Abstract
Abstract. This study focuses on daytime low-level clouds (LLCs) that occur within the first 2 km of the atmosphere 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) is 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-scale moisture flux associated with the long-lasting LLC events. Finally, the impact of LLC on the surface heat fluxes and the incoming solar irradiance is investigated. During the summer months in the Guinean region, LLC Class-1 occurrence 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, events with No LLC do not present any specific correlation with the time of the day. However, a diurnal evolution that appears to be strongly different from one region to the other is noted for the occurrence of LLC Class-2. LLC occurrence in both regions is associated with high moisture flux driven by strong southwesterly winds from the Gulf of Guinea and significant background moisture levels. LLC Class-2 in particular leads to a significant reduction in the upward transfer of energy and a net downward energy transfer caused by the release of large amounts of energy in the atmosphere during the cloud formation. In July, August, and September (JAS), most of the LLC Class-2 events may likely be the low-level stratiform clouds that occur frequently over the Guinean region, while they may be deep convective clouds in the Sahel. Additionally, 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 the Sahel, respectively.
Highlights
In West Africa (WA), the prediction of key features of the climate such as the West African Monsoon (WAM) is known to have large uncertainties (Christensen et al, 2013)
Though the focus of this study is on level clouds (LLCs) occurrence and the definition of the different classes is based on the cloud fraction (CF) in the lower atmosphere only, the convoluted cloud climatology in WA with frequent multilayer clouds (Stein et al, 2011) may mean that other clouds at higher altitudes may exist in addition to LLCs
We made use of the state-of-the-art hourly reanalysis dataset from European Centre for Medium-range Weather Forecasts (ECMWF)-ERA5 from 2006 to 2015 to analyze the occurrence of daytime LLCs in WA
Summary
In West Africa (WA), the prediction of key features of the climate such as the West African Monsoon (WAM) is known to have large uncertainties (Christensen et al, 2013). Other studies on LLCs in the region have been performed with satellite observations and model data (van der Linden et al, 2015; Adler et al, 2017; Hannak et al, 2017) These studies suggest that LLC formation is linked to a number of processes including but not limited to cooling caused by horizontal cold-air advection and the occurrence and strengthening of the nocturnal low-level jet (Schrage and Fink, 2012; Adler et al, 2017; Babicet al., 2019b).
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