Observations of the Nocturnal Boundary Layer Associated with the West African Monsoon

A set of night time tethered balloon and kite measurements from the central Sahel (15.2N,1.3W) in August 2005 were acquired and analyzed. A composite of all nights' data was produced using boundary layer height to normalize measured altitudes. The observations showed some typical characteristics of nocturnal boundary layer development, notably a strong inversion after sunset and the formation of a low-level nocturnal jet later in the night. On most nights, the sampled jet did not change direction significantly during the night. The boundary layer thermodynamic structure displayed some variations from one night to the next. This was investigated using two contrasting case studies from the period. In one of these case studies (18 August 2005), the low level wind direction changed significantly during the night. This change was captured well by two large scale models, suggesting that the large scale dynamics had a signicant impact on boundary layer winds on this night. For both case studies, the models tended to underestimate near-surface wind speeds during the night; a feature which may lead to an underestimation of moisture flux northwards by models.