Dependence of aerosol-precipitation interactions on humidity in a multiple-cloud system

Abstract. This study examines the dependence of aerosol-precipitation interactions on environmental humidity in a mesoscale cloud ensemble (MCE) which is composed of convective and stratiform clouds. The author found that increases in aerosol concentration enhance evaporative cooling, which raises not only the intensity of vorticity and entrainment but also that of downdrafts and low-level convergence. The increase in vorticity tends to suppress precipitation. The increase in low-level convergence tends to enhance precipitation by generating more secondary clouds in a muptiple-cloud system simulated here.

At high humidity, the effect of the increased vorticity on cloud-liquid mass and, thus, precipitation is outweighed by that of the increased low-level convergence. This leads to the precipitation enhancement induced by the increase in aerosol concentration. When humidity lowers to mid humidity, the effect of aerosol on low-level convergence still dominates that on entrainment, leading to the precipitation enhancement with the increased aerosol concentration. With the lowest humidity in the current work, the effect of aerosol on entrainment dominates that on low-level convergence, leading to the precipitation suppression with the increased aerosol concentration. Hence, there is not only a competition between the effect of evaporation on vorticity and that on low-level convergence at a given humidity level but also the variation of the competition with the varying humidity. This competition and variation are absent in a single-cloud system where the effect of low-level convergence on secondary clouds is absent. This exemplifies a difference in the mechanism which controls aerosol-precipitation interactions between a single-cloud system and a multiple-cloud system.