Climatology of continental stratus over the southern West African monsoon region

The southern parts of West Africa, from the coast to about 10°N, are frequently covered by an extensive deck of shallow, low (200 – 400 m above ground) stratus or stratocumulus clouds during the summer monsoon season. These clouds usually form at night in association with a nocturnal low-level jet (NLLJ) and can persist into the early afternoon hours until they are dissipated or replaced by fair-weather cumuli. Recent work suggests that the stratus decks are unsatisfactorily represented in standard satellite cloud retrievals and poorly simulated by climate models. Here we will present the first multi-year (2006–2011) July–September (JAS) climatology of the diurnal cycle of the low cloud deck based on surface observations and satellite products. Two products based on the geostationary Meteosat Second Generation (MSG) Spinning Enhanced Visible and InfraRed Imager (SEVIRI) passive instrument will be used; one false color RGB composite from three infrared (IR) channels and the cloud classification from the Satellite Application Facility on Support to Nowcasting and Very Short Range Forecasting (SAF-NWC). Both are objectively validated against surface SYNOP observations and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) measurements. In addition, observations by the active instruments CALIOP and Cloud Profiling Radar (CPR) as well as the passive instruments Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging SpectroRadiometer (MISR) aboard the polar-orbiting CALIPSO, CloudSat, Aqua, and Terra satellites will be utilized. The two products from MSG have the advantage of high spatio-temporal sampling. The CALIOP and CPR footprints are narrow, but the emitted signals can, to a certain extent, penetrate clouds. The main conclusions from this work are: (a) The observed stratus deck forms after sunset along the coast, spreads inland in the course of the night, reaches maximum poleward extent at about 10°N around 09–10 local time and dissipates in the early afternoon; (b) the low stratus deck is most extensive over the lowlands of the Ivory Coast and on the windward sides of low mountain ranges like the Mampong range in Ghana and the Oshogbo hills in Nigeria; (c) the MSG RGB product is superior to SAF-NWC at night; and (d) mid- and upper-level cloud shields hamper low-level cloud detection, particularly the frequently occurring extensive upper-tropospheric cirrus decks blown out from deep convection for example in southern Nigeria. Our climatology documents for the first time the extensiveness and the diurnal cycle of the stratus clouds during the West African wet season, but also points to our present inadequacy to fully document this phenomenon.