Aerosols vs Dry Air Entrainment in Thermally Driven Orographic Clouds

The generation of precipitation from a cumulus cloud depends sensitively on the nature of the updraft aerosols and the turbulent entrainment of dry air.  These two processes were observed in the orographic clouds during the DOMEX (Dominica Experiment) field campaign.  On days with thermally driven convection, little precipitation develops and the orographic clouds are composed on average of clouds with 125 cm^-3 droplet number concentration and 15 μm cloud droplet diameter.  Aerosol number concentrations as high as 325 cm^-3 are found in the detrained air above Dominica.  The island surface aerosol source and the relatively dry cloud layer are two independent variables that play a role in the composition and development of the observed orographic clouds.

In the present study, we use idealized 3D WRF simulations with the new aerosol-aware Thompson and Eidhammer microphysics scheme to compare with observations.  A 1 km high mountain with a constant surface sensible heat flux of 200 W m^-2 drives convection with no background wind.  Four simulations are performed to explore the parameter space with and without an aerosol source, and with a dry and moist cloud layer: (1) aerosol source / dry, (2) aerosol source / moist, (3) no source / dry, and (4) no source / moist.  The aerosol source is composed of an organic-like aerosol with a mean radius of 0.08 μm and a hygroscopicity of 0.6.  The aerosol flux comes only from the island surface at a rate of 10 aerosols cm^-3 s^-1 or 3x10⁸ aerosols m^-2 s^-1. Precipitation efficiency and microphysical conversion rates of liquid water are computed and tracked, and a full aerosol budget is completed.  Comparing the development of orographic clouds and precipitation in the four simulations leads toward an improved understanding of the observations and the relative controls on convection.