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 SARAH-2.1 data set. Mean annual SDU levels are low: less than 5h day-1 at the regional scale, with the sunniest stations in the northeast (Cameroon, Central African Republic) and the least sunny in a ∼150km wide coastal strip in Gabon and Republic of 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 2.5h day-1, especially on the windward slopes of the Massifs du Chaillu, 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 to 2h 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 two-fold problem: the number of dark days (SDU<1h day-1) is 50% lower than observed while that of sunny days (SDU>9h day-1) is 50% higher than observed.

The representation of dry-season low-level clouds over Western Equatorial Africa in reanalyses and historical CMIP6 simulations

“…Dry seasons generally attract less attention, since they are considered as periods of relatively steady, stable and cloudless weather. In Western Equatorial Africa (WEA), especially in Gabon and southern Congo-Brazzaville, the austral winter season from June to September is rainless but is unexpectedly also the time of the year showing the least sunshine and the highest cloud amounts (Bush et al 2019;Philippon et al 2021). Over the Mayombe range in Congo-Brazzaville, mean dry season sunshine duration is under one hour per day (Bouka-Biona et al, 1993).…”

Diurnal to interannual variability of low‐level cloud cover over western equatorial Africa in May–October

“…Western equatorial Africa (WEA), comprising the countries of Equatorial Guinea, Gabon, the southwestern part of the Republic of Congo (RC) and southern Cameroon ($5 S-5 N, west of 15 E), exhibits an almost absolute dry season centred on June-September combined with a persistent low-level cloud cover. The low-level cloud cover fractions (LCF) usually exceed 70%-80% over the coastal area and first hills and plateaus (Champagne et al, 2023;Dommo et al, 2018), strongly limiting the incoming solar radiation at the surface (Philippon et al, 2019(Philippon et al, , 2022, the maximum diurnal temperature range and the evapotranspiration rate (Philippon et al, 2019) with major impacts on forest functioning and composition (Réjou-Méchain et al, 2021).…”