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Earth System Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/esd-2020-33
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esd-2020-33
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 Jun 2020

Submitted as: research article | 15 Jun 2020

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This preprint is currently under review for the journal ESD.

Daytime low-level clouds in West Africa – occurrence, associated drivers and shortwave radiation attenuation

Derrick K. Danso1,2, Sandrine Anquetin1, Arona Diedhiou1,2, Kouakou Kouadio2, and Arsène T. Kobea2 Derrick K. Danso et al.
  • 1Université Grenoble Alpes, IRD, CNRS, Grenoble-INP, IGE, 38000 Grenoble, France
  • 2Laboratoire de Physique de l'Atmosphère et de Mécaniques des Fluides (LAPAMF), Université Félix Houphoüet, Boigny, Abidjan, Côte d'Ivoire

Abstract. This study focuses on low-level clouds (LLC) that occur during the daytime over West Africa (WA). These daytime LLCs play a major role in the earth's radiative balance, yet, their understanding is still relatively low in WA. We use the state-of-the-art ERA5 dataset to understand their occurrence and associated drivers as well as their impact on the incoming surface solar radiation in the two contrasting Guinean and Sahelian regions of WA. The diurnal cycle of the daytime occurrence of three LLC classes namely No LCC, LLC Class-1 (LLCs with lower fraction), and LLC Class-2 (LLCs with higher fraction) are first studied. The monthly evolutions of hourly and long-lasting LLC (for at least 6 consecutive hours) events are then analyzed as well as the synoptic and local dynamics associated with the long-lasting LLC events. The occurrence of No LLC events does not present any specific correlation with the time of the day, whatever the region while as soon as LLC coverage becomes more pronounced (LLC Class-2), a diurnal evolution is noted and appears to be strongly different from one region to the other. During the summer months in the Guinean region, the occurrence of LLC Class-1 is low while LLC Class-2 is frequent (occurrence frequency around 75 % in August). In the Sahel, LLC Class-1 is dominant in the summer months (occurrence frequency more than 80 % from June to October), however the peak occurrence frequency of Class-2 is also in the summer. In both regions, the occurrence of LLCs during the rainy season is associated with an influx of cold moist air driven by strong southwesterly winds from the Guinean Gulf. Furthermore, the occurrence of LLC Class-2 is linked to strong surface heating and evaporation of soil moisture. During the dry season on the hand, the occurrence of LLCs is linked more to turbulent upward motion of air caused by surface heating (only in the Sahel) and the convergence of air masses near the surface. The results also showed that the occurrence of LLC Class-2 causes high attenuation of the incoming solar radiation, especially during JAS where about 49 % and 44 % of the downwelling surface shortwave radiation is lost on average in Guinea and Sahel respectively.

Derrick K. Danso et al.

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Derrick K. Danso et al.

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Short summary
The atmospheric and surface conditions that exist during the occurrence of daytime low-level clouds (LLCs) and their influence on solar radiation were investigated in West Africa. These LLCs are linked to horizontal transport of cold moist air from the ocean, surface heating and evaporation during the rainy season. In the dry season they are mostly linked to converging air at the surface and subsequent upward movement. Their occurrence lead to a strong reduction of the incoming solar radiation.
The atmospheric and surface conditions that exist during the occurrence of daytime low-level...
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