Investigation of aerosol effects on a deep convection case in Southeast China with budget analysis of hydrometeor mass and latent heat

Abstract

It remains an open question which microphysical processes determine the sign of aerosol effects on deep convective rain. This lack of knowledge prevents us from understanding the uncertainties of the aerosol effects on deep convection. We try to answer this question through a cloud-resolving model (CRM) simulation on a summer storm that occurred in Southeast China, by analyzing both column hydrometeor mass and latent heat budget and quantifying the contribution of each microphysical process to the rain variations with aerosol. Our results show that warm-phase microphysical processes have stronger contributions (52% −99%) to the rain variations than ice-phase microphysical processes (1%−48%) when gradually increasing cloud condensation nuclei (CCN) concentration from 125 cm−3 (clean environment) to 16,000 cm−3 (highly polluted environment). Condensation and evaporation of cloud droplets are identified as two crucial microphysical processes, both of which are enhanced with increased aerosol loading. The competition between them (i.e., net condensation) essentially determines the sign of aerosol effects on surface rain. We also explore aerosol effects on updraft velocity from the perspective of buoyancy and latent heat budget, it reveals that the largely enhanced convective updrafts below 7 km are attributed to the increase of condensation heating. While the competition between enhanced condensation heating and suppressed deposition heating restricts the aerosol-induced convective invigoration above 7 km. This study highlights the dominant role of net condensation in the aerosol-imposed invigoration of deep convection under highly polluted environments, suggesting that an explicit treatment of vapor diffusion processes is necessary to simulate aerosol effects on deep convection more accurately.