Radiative perturbations due to dust-atmosphere interactions over north Africa and the Atlantic and their implications for global climate

The Sun is the fundamental energy source driving weather and climate. In the global mean, there is a balance between the shortwave solar radiative energy absorbed by the Earth system, and the longwave thermal radiative energy emitted by the Earth to space. Any process that changes this equilibrium has the potential to alter our weather and climate. One class of processes involve atmospheric aerosols, small liquid or solid particles, found in the atmosphere naturally (e.g. as wind blown dust), and also generated by a variety of human activities (e.g. from car exhausts). Aerosols are important because they can change the reflectivity of the Earth, altering the amount of shortwave energy available to the climate system, and can also absorb both shortwave and longwave radiation. Recently it has been shown that airborne dust can exert a large effect on the energy balance at the top of the atmosphere and the surface. Studies also suggest that the dust may alter the temperature structure of the atmosphere through heating of the dust layer. A change in dust loading, for example caused by a desert dust storm, can affect regional circulation patterns, with implications for the development of important weather phenomena such as hurricanes. On longer timescales, changes in land use occurring under climate change (e.g. desertification) could increase dust production and further perturb our climate. Dust amount and particle size vary greatly in space and time, making it difficult to quantify its overall effect on the energy balance. The aim of this project is to bring together new observations from a variety of sources to study just what the radiative effect of dust aerosol is in the real atmosphere. This study takes place in the Sahara, the most important dust source in the world. One of the new sources of observations is the Meteosat-8 satellite, located in geostationary orbit at a point over the equator, and the Greenwich meridian. On board are two novel instruments: Geostationary Earth Radiation Budget experiment (GERB), designed to measure the Earth's energy balance, and the Spinning Enhanced Visible and Infrared Imager (SEVIRI), which can provide information about dust aerosols. Because the satellite is geostationary GERB and SEVIRI see Africa continuously and can monitor dust outbreaks in great detail. Other satellite observations add different information, such as the vertical distribution of aerosol, which is important for our understanding of the radiative effects. By merging information from all of these sensors a comprehensive picture of dust-atmosphere interactions over Africa and the Atlantic will be obtained. During 2006 a number of special campaigns will take place over north-west Africa and the Atlantic, aimed at characterizing the atmospheric state using ground-based and aircraft measurements. These observations will all be used to determine the amount, extent and variability of the dust, the associated atmospheric conditions and the total effect on the regional radiative balance. Finally, to help with the interpretation of the observations, colleagues at the UK Meteorological Office will provide simulations of the dust events, meteorological conditions and radiative fields. In the end, we hope to have developed a much more complete understanding of how these fascinating dust storms might be influencing our climate, and how this might be changing with time.