Investigation of atmospheric boundary layer characteristics for different aerosol absorptions: Case studies using CAPS model

This study investigates the direct radiative effects of aerosols on the evolution of atmospheric boundary layer (ABL) over a tropical site, Anand, for 4 days, representative of each season, using land surface processes experiment (LASPEX) data sets used in a one-dimensional ABL and radiative-transfer models. Simulations with the ABL model incorporating fixed aerosol loading with three different levels of absorption and its perturbation with aerosol-free conditions were analyzed. The reduction in net available flux (NAF) increases with increase in aerosol absorption, resulting in maximum reduction for strongly absorbing type. In dry seasons, soil being dry, NAF is partitioned almost equally by latent (LE) and sensible (H) heat fluxes. In wet season, since soil moisture is abundant, LE dominates about 75% in compensating the reduction in NAF. The larger the absorption, the lesser the gradient between the surface and 2-m air temperature and hence more stable the surface layer. The reduction in vertical temperature gradient ranges from 1.74 (dry season) to 0.6 K (wet season). This stabilization of the surface layer reduces sensible heat flux and surface evaporation. Aerosol absorption decreases the turbulent heating but simultaneously increases the solar heating, and in turn increases the air temperature. This affects the inversion layer and hence the ABL height. Reduction in NAF at the surface decreases the ABL height, and the average decrease during the daytime is 167, 204 and 247 m for SSA=1.0, 0.9 and 0.8, respectively, during dry seasons. It is also found that absorbing aerosols delay the growth and promote early collapse of the ABL in all seasons.