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. A ground-based field campaign was conducted insouthern West Africa from mid-June to the end of July 2016 within theframework of the Dynamics–Aerosol–Chemistry–Cloud Interactions in WestAfrica (DACCIWA) project. It aimed to provide a high-quality comprehensivedata set for process studies, in particular of interactions betweenlow-level clouds (LLCs) and boundary-layer conditions. In this regionmissing observations are still a major issue. During the campaign, extensiveremote sensing and in situ measurements were conducted at three supersites:Kumasi (Ghana), Save (Benin) and Ile-Ife (Nigeria). Daily radiosoundingswere performed at 06:00 UTC, and 15 intensive observation periods (IOPs) wereperformed during which additional radiosondes were launched, andremotely piloted aerial systems were operated.Extended stratiform LLCs form frequently in southern West Africa during thenighttime and persist long into the following day. They affect theradiation budget and hence the evolution of the atmospheric boundary layerand regional climate. The relevant parameters and processes governing theformation and dissolution of the LLCs are still not fully understood. Thispaper gives an overview of the diurnal cycles of the energy-balancecomponents, near-surface temperature, humidity, wind speed and direction aswell as of the conditions (LLCs, low-level jet) in the boundary layer at thesupersites and relates them to synoptic-scale conditions (monsoon layer,harmattan layer, African easterly jet, tropospheric stratification) in theDACCIWA operational area. The characteristics of LLCs vary considerably fromday to day, including a few almost cloud-free nights. During cloudy nightswe found large differences in the LLCs' formation and dissolution times aswell as in the cloud-base height. The differences exist at individual sitesand also between the sites. The synoptic conditions are characterized by amonsoon layer with south-westerly winds, on average about 1.9 km deep, andeasterly winds above; the depth and strength of the monsoon flow show greatday-to-day variability. Within the monsoon layer, a nocturnal low-level jetforms in approximately the same layer as the LLC. Its strength and durationis highly variable from night to night. This unique data set will allow usto test some new hypotheses about the processes involved in the developmentof LLCs and their interaction with the boundary layer and can also be usedfor model evaluation.

atmospheric chemistry and physics

An overview of the diurnal cycle of the atmospheric boundary layer during the West African monsoon season: results from the 2016 observational campaign