Ultra‐low clouds over the southern West African monsoon region: causes and challenges in representation in models and satellite data

Cloud observations from weather stations in combination with new spaced-based observations show that, during the West African summer monsoon, an extended cover of shallow, non-precipitating, extremely low clouds often form at night in the southern, moister portions of the region. New ground-based observations show that these clouds appear to develop in association with turbulence produced by a nocturnal low-level jet. This turbulence reduces the stability of the nighttime boundary layer, lowering the Richardson number of the layer and converting water vapor that has accumulated in the near-surface layer (due to evaporation or moisture flux convergence) into stratus and stratocumulus clouds. Several large-scale models do not correctly simulate these clouds at the present time, nor are they well-sampled by new satellite sensors (e.g., METEOSAT SEVIRI, CALIPSO, or Cloudsat) or well-represented in satellite archives such as ISCCP due to their very warm temperatures. More efforts are needed in the future to improve monitoring, modeling, and physical understanding of these ultra-low clouds and their role for the West African monsoon system.