What Are the Differences in Atmospheric Conditions between Nights with and without Low-Level Stratus Clouds over Southern West Africa?

During the summertime monsoon season southern parts of West Africa are frequently affected by an extensive cover of stratus clouds, whose cloud-base height is only few hundred meters above the ground. Although these clouds form frequently and cover large areas, recent studies suggest that they are unsatisfactorily represented by standard satellite retrievals as well state-of-the-art climate models. Based on satellite data, synoptic observations and few modeling studies recent results indicate that these low-level clouds (LLC) are associated with a nocturnal low-level wind maximum and that they can persist long into the following day. Up to now, high-quality observational data sets were rare in this region, thus many uncertainties about the relevant factors and processes which control their formation, maintenance and dissolution still remain.

The aim of a recent comprehensive ground-based field campaign, which was conducted within the framework of the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions over West Africa) project in June and July 2016 at three supersites in Ghana, Benin and Nigeria, was to obtain comprehensive and high-quality data needed for the process studies of the interaction between LLC and boundary-layer conditions. This 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. During the 7 weeks of the campaign, at the supersite in Savè (Benin) the LLC formed quite frequently, but their characteristics, such as formation and dissolution times as well as cloud-base height varied considerably from day to day. Additionally, several stratus-free nights occurred during the campaign. In this study we analyze conditions during cloud-free nights and compare them to nights when stratus decks formed. This comparison allows us to quantify the importance of certain processes, such as horizontal cold-air advection associated with a low-level jet, shear generated vertical mixing as well as radiative cooling on the formation of LLC.