The Influence of Aerosols on the Characteristics of Low-level Clouds in Southern West Africa

Low-level stratocumulus clouds are a common occurrence over southern West Africa during the summer West African Monsoon (WAM) period. These layer clouds typically form following the onset of the nocturnal southwesterly low level jet, and can persist until late morning or early afternoon. The presence of such cloud layers over land has implications for the surface radiation budget and the wider WAM circulation, although General Circulations Models struggle to represent these clouds adequately. A better understanding of the factors that determine their evolution and dissipation is urgently needed. It is hypothesised that aerosols resulting from anthropogenic activities in southern West Africa (specifically fossil fuel burning and biomass burning) are modifying the cloud properties and hence the climate of the region. To test this hypothesis, large eddy simulations are performed using the Met Office NERC Cloud model (MONC) with the Cloud-AeroSol-Interactions Microphysics scheme (CASIM). Specific case studies are selected from the Dynamics-Aerosols-Chemistry-Cloud-Interactions in West Africa (DACCIWA) project, providing a wealth of ground-based and in-situ measurements for model evaluation. A suite of sensitivity experiments are designed to quantify the impact of aerosols in terms of cloud fraction, liquid water content, cloud base height and precipitation. Preliminary results show that the cloud evolution is strongly influenced by the number of available cloud condensation nuclei (CCN), with increasing CCN concentrations producing more numerous smaller droplets, suppressing collision and coalescence and delaying the break-up of the stratocumulus layer. The detail of the processes involved which include depletion of liquid water path, boundary layer decoupling and the role of washout of CCN are being investigated.