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.

CORDEX Multi‐RCM Hindcast Over Central Africa: Evaluation Within Observational Uncertainty

“…These features, which are generally induced by topography, are also associated to pressure gradient between CA land and the Atlantic Ocean surface (Vondou & Haensler, ; Vondou et al, ). Thus, the wet biases found here highlight the challenge for ERA‐Interim reanalysis to adequately simulate low‐level clouds and precipitation in CA as recently shown by Dommo et al (). The excessive precipitation intensity, particularly found in UC‐WRF and UCT‐PRECIS are reduced in other RCMs.…”

Assessing the reliability and uncertainties of projected changes in precipitation and temperature in Coupled Model Intercomparison Project phase 5 models over the Lake Chad basin

“…Generally, the models were biased in simulating historical and future projected average temperature in the LCB. The poor performance by GCMs in simulating average surface temperature in this study may be attributed to increased insolation over region considering that the region is known to be one of the cloudiest in the tropics; it could also be attributed to large biases in GCMs in simulating cloud climatology (Lauer and Hamilton, ; Diallo et al ., ; Dommo et al ., ).…”

Identification of processes that control the stable isotope composition of rainwater in the humid tropical West-Central Africa

“…To document the cloud types and convection organization in Western Central Africa, we use a cloud type (CT) product issued from geostationary MSG (Meteosat Second generation) satellite data and developed by SAFNWC (Satellite application facilities in support to nowcasting) /MSG algorithms (for more information see http://www.nwcsaf.org/web/guest/scientific-documentation). Clouds types are determined from their top temperature or pressure and, for high level clouds from their opacity (Dommo et al 2018;Seze 2015). The SAFNWC CT offers a classification of clouds into 12 classes: free land, free sea, very low clouds, low clouds, medium clouds, high clouds, very high clouds, thin cirrus, medium cirrus, thick cirrus, fractional clouds and semi-transparent above low or medium clouds, at 3-km spatial resolution for regions close to MSG sub-satellite point (0N, 0E) and a time step of 15-min.…”

“…Dommo, A., Philipon, N., Vondou, D.A., Seze, G., Eastman, R., 2018. The June-September Low Cloud Cover in Western Central Africa: Mean Spatial Distribution and Diurnal Evolution, and Associated Atmospheric Dynamics.…”