Abstract. Realistically simulating the West African monsoon system still poses a
substantial challenge to state-of-the-art weather and climate models. One
particular issue is the representation of the extensive and persistent
low-level clouds over southern West Africa (SWA) during boreal summer. These
clouds are important in regulating the amount of solar radiation reaching the
surface, but their role in the local energy balance and the overall monsoon
system has never been assessed. Based on sensitivity experiments using the
ICON model for July 2006, we show for the first time that rainfall over SWA
depends logarithmically on the optical thickness of low clouds, as these
control the diurnal evolution of the planetary boundary layer, vertical
stability and finally convection. In our experiments, the increased
precipitation over SWA has a small direct effect on the downstream Sahel, as
higher temperatures due to increased surface radiation are accompanied by
decreases in low-level moisture due to changes in advection, leading to
almost unchanged equivalent potential temperatures in the Sahel. A systematic
comparison of simulations with and without convective parameterization
reveals agreement in the direction of the precipitation signal but larger
sensitivity for explicit convection. For parameterized convection the main
rainband is too far south and the diurnal cycle shows signs of unrealistic
vertical mixing, leading to a positive feedback on low clouds. The results
demonstrate that relatively minor errors, variations or trends in low-level
cloudiness over SWA can have substantial impacts on precipitation. Similarly,
they suggest that the dimming likely associated with an increase in
anthropogenic emissions in the future would lead to a decrease in summer
rainfall in the densely populated Guinea coastal area. Future work should
investigate longer-term effects of the misrepresentation of low clouds in
climate models, e.g. moderated through effects on rainfall, soil moisture and
evaporation.
Seasonal Evolution and Variability Associated with the West African Monsoon System