Western central Africa (WCA) was recently shown to be one of the cloudiest areas of the tropics. Analyzing an ensemble of satellite products and surface cloud observations, we show that in June–September, WCA cloud cover is dominated by single-layered low stratiform clouds. Despite an underestimation of low cloud frequency in satellite estimates at night, comparisons with surface observations bring insights into the spatial distribution and diurnal cycle of low clouds. Both appear strongly influenced by orography: to the west, the coastal plains and the ocean-facing valleys have the largest cloud cover and a lower-amplitude diurnal cycle with a maximum cloud phase at 0400 local time (LT). To the east, across the windward slopes, plateaus, and downwind slopes, the cloud cover becomes progressively reduced and the diurnal cycle has a larger amplitude with a maximum cloud phase at 1000 LT. In terms of atmospheric dynamics, the east/west gradient observed in low cloud frequency and amount is related to a foehn effect without substantial rainfall on windward slopes. The diurnal cycle of low clouds on the windward slopes and plateaus is related to the reversal, from mean subsidence at 0700 LT over the Atlantic and inland to rising motion inland at 1300 LT. In addition, the airmass stability in low levels prevents the vertical development of cloud cover. Last, we could not detect in the European reanalyses any nocturnal jet as observed in southern West Africa (SWA), suggesting different mechanisms triggering low cloud formation in WCA compare to SWA.

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This paper analyzes the diurnal cycle of low cloud cover (LCC) and the atmospheric conditions under which it grows over Western Central Africa during the cloudiest season (June-September). Moderate Resolution Imaging Spectroradiometer (MODIS) observations, Extended Edited Clouds Reports Archive (EECRA) and the fifth generation of reanalysis of the European Centre for Medium Range Weather Forecasts (ECMWF), i.e., ERA5 are used. LCC peaks between 04LT and 07LT and tends to be less dense during the afternoon. The associated dynamic and thermodynamic ERA5 conditions reveal different processes. The strong low level (below 1000 m) southwesterly flow in the evening supplies the region with humidity from the ocean and leads to cloud formation. Relative humidity (RH) tendencies show that temperature contributes to 100% of RH changes : the strong cooling observed after sunset at 19LT increases RH in the area of about 8%/h in the lower layer (below 1000m). The nighttime cooling shows strong cooling rates of about -1.4K/h after sunset till 22 LT, then rates decrease during the night to reach a value of about -0.3K/h between 22LT and 07LT. The cloud formation is mostly related to horizontal air advection, strong convergence in the lower layer and turbulent upwards mixing of moisture, while cooling at the cloud-top helps to maintain the cloud deck once it has formed. During daytime, solar radiation suppressed cooling at the cloud-top, thereafter strong turbulent kinetic energy acts to partly destroy the cloud deck and cloud fraction.

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