The Impact of Aerosols on the Stratiform Clouds over southern West Africa: A Large-Eddy Simulation Study

Abstract. Low level stratiform clouds (LLSCs) covering a large area appear frequently during the wet monsoon season in southern West Africa. This region is also a place where different types of aerosols coexist, including biomass burning aerosols coming from Central and South Africa and anthropogenic aerosols emitted from local activities. We investigate the semi-direct and indirect effects of these aerosols on the diurnal cycle of LLSCs by constructing a case study based on airborne and ground-based observations from the Dynamic-Aerosol-Chemistry-Cloud-Interaction in West Africa (DACCIWA) field campaign. This case is modelled using a Large Eddy Simulation (LES) model with fine scale resolution and in-situ aerosol measurements including size distribution and chemical composition. The model has successfully reproduced the observed life cycle of the LLSC, from stratus formation to stabilization during the night, to upward development after sunrise until breakup of cloud deck in late afternoon. Various sensitivity simulations using different measured aerosol profiles also suggest that aerosols can affect the cloud life cycle through both the indirect and semi-direct effect. Despite precipitation produced by the modeled cloud is nearly negligible, cloud lifetime is still sensitive to the aerosol concentration. As expected, modeled cloud microphysical features including cloud droplet number concentration, mean radius, and thus cloud reflectivity are all controlled by aerosol concentration. However, it is found that the difference in cloud reflectivity is not always the only factor in determining the variation of the incoming solar radiation at ground and cloud life cycle specifically beyond sunrise. Instead, the difference in cloud-void space brought by dry air entrainment from above and thus the speed of consequent evaporation – also influenced by aerosol concentration, is another important factor to consider. Results have shown that clouds in the case with lower aerosol concentration and larger droplet size appear to be less affected by entrainment and convection. In addition, we have found that an excessive atmospheric heating up to 12 K day⁻¹ produced by absorbing black carbon aerosols (BC) in our modeled cases can also affect the life cycle of modeled clouds. Such a heating is found to lower the height of cloud top and stabilize the cloud layer, resulting a less extent in vertical development and accelerating cloud breakup. The semi-direct effect impacts on indirect effect by reducing cloud reflectivity particularly in case of polluted environment. Finally, semi-direct effect is found to contribute positively to the indirect radiative forcing due to a decreased cloud-void space, and negatively by causing thinner clouds that would break-up faster in late afternoon, all depending on the phase in stratiform cloud diurnal cycle.