Dynamics-aerosol-chemistry-cloud interactions in West Africa
Work Packages
Radiative Processes

Radiative processes are crucial in determining the forcing and feedbacks operating on a variety of timescales and strongly influence synoptic-scale evolution and climate change. Knowing the properties of the surface and of gases, clouds, precipitation and aerosols in the atmosphere, the radiative energy emitted and reflected by the Earth-atmosphere system can be accurately predicted. Therefore, radiation measurements provide an excellent pathway to evaluate and improve the accuracy of physical processes represented in weather forecast and climate models (as shown in the image below). In particular, models have displayed substantial biases over the West Africa region in both reflective shortwave and outgoing longwave radiation. These radiative biases are mainly due to the deficiency in modelling surface albedo, aerosol distribution and its interactions with cloud and precipitation, which leads to a large uncertainty in radiative effects of cloud and aerosol.

The image above shows the performance of 2-day (upper panel), 4-day (middle) and 7-day (bottom) forecast made by the Met Office global numerical weather prediction model in outgoing longwave radiation (OLR; left two columns) and precipitation (right two columns) at 00 UTC (around midnight) and 12 UTC (around local noon). The performance is evaluated against Geostationary Earth Radiation Budget (GERB) data. The precipitation bias in southern West Africa, underestimation at midnight and overestimates at noon, reveals the outstanding problem that the predicted diurnal cycle of precipitation peaks too early at noon, rather than in the afternoon as we see in observations. (Courtesy of Richard Allan, University of Reading.)
Science Activities:
The objective of this work package is to quantify the radiative forcing of clouds and aerosols (direct and indirect) using two complimentary approaches: a regional multi-year multi-satellite analysis, and a detailed radiative closure study including airborne and ground-based measurements from DACCIWA. The specific objectives are to:
  1. contribute the satellite-based component of the DACCIWA climatological dataset;?
  2. provide a comprehensive product of aerosol and cloud properties that are important for understanding radiative processes;
  3. perform broadband and spectral radiation closure at the surface, the aircraft altitudes and at the top of the atmosphere (TOA) for all-sky situations;
  4. ?derive recommendations for potential improvement in satellite-based retrievals for aerosol, cloud and precipitation, based on DACCIWA observations; and
  5. assess aerosol and cloud radiative effects on the West African monsoon circulation and water budget including possible feedbacks.
The multiple-satellite dataset combined with radiative closure calculations will be used to evaluate predictive skills of models across various temporal and spatial scales, and to deliver a new understanding of the interactions and potential feedbacks between cloud and aerosol radiative effects and atmospheric circulation.
Field Activities:
This work package will be responsible for planning aircraft radiation measurements, making ground-based sunphotometer measurements, retrieving cloud properties, and performing radiation closure study using a synergetic dataset from the field campaign and various satellite overpasses.
The modelling activities in this work package mainly involve radiative transfer, including shortwave and longwave, broadband and spectral.
Project Lead:
Dr Christine Chiu
University of Reading
United Kingdom
Other Partners:
  • MO