Entrainment Makes Pollution More Likely to Weaken Deep Convective Updrafts Than Invigorate Them

Are the results of aerosol invigoration studies that neglect entrainment valid for diluted deep convective clouds? We address this question by applying an entraining parcel model to soundings from tropical and midlatitude convective environments, wherein pollution is assumed to increase parcel condensate retention. Invigoration of 5%-10% and <2% is possible in undiluted tropical and midlatitude parcels respectively when freezing is rapid. This occurs because the positive buoyancy contribution from freezing is larger than the negative buoyancy contribution from condensate loading, leading to positive net condensate contribution to buoyancy. However, aerosol-induced weakening is more likely when realistic entrainment rates occur because water losses from entrainment more substantially reduce the latent heating relative to the loading contribution. This leads to larger net negative buoyancy contribution from condensates in polluted than in clean entraining parcels. Our results demonstrate that accounting for entrainment is critical in conceptual models of aerosol indirect effects in deep convection. Plain Language Summary Past theoretical studies of how pollution affects thunderstorms typically ignore the mixing of clouds with their surrounding environment. Some of these studies argue that pollution should make thunderstorms more vigorous. Here, we use a theoretical model to investigate how pollution and mixing affects the interplay within the region of a cloud where both supercooled liquid water and ice are present. Our results suggest that pollution becomes increasingly detrimental to storm updrafts as the rate of mixing of thunderstorm air with the storm's surroundings increases. Consequently, because mixing is common in thunderstorms (especially those occurring in the tropics), our results suggest that pollution should weaken (not strengthen) storm updrafts. Our results emphasize the importance of considering mixing when studying pollution effects on storm updrafts.