The June–September Low Cloud Cover in Western Central Africa: Mean Spatial Distribution and Diurnal Evolution, and Associated Atmospheric Dynamics

Abstract

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.