Controls on precipitation in thermally driven orographic clouds

Active orographic convection was observed over a small tropical island and yet a negligible amount of precipitation was formed. The generation of precipitation from a cumulus cloud depends sensitively on the nature of the updraft, the updraft aerosols, and the turbulent entrainment of dry air, all of which were observed during the DOMEX (Dominica Experiment) field campaign. Observations suggest aerosol-cloud-precipitation interactions may be important in retarding precipitation formation in thermally driven orographic convection but differences in the observed cloud layer moisture content confound the relationship. Early test simulations also suggest that the island geometry and presence of wind shear may influence the aerosol impact. This leads to the question of under what circumstances do aerosol-cloud-precipitation interactions impact orographic precipitation and to what extent?

The WRF model with the aerosol-aware Thompson microphysics scheme is used to further explore the relationships between simulations with and without a surface aerosol source. Idealized cloud resolving 3D simulations with a variable sensible heat flux simulating one thermal day are performed with a set of terrain configurations, wind speeds with and without shear, and cloud layer moisture contents in addition to the changing surface aerosol source. It is found that aerosols have the largest impact on precipitation when it is struggling to form. A water budget, an aerosol budget, and a close look at microphysical conversion rates from cloud to rain water help to shed light on the processes at work.