The vertical transport of black carbon aerosol and its effect on shallow cumulus clouds

Black carbon (BC) aerosols influence clouds through absorbing solar radiation and hence heating the air and the cloud. This is the so-called semi-direct effect. This work uses the UCLA-LES model with bulk microphysics scheme to study the semi-direct effect of BC on shallow cumulus. Two scenarios are investigated: (1) BC are initially in the inversion layer, to represent the smoke transported to the area from aloft; (2) BC are initially at the surface, to represent locally-emitted smoke. The vertical transport of BC and the subsequent effects on cumulus cloud development are investigated. A simple shortwave radiation scheme is used, in which the heating effect due to BC is proportional to BC mass concentration. For BC initially in the inversion layer, the results show that a small portion of BC are transported to the cloud layer while a large portion are still in the inversion layer after 8 hours. Compared to the case without BC, there is no significant change in cloud fraction or liquid water content (LWC). For BC initially at the surface, BC are well mixed in the boundary layer after 8 hours. However, cloud fraction and LWC still remain almost unchanged compared to the case without BC. The reason might be that the evaporation of cloud water caused by BC heating is not significant. On the other hand, the little change in cloud properties due to BC may also be caused by two or more processes that cancel out the effects. The work is now investigating the relative importance of the processes and the interactions between microphysics and dynamics for both BC scenarios.