The Atlantic inflow to the Saharan heat low: observations and modelling

The inflow of relatively cold and stably stratified air from the Atlantic Ocean into western Mauritania and into the southwestern part of the Saharan heat low is studied using the mesoscale COSMO model. This model was used to provide operational forecasts for the GERBILS field campaign, which was conducted by the Met Office in West Africa in June 2007. The forecasts were validated against airborne measurements as well as satellite imagery and were found to represent the main synoptic features of the region accurately. A complex mesoscale feature in western Mauritania, which we call the Atlantic Inflow, was identified in the COSMO model output. The main component of the Atlantic Inflow is the sea breeze and associated front. The sea breeze interacts with larger‐scale, higher‐altitude fluctuations in the thermal and humidity advection. During the day the balance between horizontal advection of cool maritime air and turbulence in the convective boundary layer over land results in a stationary sea breeze front at the coast. Once turbulence dies down in the evening, the sea breeze front penetrates inland. Above the sea breeze layer, thermal advection in the Saharan Atmospheric Boundary Layer (SABL) also controls the structure of the Atlantic Inflow. A marked baroclinic zone was observed, in which the temperature and humidity made a relatively smooth transition from values typical of the Atlantic air to values characteristic of the SABL. Budget calculations showed that, through its cooling and occasional moistening at low levels, the Atlantic Inflow has an important impact on the regional heat and moisture budgets.