Microphysical properties of low level clouds in West Africa and the tropical Atlantic and their sensitivity to aerosol and updraft distributions
Résumé
Low-level clouds have an important cooling effect on climate and cover wide areas of West Africa and the tropical Atlantic during the summer monsoon period. Over this region, aerosol and cloud microphysical properties are poorly constrained by observations, limiting the quality of climate predictions and weather forecasts. Here, we present comprehensive in situ measurements of microphysical properties of low level clouds over southern West Africa, measured from three aircraft during the DACCIWA (Dynamics-aerosol-chemistry-cloud interactions in West Africa) campaign in June and July 2016 with a focus on cloud observations from the DLR Falcon. An overview of the DACCIWA campaign is presented. A large variability of cloud properties is detected in the low level clouds with cloud droplet number concentrations of particles larger than 3µm between 100 and 800 cloud droplets per cm³. Simulations suggest that the updraft speed has a large impact on cloud droplet number concentrations. At similar updraft speeds, we measured enhanced cloud droplet number concentrations and smaller particle sizes in continental low level clouds that were affected by urban outflow and by biomass burning plumes transported over longer distances from Southern Africa. In contrast, oceanic low level clouds contained significantly lower aerosol and cloud droplet number concentrations (less than 200 per cm³) and larger droplets compared to continental clouds. The sensitivity of the continental clouds to anthropogenic aerosol emissions might be attenuated by high background aerosol concentrations over the continent, while oceanic clouds may show stronger responses.