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.
