Mid-level clouds over the Sahara in a convection-permitting regional model

The simulation of Saharan mid tropospheric clouds is investigated with the weather research and forecasting (WRF) regional atmospheric model at convection permitting (4 km) horizontal grid-spacing. We identify two potential problems in such simulations: one that affects cloud cover, and another that affects the mean and geographic patterns of both cloud and precipitation. Our simulations show that using a vertical grid typical of GCMs (38 levels) inhibits the formation of Saharan mid-level clouds. In particular, it underestimates the supercooled water content that often resides at the top of these clouds, in favour of ice which falls out of the cloud quickly. When the vertical resolution becomes high enough to allow layers of supercooled water and ice to exist separately, the simulation of the Saharan mid-level clouds improves significantly. Additional improvement is achieved by using realistic high resolution surface albedo, which also shows that low albedo areas favour the formation of mid-level clouds much more than high albedo ones. The simulation of precipitation on the northern edge of the Sahel is also improved with the use of realistic surface albedo. Overall, despite the disagreement of the simulated and the observed clouds, our results show that using increased resolution and realistic surface albedo seems to fully reproduce their observed radiative effect.