study. First, the domain-averaged profiles of heat flux do not change linearly with height. Second, the potential temperature gradients within the entrainment zone and the entrainment fluxes decrease as AOD increases. Third, the local entrainment ratio for heat was found to decrease with increasing AOD. The ratios decrease by about 19% as AOD increases from 0 to 1.5. This indicates that the assumption of constant entrainment ratio, often used in the regional numerical models, does not hold in the ABL under hazy conditions. In the absence of AOD, the entrainment ratio showed growing trend with decreasing surface heat flux. Specifically, the 40% increase in the ratio corresponded to the surface kinematic heat flux reduction from 0.17 K·m·s^-1 to 0.06 K·m·s^-1. Relationships between the entrainment parameters need to be modified to account for the impact of aerosol radiation effect. These relationships will be evaluated using the LES output data.