Quantifying the direct radiative effect of Saharan dust over north-west Africa and the tropical Atlantic.

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

The fundamental energy source driving our weather and climate is the sun. In the global mean, around one-third of the solar (or shortwave) energy incident at the top-of-the-atmosphere is reflected by a combination of the Earth's surface and clouds back out to space. The Earth and its atmosphere absorb the remaining shortwave energy, called radiation or electromagnetic radiation, while radiating (or emitting) their own terrestrial (or longwave) radiation to space. In the long term the rates of absorption and emission are approximately equal, and the Earth-atmosphere system can be said to be in balance, or radiative equilibrium with the sun. Any process that can change this equilibrium state has the potential to alter our weather and climate. Atmospheric aerosols are small liquid or solid particles which can be found in the atmosphere naturally (for example, as wind blown dust), but can also be generated by a variety of human activities (for example, from car exhausts or from the burning of vegetation). They are important because they can change the reflectivity (or albedo) of the Earth, hence altering the amount of shortwave energy available to the climate system. Dependent on their composition and size they may also absorb both shortwave and longwave radiation. Recently it has been shown that over ocean, mineral dust (such as that blown from the Sahara desert) can exert a very large effect not only on the energy balance at the top-of-the-atmosphere but also on that at the surface, cooling the surface temperature. Unfortunately, quantities such as dust amount and dust particle size vary greatly in space and time, making it very difficult for climate scientists to estimate the overall effect of dust on the energy balance with any great certainty. In addition, the changes seen to the energy balance over ocean will be quite different to those seen over land due to differences in the surface properties of water and vegetation or desert. North Africa is home to the Sahara desert, the most important desert dust source on Earth. The Meteosat-8 satellite is a space-platform placed in a geostationary orbit over the equator, near to the Greenwich meridian. Two new instruments are flying on Meteosat-8, the Geostationary Earth Radiation Budget (GERB) experiment, designed to measure the Earth's energy balance with high accuracy, and the Spinning Enhanced Visible and Infrared Imager (SEVIRI), which looks at the planet at several different wavelengths and so is able to provide information about quantities such as water vapour, clouds and aerosols. Because the satellite is geostationary GERB and SEVIRI see the same portion of the Earth at all times of the day, and its position means that they are ideally placed to view the northern part of the African continent, and consequently monitor dust outbreaks in great detail. In this project we intend to make use of the fact that GERB and SEVIRI view Africa continuously in space and time in order to perform a novel investigation of the impact of desert dust on the energy balance at the-top-of-the-atmosphere and at the surface. The timing of the project is particularly good because it coincides with a major field campaign over the same region. The local or point measurements taken in the course of this campaign will provide us with the information needed to check that the methods we use to obtain dust and surface flux information from SEVIRI are correct. Once we are confident that these methods work we will be able to apply them to the complete satellite data record in order to obtain a unique record of the influence of desert dust on the Earth's radiative balance over north-west Africa and the tropical Atlantic. This information will substantially improve our understanding of dust-radiation interactions, and could be used to improve both short-term weather forecasting, and our predictions of future climate change.