Cloud-resolving simulations of intense tropical Hector thunderstorms:: Implications for aerosol-cloud interactions

The Hector thunderstorm is studied with a numerical cloud-resolving model. Special attention is given to modelling the mixed-phase and glaciated cloud microphysical processes (along with the implications of aerosols) and their influence on the resulting microphysical and dynamical storm structure. Radiative impacts are also calculated. Simulations are performed for a typical storm case from the EMERALD-II convective cloud experiment in November and December 2002. It is found that, for intense thunderstorms, aerosol indirect effects are generally modified from recently proposed theoretical considerations. Specifically, the proposed 'glaciation' indirect effect, resulting from increasing ice nuclei concentrations, is small for intense convection. More importantly, increasing ice number concentrations results in a 'collection' indirect effect (where aggregation and accretion processes lead to precipitation) rather than the 'glaciation', Bergeron-Findeisen process. There is a 'thermodynamic' indirect effect for Hector, as increasing the cloud droplet number concentration from maritime to continental values resulted in a suppression of the heterogeneous freezing process. However, for extreme continental cases, liquid and raindrop freezing by collection processes acquires higher importance; hence there is an optimal value for strong cumulonimbus development. The 'glaciation' indirect effect is found to be similar to increasing the rate of ice production by the Hallett-Mossop process. Another aspect of this study shows that there is a significant impact of microphysics on cloud dynamics, and so studying aerosol-cloud effects must also consider dynamical feedback, a strong component of which arises from the latent heat released during homogeneous freezing. The important indirect effects may be well described by recent theory for smaller, more common stratiform and cumulus clouds; however, in the tropics, the importance of Hector-type storms cannot be ignored as they, and other similar storms, provide a mechanism for the production of widespread cirrus and the release of a large amount of precipitation.