Nocturnal stratiform cloudiness during the West African monsoon

An analysis of METEOSAT satellite images and synoptic reports from Parakou, Benin suggests that those nights that lack deep convection during the West African monsoon are often either completely clear or completely overcast at low levels. Using radiosonde observations gathered at Parakou during the IMPETUS field campaign of the summer of 2002 and ECMWF operational analyses, the composite structure of the atmosphere for both cloudy and clear nights is presented. Cloudy nights are found to occur when turbulent processes result in large-scale speed convergence in the boundary layer and a positive net moisture flux convergence. Conversely, the clear conditions occur when a nocturnal inversion decouples the boundary layer from the surface. The cloudy and clear conditions are also shown to be related to large-scale changes in the configurations of the African Easterly Jet and the Tropical Easterly Jet, although the nature of this relationship remains unclear.