The structure of the atmospheric boundary layer (ABL) depends mainly on the energy exchange at the Earth’s surface. However, due to the high natural and anthropogenic emissions in Southwest Africa, gaseous and aerosol air pollutants also affect the diurnal cycle of the ABL, as do sea breeze and monsoon flows from the Gulf of Guinea.
Characteristic features, for example nocturnal low-level jet (LLJs), deep daytime ABLs, and various types of boundary-layer clouds often occur and during the course of the day a transition from nocturnal low-level stratus to stratocumulus, congestus and cumulonimbus clouds is observed. The atmospheric processes driving this transition is sensitive to the conditions mentioned above and although the nocturnal low-level stratus and the transition to broken clouds appear quite frequently, little attention has been paid to the phenomenon so far.
In this work package, the intention is to identify the meteorological controls on the whole process chain from the formation of nocturnal stratus clouds, via the daytime transition to convective clouds and the formation of deep precipitating clouds. This will be achieved by performing detailed intensive in-situ and remote sensing observations in addition to highly resolved model simulations (large eddy simulations, LES). The intensive observational period will include measurements of the energy-balance components at the Earth’s surface, the mean and turbulent conditions in the nocturnal and daytime ABL as well as measurements of the de- and entrainment processes between the ABL and the free troposphere. The meteorological measurements will be supplemented by air chemical observations.
In June and July 2016, comprehensive measurements were successfully performed at the three DACCIWA-supersites at Savè (Benin), Kumasi (Ghana), and Ile-Ife (Nigeria). Besides near-surface observations, the boundary layer was scanned by various active and passive remote sensing systems as well as surveyed by radiosondes, tethered sondes and remotely piloted aircraft systems. The extensive data set allows to study the temporal and spatial characteristics of the low-level stratus, the breakup in the morning and the convective boundary layer as well as the relevant processes. The four institutes involved in this ground campaign are now in the stage of data quality control and process in order to make the dataset available to everyone in the DACCIWA database by the end of January 2017.
Radiosonde, X-band radar and Doppler lidar at Savè supersite.
© Sébastien Chastanet/LA/OMP/UPS