Spatio-temporal variability of clouds and associated shortwave radiative effects in West Africa with a satellite-based and reanalysis data

This study aims to understand and document the occurrence and variability of cloud cover in West Africa (WA) essential for assessing feasibility of planned large-scale solar energy projects. Investigations are carried out with a 10-year hourly record of two cloud cover data products: Clouds and the Earth Radiant Energy System passive satellite observations (CERES SYN1deg) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5). The seasonal evolution of high-level, mid-level and low-level clouds (HCC, MCC and LCC) as well as the total cloud cover (TCC) over the region is examined. The results show that both products agree on the average seasonal and spatial distribution of cloud cover over the region, although the CERES SYN1deg product presents lower cloud fractions than ERA5. This is partly attributed to the inability of the satellite sensor to detect optically thin clouds in the atmosphere. Southern WA is found to be cloudier than other parts of the region in all seasons. During the monsoon season (June, July, August and September) cloudiness increases over southern WA (mean cloud fraction up to 70% in CERES SYN1deg and 80% in ERA5) but also extends northwards and strengthens over the Sahelian region. The intense cloud coverage over southern WA during the monsoon season leads to a large regional mean reflectance of incoming solar radiation of about 55%. In all seasons, the presence of LCC over large areas of the Sahel/Sahara region is highlighted in the CERES SYN1deg product. This could be due to a potential misinterpretation of Saharan dust as low clouds by the satellite sensor which may have overestimated their occurrences and fractions and may lead to errors in the estimation of cloud radiative effects over the region. Northern WA is associated with higher frequencies of no cloud occurrence events unlike the south where cloudless skies are rare. Furthermore, in southern WA, overcast conditions of LCC which significantly reduce incoming solar radiation are observed for a significant number of times (up to 10% of the time in CERES SYN1deg and 20% in ERA5).