Characterization of Sunshine Duration in Western Equatorial Africa: In Situ Measurements versus SARAH-2 Satellite Estimates

Abstract

Western Equatorial Africa is one of the least sunny areas in the world. Yet, this has attracted little research so far. As in many other parts of Africa, light availability is mainly estimated using in situ measurements of sunshine duration (SDU). Therefore, this study conducts the first characterization of SDU evolution during the annual cycle for the region. It also evaluates the skill of satellite-based estimates of SDU from the Surface Solar Radiation Data Set–Heliosat, edition 2.1 (SARAH-2.1). Mean annual SDU levels are low: less than 5 h day−1 at the regional scale, with the sunniest stations in the northeast (Cameroon and Central African Republic) and the least sunny in an ∼150-km-wide coastal strip in Gabon and Republic of the Congo (RoC). For most of the stations except the southeast ones in the Democratic Republic of Congo, the lowest SDU levels are recorded in July–September, during the main dry season, with persistent overcast conditions. They are as low as 2.5 h day−1, especially on the windward slopes of the Massifs du Chaillu and du Mayombé, and of the Batéké Plateaus in Gabon and RoC. Although the mean annual and monthly spatial patterns are well reproduced in SARAH-2.1, SDU levels are systematically overestimated by 1–2 h day−1. The largest positive biases are recorded during the December–February dry season, especially at the northernmost stations. Analyses at the daily time scale show that SARAH-2.1 biases arise from a twofold problem: the number of dark days (SDU < 1 h day−1) is 50% lower than observed whereas that of sunny days (SDU > 9 h day−1) is 50% higher than observed.