Parcel model simulations of aerosol – warm phase cloud microphysics interactions over the Amazon

Abstract. Recent observational studies have shown that biomass burning aerosol can modify the microstructure of convective clouds in tropical regions such as Indonesia and the Amazon. In such regions warm rain development is evidently suppressed during the burning season 5 relative to the wet season, as numerous condensation nuclei produce more numerous but smaller cloud droplets. It is not clear, however, whether rain formation is affected only by CCN or whether other factors such as giant CCN, updraft speeds, and atmospheric moisture also play important roles in the observed differences. These issues are addressed here using a simple parcel model to explain data collected during 10 the LBA-SMOCC-EMfiN! field campaign. The results suggest that polluted clouds in particular tend to be sensitive to each of these factors. When droplet concentrations exceed 500–1000 per cc, giant CCN (GCCN) become important by generating droplets that may eventually grow to precipitation size, though in cleaner environments warm rain occurs readily regardless of GCCN. Variations in the vertical velocity and in the 15 low-level moisture are also shown to affect polluted clouds, and should be taken into account in interpreting microphysical differences observed among continental clouds.