Convective cloud inhibition attributed to dust and smoke aerosols in sub-Saharan African

Mobilized Saharan dust aerosols and smoke from anthropogenic savanna fires in sub-Saharan Africa affect convective clouds in the region. Here we measured the temporal cloud response to dust and smoke aerosols in northern Africa using aerosol data from the Multi-angle Imaging SpectroRadiometer (MISR) and cloud fraction data from the spatially coincident morning and afternoon overpasses of the MODerate resolution Imaging Spectroradiometer (MODIS) instrument. We estimated the relative daily atmospheric burden of each aerosol type (dust, smoke, sea salt) from 2006 to 2010 using MISR, and then selected sixty-nine Level 2 scenes that exhibited either high fire (December) or high dust (February-March) aerosol optical thickness. We selected thirty-five aerosol-neutral scenes to control for local meteorology, and then quantified changes in MODIS cloud fraction from morning (10:30am local time) to afternoon (1:30pm local time) in the presence of different morning aerosol burdens (from MISR). High aerosol optical thickness was associated with reduced cloud fraction in both the raw and meteorologically-normalized data. Reduction in cloud fraction was also positively correlated with aerosol optical thickness. These results, combined with observed changes in meteorological variables, provide observational evidence of a localized semi-direct cloud decimation (convective inhibition) aerosol effect in tropical Africa, and suggest a positive feedback loop between anthropogenic burning and cloudiness.