Climatology of low-level clouds during the main dry season over Western Equatorial Africa: Comparison between ground observations and satellites

During the long dry season (June-September) Western Central Africa (WCA) is a region with an extensive and persistent stratocumulus or stratus deck. Previous studies have shown that this extensive cloud cover is crucial for the existence of a biodiverse, light-deficient tropical rainforest ecosystem in Gabon. Yet, its climatological behaviour, the cloud genesis and lysis mechanisms, and thus its persistence under the ongoing climate change has not been intensively studied yet.

In the present study, we created various climatologies of Low Cloud Cover (LCC) in the region based on (a) 3-hourly data from in-situ eye observations by synoptic stations, (b) on 15-minute data from the SEVIRI instrument aboard the METEOSAT Second Generation geostationary satellite, and (c) twice daily data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat’s Cloud Profiling Radar (CPR). To validate the satellite products against eye observations of cloudiness at synoptic stations, the latter are spatially weighted in the observer’s field of view and compared against co-located pixel averages of the satellites. To obtain an as accurate depiction of the diurnal cycle of LCC as possible, we combined the SAFNWC cloud type product during the day with the Night Microphysical Scheme during the night. Both product use various SEVIRI spectral channels. Vertical profiles of cloudiness at 250 m resolution are derived from the Calipso-CloudSat 2B-GEOPROF-LIDAR product.

The mean climatology shows more clouds at the coast and the coastal plains, decreasing cloud occurrence frequency landward. The closer to the Congo basin, the less persistence is the LCC deck. The diurnal cycle of the LCC has a smaller amplitude on the coastal plains of Gabon, while at the leeward site of the Chaillu Mountains, an up to 1000 m high low mountain range in southern Gabon, a higher amplitude with substantial clearing in the afternoon and cloud formation in the night prevails. The persistence of the LCC on the windward side of the Chaillu Mountains might be related to upslope winds. Further, a change in the cloud genus from stratocumulus to cumulus is observed on the plateau during the afternoon in association with the clearing. It is speculated that this is associated with an increased boundary layer height. Another cause might be a Foehn-effect, dissipating the Low Cloud Cover behind the Chaillu Mountains and being responsible for the higher amplitude in the diurnal cycle above the plateau. On the windward side of the Chaillu Mountains, there is no transition observed from stratocumulus to cumulus clouds, presented with its persistence cloud deck.

In summary, the present study constitutes the hitherto most comprehensive sub-daily station- and satellite-based, dry-season climatology of LCC over western equatorial Africa.