Analysis of Processes Relevant for the Development of Nocturnal Low-Level Clouds over Southern West Africa

During the summer monsoon season, the nocturnal boundary layer over southern West Africa is often characterized by stratiform low-level clouds (LLC), which typically form few hundred meters above the ground. These clouds have a distinct diurnal cycle, often form around midnight persisting long into the following day and transitioning into convective clouds. Thus, they affect the surface radiation budget and consequently the regional climate. Up to now, the investigations of LLC characteristics were based on satellite data, synoptic observations and few modeling studies, while high-quality observational data sets were rare in this region. Due to these limitations, the understanding of processes which control the formation, maintenance and dissolution of LLC is still limited. Moreover, understanding of these processes has important practical implications, such as improving operational forecast and predictions of West African monsoon in weather, seasonal and decadal climate simulations.

With the aim to provide required high-quality observations in the region, a comprehensive field campaign within the framework of the Dynamics-Aerosol-Chemistry-Cloud-Interactions over West Africa (DACCIWA) project was conducted in June and July 2016 at three supersites in Ghana, Benin and Nigeria. The comprehensive data set consists of remote sensing and in-situ data, which enable the investigation of cloud characteristics, dynamic and thermodynamic conditions at high temporal and vertical resolutions.

In this study we analyze processes relevant for the development of LLC during one typical night in July 2016, using the data from the supersite at Savè (Benin). The LLC formed around 00 UTC at a height of about 300 m above ground and persisted until 07 UTC. Subsequently, the cloud base started to rise due to the growing convective boundary layer. A marked feature observed during this night is the arrival of the low-level jet (LLJ) several hours before the formation of LLC. Therefore, we assessed the role of the LLJ and the associated horizontal cold-air advection of a maritime air-mass on the formation of LLC. Further, based on radiosonde profiles obtained every 1.5 hours during the night, the effect of temperature and moisture change on the relative humidity tendency is examined. Finally, the heat-budget analysis reveals the importance of different processes, such as radiative cooling, cold-air advection, shear generated mixing, for the formation, maintenance and dissolution of LLC.