Abstract. A ground-based field campaign was conducted in southern West Africa from mid-June to the end of July 2016 within the framework of the Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) project. It aimed to provide a high-quality comprehensive data set for process studies, in particular of interactions between low-level clouds (LLCs) and boundary-layer conditions. In this region missing observations are still a major issue. During the campaign, extensive remote sensing and in situ measurements were conducted at three supersites: Kumasi (Ghana), Savè (Benin) and Ile-Ife (Nigeria). Daily radiosoundings were performed at 06:00 UTC, and 15 intensive observation periods (IOPs) were performed during which additional radiosondes were launched, and remotely piloted aerial systems were operated. Extended stratiform LLCs form frequently in southern West Africa during the nighttime and persist long into the following day. They affect the radiation budget and hence the evolution of the atmospheric boundary layer and regional climate. The relevant parameters and processes governing the formation and dissolution of the LLCs are still not fully understood. This paper gives an overview of the diurnal cycles of the energy-balance components, near-surface temperature, humidity, wind speed and direction as well as of the conditions (LLCs, low-level jet) in the boundary layer at the supersites and relates them to synoptic-scale conditions (monsoon layer, harmattan layer, African easterly jet, tropospheric stratification) in the DACCIWA operational area. The characteristics of LLCs vary considerably from day to day, including a few almost cloud-free nights. During cloudy nights we found large differences in the LLCs' formation and dissolution times as well as in the cloud-base height. The differences exist at individual sites and also between the sites. The synoptic conditions are characterized by a monsoon layer with south-westerly winds, on average about 1.9 km deep, and easterly winds above; the depth and strength of the monsoon flow show great day-to-day variability. Within the monsoon layer, a nocturnal low-level jet forms in approximately the same layer as the LLC. Its strength and duration is highly variable from night to night. This unique data set will allow us to test some new hypotheses about the processes involved in the development of LLCs and their interaction with the boundary layer and can also be used for model evaluation.

The European Union (EU)-funded project Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) investigates the relationship between weather, climate, and air pollution in southern West Africa—an area with rapid population growth, urbanization, and an increase in anthropogenic aerosol emissions. The air over this region contains a unique mixture of natural and anthropogenic gases, liquid droplets, and particles, emitted in an environment in which multilayer clouds frequently form. These exert a large influence on the local weather and climate, mainly owing to their impact on radiation, the surface energy balance, and thus the diurnal cycle of the atmospheric boundary layer. In June and July 2016, DACCIWA organized a major international field campaign in Ivory Coast, Ghana, Togo, Benin, and Nigeria. Three supersites in Kumasi, Savè, and Ile-Ife conducted permanent measurements and 15 intensive observation periods. Three European aircraft together flew 50 research flights between 27 June and 16 July 2016, for a total of 155 h. DACCIWA scientists launched weather balloons several times a day across the region (772 in total), measured urban emissions, and evaluated health data. The main objective was to build robust statistics of atmospheric composition, dynamics, and low-level cloud properties in various chemical landscapes to investigate their mutual interactions. This article presents an overview of the DACCIWA field campaign activities as well as some first research highlights. The rich data obtained during the campaign will be made available to the scientific community and help to advance scientific understanding, modeling, and monitoring of the atmosphere over southern West Africa.

No abstract

RESUMENSe presentan datos de radiación neta en todas las longitudes de onda promediados cada hora, registrados en una estación meteorológica situada en el campus de la Universidad Obafemi Awolowo, Ile-Ife (7.52º N, 4.52º E), Nigeria, durante un periodo completo de tres años (2010)(2011)(2012), para estudiar sus variaciones diurnas y estacionales. Esta información, obtenida con un radiómetro neto de alta sensibilidad de cuatro componentes, representa hasta hoy la información más consistente y detallada disponible para una localidad tropical de África Occidental. De acuerdo con los datos obtenidos, la radiación neta máxima ocurrió a las 14:00 LT (GMT + 1), y sus valores se incrementaron de manera considerable de 337.6 ± 146.4 Wm -2 en julio, que es el pico de la estación lluviosa, a 441.7 ± 82.4 Wm -2 durante marzo, en el final de la estación seca. En los meses de abril y octubre, que marcan el principio y fin de la estación lluviosa en Ile-Ife, se han registrado los valores más altos de radiación neta: 584.7 y 612.2 Wm -2 , respectivamente. Se observó una importante variación inter e intraestacional en los valores medios mensuales de radiación neta, debida principalmente a fluctuaciones de nubosidad y humedad. En el área de estudio, los datos revelaron la presencia de calentamiento radiativo neto en la superficie, cuya tendencia anual sigue una distribución bimodal. Los datos de este estudio respaldan los aportados por otros autores. ABSTRACTHourly averaged net all-wave radiation data spanning a complete three-year period (2010-2012) at a meteorological station located inside the Obafemi Awolowo University campus in Ile-Ife (7.52º N, 4.52º E), Nigeria is presented in this study to investigate its diurnal and seasonal variations. Using a high-sensitivity four-component net radiometer, the data represents so far the most consistent and detailed information available for a tropical location in West Africa. From the dataset, hourly maxima of the net radiation occurred at 14:00 LT (GMT + 1), whose values increased considerably from 337.6 ± 146.4 Wm -2 in July, which is the peak of the wet season, to 441.7 ± 82.4 Wm -2 in March, the end of the dry season. April and October, both of which mark the beginning and end of the raining season at Ile-Ife have recorded the highest values of 584.7 and 612.2 Wm -2 , respectively. There was strong intra/inter-seasonal variation observed in the monthly mean values of the net radiation due mainly to the fluctuations in cloudiness and humidity. In the study area, the data indicated a net radiative heating taking place at the surface, whose annual trend follows a bimodal distribution. The present data supports the results published in earlier studies by other authors.

The diurnal and seasonal variations of the incoming solar radiation have been studied by analysing two years data measured between January, 2016 to December, 2017 at a Tropical station, Ile-Ife (7.53°N; 4.54°E), Nigeria. The maximum incoming solar radiation flux which occurs between 13:00 – 14:00 LT and varies in the course of the year from 639.5 ± 171.6 Wm -2 (with large fluctuations) in the wet months (March – October) to 700.7 ± 105.2 Wm -2 in the dry months (November – February). The large differences in the values, diurnal and seasonal variation of the measured incoming solar radiation between the dry and wet seasons are attributed to the attenuation of the flux by aerosol particles in the dry season and increased cloudiness and humidity in the wet season. The monthly maximum values of 760.3 Wm -2 and 732.8 Wm -2 indicated a double peak from March to May and October to November respectively while a minimum of about 492.7 Wm -2 was recorded from July to August. Similarly, the daytime average had a double peak of 412.5 Wm -2 and 361.3 Wm -2 in March/April/May and October/November respectively, equally a minimum value of about 249.8 Wm -2 was recorded in July/August. The maximum value of the air temperature (which occurs around 15:00 LT) was observed to lag behind the maximum value of the incoming solar radiation (which occurs around 13:00 LT) by 2 hours at the study site. The statistical analysis of the monthly daytime averages of the incoming solar radiation showed that the intensity of the flux received at Ile-Ife (a tropical location) is high (about 67% of the incoming solar radiation are between the interval 325 and 400 Wm -2 ) throughout the year.