Modeling the impact of aerosols on tropical overshooting thunderstorms and stratospheric water vapor

Overshooting deep convection plays an important role in regulating the water vapor content of the tropical tropopause layer and is also an important mechanism for transporting water vapor into the lower stratosphere (LS). The aim of this study is to examine the effect of aerosols as cloud condensation nuclei (CCN) on the water vapor content of the LS via single isolated overshooting thunderstorms. The development of a severe Hector thunderstorm in northern Australia observed during the Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere/Aerosol and Chemical Transport in Tropical Convection (SCOUT-O3/ACTIVE) campaign is simulated using a three-dimensional nonhydrostatic convective cloud model with a double-moment bulk microphysics scheme. The results show that the ice hydrometeors account for over 50% of the total condensate mass, indicating that ice processes play an important role in regulating the structure of thunderstorms in the tropics. A large number of ice particles occurring in the LS are not formed in situ but are transported upward by convective overshooting with subsequent mixing. Sensitivity tests show that the increase in cloud droplet numbers induced by increasing CCN concentrations would increase the number concentrations of the ice crystals transported to the LS, which had the effect of reducing the sizes and fall speeds of the ice crystal, thereby causing a moistening of the LS by sublimation of the injected ice particles. This result suggests that aerosols in the boundary layer can affect stratospheric water vapor via overshooting deep convection.