By Peter Hill
In my career as an atmospheric scientist I’ve relied on observational data from a wide range of sources including satellite imagery, surface measurements, ground-based and satellite based radar, and aircraft measurements. Last July I had my first opportunity to contribute to the available data when I took part in the aircraft field campaign for the EU-funded DACCIWA (Dynamics-Aerosols-Chemistry-Cloud Interaction in West Africa) project.
The DACCIWA project is investigating pollution in southern West Africa (SWA) and how this affects health and the regional climate. This region is very reliant on agriculture which is highly sensitive to the amount of rainfall. Any changes in rainfall due to pollution may have important implications for the worlds’ supply of cocoa, not to mention the livelihoods of millions of people in SWA.
My role in DACCIWA is focused on atmospheric radiation; how sunlight and thermal radiation interact with the atmosphere over SWA. Radiation is important because it is a key component of the atmospheric energy budget (see Figure 1). Consequently radiation changes can lead to circulation changes which may in turn affect more obviously societally relevant processes such as precipitation.
Figure 1: Key terms in the atmospheric energy budget for southern West Africa (defined here as 8°W – 8°E and 5 – 10°N). Units are W m-2. Values shown are June-July means for 2000-2015. Divergence of dry static energy and sensible heating are derived from ERA-Interim. Radiation values (i.e. shortwave SW heating and longwave LW cooling) are from the CERES-EBAF dataset and latent heating is from the TRMM dataset. Adapted from Hill et al, 2016.
Pollution particles (aerosols) reflect and absorb radiation directly, but may also affect radiation by changing cloud properties. Radiation measurements are important to understand the extent to which both occur. These measurements can also be used as an additional check on aerosol and cloud measurements made during the campaign by both the aircraft and by satellites. Radiative transfer is a relatively well understood process. If the measured cloud and aerosol properties are correct, we should be able to predict the measured radiation quite accurately using computer-based models.
The campaign involved three aircraft, two of which were equipped with instruments to measure radiation (in addition to many other instruments). Each had two pyranometers, which measure solar radiation, and two pyrgeometers, which measure thermal radiation. One of each was mounted above the aircraft pointing upwards to measure downwelling radiation and one of each was mounted below the aircraft pointing downwards to measure upwelling radiation. During the campaign, out of a total of 50 flights, seven were made with the primary objective of making radiation measurements. I was lucky enough to fly on the British Antarctic Survey Twin Otter (Figures 2 and 3) on one such flight, which was an exhilarating and surprisingly comfortable experience.
Figure 2: The British Antarctic Survey Twin Otter aircraft outside the hangar at Gnassingbé Eyadéma airport in Lomé, Togo during the DACCIWA aircraft field campaign.
Figure 3: Airborne view of Lomé from the Twin Otter aircraft. Note the haze due to pollution.
Together with observations from three highly-instrumented field sites, the aircraft campaign has provided a wealth of measurements. These measurements provide an indispensable dataset for understanding pollution, weather, and climate in this region. The measurements will facilitate lots of exciting scientific research and scientists across Europe and SWA will be working with this dataset for at least the next two years. Keep up to date with our progress via the DACCIWA newsletter, or twitter feed.
REFERENCES
Hill, P. et al, 2016. A multisatellite climatology of clouds, radiation, and precipitation in southern West Africa and comparison to climate models – http://onlinelibrary.wiley.com/doi/10.1002/2016JD025246/full, where further details can be found.