Nocturnal Stratiform Cloudiness and the Structure of the Atmosphere during the West African Summer Monsoon

Nonprecipitating nights in sub-Saharan West Africa are often characterized by completely overcast conditions at very low levels. Using radiosonde observations collected during the IMPETUS field experiment in 2002 and ECMWF operational model analyses, the authors have shown that the cloud cover is related to strong moisture flux convergence in the monsoon layer along about 10°N. The moisture flux convergence appears to be driven by frictional processes that are enhanced by the strong coupling of the surface to the atmospheric boundary layer on cloudy nights. In this study, the nocturnal stratiform cloud decks are examined in the context of the broader atmospheric environment of West Africa. Cloudy nights are shown to have a weaker African Easterly Jet and stronger southerlies throughout the lower troposphere. The low-level dryness on the clear nights is found to extend throughout the depth of the troposphere, despite the limited vertical scale of the stratiform clouds themselves. Changes in the strength and configuration of the heat low are also examined.