Abstract:

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 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.  This is because buoyancy in polluted clouds relies more on the latent heat released when liquid condensate freezes, than in clean clouds.  This latent heat release is reduced by water losses from entrainment. Hence, the negative buoyancy from condensate loading in polluted clouds more than offsets the positive buoyancy gained from freezing, and polluted clouds are more adversely affected by entrainment. 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 demonstrate that accounting for entrainment is critical in conceptual models of aerosol indirect effects in deep convection.