Dynamics-aerosol-chemistry-cloud interactions in West Africa
Rural measurement
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The ground campaign

During June and July 2016, the DACCIWA ground campaign took place at three supersites in southern West Africa: Savè (Benin), Kumasi (Ghana) and Ile-Ife (Nigeria). The entire measurement team included 51 people working in the field at all three supersites.

 
Measurement sites location map
 
Savè supersite Benin © Sébastien Chastanet
 
Kumasi supersite Ghana
 
Ile-Ife supersite Nigeria

The main objective of the ground campaign is to study nocturnal low-level stratus, i.e. by identifying the processes, which determine their formation and dissolution.
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During 7 weeks of continuous measurements and 15 Intensive Observation Periods (IOPs), a comprehensive and unique data set was collected allowing the investigation of the state of the atmosphere from the ground through the boundary layer (BL) to the free troposphere. On IOP days, normal radiosondes (every 6 hours up to 20 km) and frequent radiosondes (every 1 to 3 hours up to 1.5 km) were launched and two Remotely Piloted Aerial Systems (RPAS) were additionally operated. The data set includes near-surface observations (surface energy balance station and chemistry compound measurements) for studying surface-atmosphere interactions. The boundary layer data include observations from various active and passive remote sensing systems, such as sodar, UHF wind profiler, wind lidar, microwave radiometer as well as from normal and frequent radiosondes. This gives information on the mean meteorological conditions as well as on turbulence in the BL. To investigate clouds characteristics and precipitation ceilometers and cloud cameras are used, as well as different types of radars (micro-rain, cloud and x-band) and disdrometers.

The collected data set allows us 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.

Backscatter data from Ceilometer (colour coded) and cloud base height (black dots) at Savè site on two days.

Measurements show that low-level clouds appear frequently at all three sites and are often accompanied by the formation of the low-level jet. The measurements show large variability in the onset, depth and dissolution of the nocturnal stratus indicating the need to investigate the dependence of nocturnal low-level stratus on different atmospheric conditions.

Supersite Instrumentation

The Radiosonde campaign

Within the DACCIWA campaign, some 750 extra meteorological sondes have been released into the West African sky in June-July 2016. Not only have these sondes provided temperature, humidity, wind, and pressure measurements every second from the ground to altitudes of 20 km to the DACCIWA research data base, but coarser resolved vertical profiles of these variables were successfully submitted in real time to the weather forecasting centres worldwide. This success story would not have been possible without the grand support of African Weather Services, African academic colleagues, and students from Europe and Africa.

The resulting DACCIWA radiosonde network is shown in the map.

DACCIWA Radiosonde network and deployed sonde types. Blue: Operational or re-activated AMMA stations with enhanced sounding frequency; Black: DACCIWA super-sites; Red: DACCIWA stations operated by KIT, GMet, and UFHB; Yellow: Operational ASECNA and NWS stations outside the DACCIWA network

The first DACCIWA radiosonde went into the African sky in Kumasi on 11 June 2016, with Savé following on 13 June. Accra started soundings just a few days later. Between 20 and 27 June 2016, Abidjan, Cotonou, and Parakou slowly increased launching frequency to the planned four-times daily ascents. Between 29 June and 17 July 2016, the period of aircraft operations, all stations were fully active, except Lamto that started not until 06 July 2016. The last DACCIWA sonde was launched on 31 July 2016 at Accra.


Radiosonde launch at Lamto (Côte d‘ivoire)
 
A cohort of KNUST meteorology students went on an internship to GMET Headquarters

High priority was given to submit the radiosonde data in real-time into the GTS such that the data are used at ECMWF (European Centre for Medium-Range Weather Forecast), Météo France, UK MetOffice and DWD in data assimilation of operational numerical weather prediction models. This turned out to be another success story, not at least due to the great support from Bernd Richter (DWD) and his team. In the meantime, feedback from all the above-mentioned weather services was obtained, clearly showing that the DACCIWA observations had a substantial positive impact on their operational analyses. Currently the data are uploaded to the DACCIWA data base for usage by DACCIWA and associated partners, and will be freely available after the agreed two-year embargo period. As a cautionary note, it must be stated that at low levels, some quality issues with relative humidity at near saturation environments at Accra and Lamto have been identified. Pending on finding a solution for a posteriori correction, these parameters will be updated for these stations.

A great experience at several sites was the collaborations with local colleagues and the involvement of African students. They showed a great interest and engagement to help in sounding preparations and timely real-time submissions. For example, at Lamto and Accra, altogether about 10 students from Karlsruhe Institute of Technology (KIT) worked together with African colleagues and students and very positive experiences were received from all of them. At Accra a cohort of KNUST meteorology students went on an internship to GMET Headquarters and took part in DACCIWA soundings. From the photo, it is clearly apparent that a high interest was shown by the KNUST students. Thus, the DACCIWA RS campaign was not only a science success, but also tightened collaborations between African and European researchers and students, thus contributing to capacity development and human networking.