Abstract
Abstract. Nocturnal low-level stratus clouds (LLCs) are frequently observed in the atmospheric boundary layer (ABL) over southern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy and radiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate. However, an adequate representation of LLCs in the state-of-the-art weather and climate models is still a challenge, which is largely due to the lack of high-quality observations in this region and gaps in understanding of underlying processes. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-aerosol-chemistry-cloud interactions in West Africa) ground-based field campaign was used for the first observational analyses of the parameters and physical processes relevant for the LLC formation over SWA. However, occasionally stratus-free nights occur during the monsoon season as well. Using observations and ERA5 reanalysis, we investigate differences in the boundary-layer conditions during 6 stratus-free and 20 stratus nights observed during the DACCIWA campaign. Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLCs during the monsoon season: (i) the onset time and strength of the nocturnal low-level jet (NLLJ), (ii) horizontal cold-air advection, and (iii) background moisture level. Namely, weaker or later onset of NLLJ leads to a reduced contribution from horizontal cold-air advection. This in turn results in weaker cooling, and thus saturation is not reached. Such deviation in the dynamics of the NLLJ is related to the arrival of a cold air mass propagating northwards from the coast, called Gulf of Guinea maritime inflow. Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from, for example, central or south Africa, reduce the background moisture over large parts of SWA. Backward-trajectory analysis suggests that another possible reason for clear nights is descending air, which originated from drier levels above the marine boundary layer.
Highlights
The nocturnal boundary layer (NBL) over southern West Africa (SWA) during the summer monsoon season is typically characterized by low-level stratus clouds (LLCs), which cover extensive areas stretching over a region of approximately 800 000 km2 at a maximum coverage
We have used observational data collected during the Dynamics-aerosol-chemistry-cloud-interactions in West Africa (DACCIWA) campaign as well as the newest ERA5 reanalysis data set
Conditions during 6 stratus-free nights are compared with 20 nights with LLCs using in situ and remote-sensing measurements at the Savè supersite and radiosonde measurements from coastal stations (Abidjan, Accra and Cotonou) and inland stations in Lamto and Parakou
Summary
The nocturnal boundary layer (NBL) over southern West Africa (SWA) during the summer monsoon season is typically characterized by low-level stratus clouds (LLCs), which cover extensive areas stretching over a region of approximately 800 000 km at a maximum coverage (van der Linden et al, 2015). Based on the above-mentioned studies, we have a better understanding of mechanisms and factors which control the formation and maintenance of LLCs. The results show that relevant processes include the horizontal advection of cool maritime air from the Gulf of Guinea embedded in the southwesterly monsoon layer, formation of a NLLJ and turbulent mixing related to the strong wind shear underneath the NLLJ (Schuster et al, 2013; Adler et al, 2017, 2019; Babicet al., 2019). Based on the highresolution regional simulations performed with the Weather Research and Forecasting model for the July–September 2006 period, Schuster et al (2013) analyzed differences between the 15 cloudiest and 15 clearest nights at 6.2◦ N Their model results indicate that cooling during cloudy nights is caused by cold-air advection from the south up to about 1000 m a.g.l.
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