Ultra-low clouds over the southern West African monsoon region

New ground-and space-based observations show that summertime southern West Africa is frequently affected by an extended cover of shallow, non-precipitating clouds only few hundred meters above the ground. These clouds are associated with nocturnal low-level wind speed maxima and frequently persist into the day, considerably reducing surface solar radiation. While the involved phenomena are well represented in re-analysis data, climate models show large errors in low-level wind, cloudiness, and solar radiation of up to 90 W m -2. Errors of such a magnitude could strongly affect the regional energy and moisture budgets, which might help to explain the notorious difficulties of many models to simulate the West African climate. More effort is needed in the future to improve the monitoring, modeling, and physical understanding of these ultra-low clouds and their importance for the West African monsoon system.