A Note on Aerosol Processing by Droplet Collision-Coalescence

Abstract

The processing of aerosol by droplet collision-coalescence is analyzed in three-dimensional simulations of drizzling stratocumulus using a Lagrangian cloud model for the representation of aerosol and cloud microphysics. Collision-coalescence processing is shown to create a characteristic bimodality in the aerosol size distribution. We show that the large-scale dynamics of the stratocumulus-topped boundary layer are key to understanding the amount of time available for collision-coalescence processing. The large-scale dynamics enable aerosol particles to repeat a cycle of droplet condensation, collision-coalescence, and evaporation, which causes a steady increase in aerosol size. This process is continued until the aerosol grows so large that droplet growth is substantially accelerated and multiple collisions occur within one cycle, forming precipitation-sized droplets that are lost to the surface, including the aerosol. An analytical relationship is derived that captures the fundamental shape of the processed aerosol size distribution.