engleski

Nocturnal low-level stratus clouds over southern West Africa: Insights from the DACCIWA field campaign

During the summer monsoon season in southern West Africa, low-level stratus clouds (LLC) form frequently in the boundary layer during the night. These clouds have a distinct diurnal cycle, as they often persist long into the following day and consequently transition into convective clouds. These LLC cover a large area and affect the surface radiation budget and thus the regional climate. Up to now, investigations of LLC in this region have been performed based on satellite images, synoptic observations and few modeling studies, however, little is known about the processes controlling their evolution, maintenance and dissolution. In order to fulfill the existing gap, a comprehensive field campaign within the Dynamics-aerosol-chemistry-cloud-interactions over West Africa (DACCIWA) project was conducted during 2016 monsoon season at three supersites in Ghana, Benin and Nigeria. The comprehensive and unique data set, which consists of remote sensing and in situ data, enables, for the first time, the investigation of cloud characteristics, dynamic and thermodynamic conditions at high temporal and vertical resolutions. The aim is to study LLC characteristics, the intra-night variability of boundary layer conditions and physical processes relevant for LLC, as well as to assess their importance using measurements from Savè (Benin) supersite. We find that during the DACCIWA campaign typically five different phases of the LLC diurnal cycle can be identified: the Stable, Jet, Stratus fractus, Stratus and Convective phase and mean profiles calculated for the individual phases reveal pronounced differences. The analysis shows that relevant processes include the horizontal advection of cool maritime air from the coast in the monsoon layer, formation of a nocturnal low-level jet (NLLJ) and shear-related turbulent mixing. Cooling is a dominant process for LLC formation, which leads to a continuous increase of relative humidity, until finally saturation is reached and LLC form with a cloud-base height near the height of NLLJ maximum. About 50 % of the cooling prior to the LLC formation is caused by horizontal cold air advection, roughly 20 % by radiative flux divergence and about 22 % by sensible heat flux divergence in the presence of a NLLJ (Jet phase). After the LLC form (Stratus phase), turbulent mixing supports the cooling below the cloud base, while strong radiative cooling at the cloud top helps to maintain thick stratus. The outcomes of this study contribute to the development of a conceptual model on LLC formation, maintenance and dissolution over southern West Africa.

Low-level clouds, monsoon season, low-level jet, maritime inflow, West Africa

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