Vertical Redistribution of Aerosol by Shallow Cumulus Clouds: Observations and Modeling

The vertical redistribution of aerosol by clouds is a well documented phenomenon for deep convective clouds, but in the case of shallow cumulus, has received little attention. We present observations and modeling of this process based on measurements performed during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) in the summer of 2006. Aerosol properties (total concentration, light scattering and absorption, and composition) were measured on board the CIRPAS Twin Otter. Radiation and surface infrared temperature measurements were used to identify the sections of the highest, above-cloud legs that were influenced by clouds. A comparison is made between the aerosol properties in regions affected by cloud vertical transport, vs. those unaffected by clouds. It is shown that on average there are distinct increases in aerosol concentration and light scattering, as well as changes in composition, in regions above clouds. Large eddy simulations of this convective redistribution indicate increases in aerosol extinction on the order of 15-20%, in broad agreement with the observations.