Controls on Boundary-Layer Thermodynamics and Dynamics in Coastal West Africa During the Rainy Season of 2006

Abstract

We investigate dominant processes modulating the coastal West African atmospheric boundary layer during August and September 2006. We evaluated boundary-layer attributes using upper air soundings, tower-based observations, and information from the European Centre for Medium-Range Weather Forecasts reanalyses. Boundary-layer thermodynamics exhibited continental and maritime attributes in response to influences from regional onshore (sea to land) flows and local land–atmosphere exchanges of energy and moisture. Onshore flows transported maritime air inland and gave rise to deep (>1 km) nighttime mixed layers whose heat and moisture content resulted in maximum virtual potential temperatures of 306 K and specific humidities up to 20 g kg−1. The presence of the Saharan Air Layer corresponded with capping inversions greater than 4 K and lapse rates exceeding 7 K km−1 above the mixed layer. Mixed layers at these times became deeper than expected (≈1 km) because dust layer events were often concurrent with strong onshore flows. Despite diurnally variable land–atmosphere fluxes of sensible and latent heat that reached maximum values of 200 and 400 W m−2, respectively, the mixed-layer depth exhibited little diurnal variation due to the influences of onshore flows. Daytime heating of the land, the upward transport of moisture, and onshore flows produced boundary layers with high convective available potential energy that often exceeded 3,000 J kg−1. These results demonstrate that the atmospheric boundary-layer thermodynamics in western Senegal can be favorable for storm development during both day and night. Mesoscale and regional models applied in this region should include several processes controlling the boundary-layer attributes to realistically estimate the energy available for storm development.