Article Dans Une Revue Journal of Applied Meteorology and Climatology Année : 2024

Diurnal Changes in Cloud Cover in Eastern Gabon and Their Impacts on Energy Balance, Light Availability, and Water Demand: A Case Study of the 2022 Dry Season

Résumé

Western central Africa is atypical of the equatorial domain as the main dry season is cloudier than the rainy seasons. To understand this cloud cover's diurnal evolution, we set up an infrared camera and acquired measurements of the total cloud cover fraction (TCF) and cloud optical depth at Bambidie, Gabon (0844 30.5 S, 12858 12.4 E), from May to October 2022. Diurnal variations in TCF can be summarized into four types, mostly discretized through the timing and duration of clouds clearing in the afternoon [early afternoon clearing (EaC), late afternoon clearing (LaC), and clear night (CNi)], while one type [no clearing (NoC)] shows overcast conditions all day long. Meteorological measurements show that NoC days record 50 W m 22 less shortwave incoming surface radiation, resulting in daytime temperatures 18C lower than the seasonal norm, but 20% more diffuse light and 0.5 mm day 21 less ETo. Conversely, EaC days record 50 W m 22 more shortwave incoming surface radiation, leading to temperatures 1.58C higher than the seasonal norm, but 40% more direct light. The larger water demand (0.5 mm day 21 more ETo) is partly compensated by more frequent rainfall at nighttime. The satellite estimates of Satellite Application Facilities for supporting Nowcasting and very short-range Forecasting (SAFNWC) capture the TCF variations for most of the four types well. They confirm that TCF is dominated by very low and low clouds whose dissipation in the afternoon and evolution into fractional and cumuliform convective clouds explains the clearings on EaC and LaC days. Satellite estimates also show that the four types of days extracted at Bambidie are representative of a larger-scale cloud cover evolution in western central Africa, with a west-east gradient in the timing of afternoon cloud dissipation.

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hal-04820196 , version 1 (14-01-2025)

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Martin Magnan, Nathalie Philippon, Vincent Moron, Armand Mariscal, Olivier Liandrat. Diurnal Changes in Cloud Cover in Eastern Gabon and Their Impacts on Energy Balance, Light Availability, and Water Demand: A Case Study of the 2022 Dry Season. Journal of Applied Meteorology and Climatology, 2024, ⟨10.1175/JAMC-D-23-0225.s1⟩. ⟨hal-04820196⟩
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