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