Effects of Aerosol on the Diurnal Variation of Clouds and Precipitation in a Mesoscale System of Deep Convective Clouds

Analyses of observational data of deep convective clouds showed that larger aerosol concentrations (polluted conditions) resulted in suppressed precipitation before the mid-afternoon, while resulting in enhanced precipitation after the mid-afternoon when compared to precipitation with smaller aerosol concentrations (clean conditions). This suggests that there is a tipping point in the transition from suppressing to enhancing precipitation with increases in aerosol concentration. This work aims to identify mechanisms that control the tipping point by performing simulations for an observed mesoscale system of deep convective clouds. Simulations show that during the first three-quarters of the 12-hour simulation period, aerosol as a radiation absorber suppresses convection and precipitation by inducing greater radiative heating and stability. Convection weakens and precipitation reduces more under polluted conditions than under clean conditions. Due to the suppressed convection, the depletion of convective energy decreases. The reduced depletion of convective energy during the period of the suppressed convection boosts the level of stored energy after this period. The boosted level of stored energy enables updrafts to be strong enough to transport a greater amount of cloud liquid to the freezing level and to levels above it under polluted conditions than under clean conditions. This in turn induces more invigoration of convection and results in the transition from lower precipitation rates during the first three-quarters of the simulation period to higher precipitation rates during the last quarter of the period under polluted conditions than under clean conditions.