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

驱动天气和气候的基本能源是太阳。在全球平均水平上，大约三分之一的太阳（或短波）能量入射到大气层顶部，被地球表面和云层反射回太空。地球及其大气层吸收剩余的短波能量，称为辐射或电磁辐射，同时向太空辐射（或发射）它们自己的地面（或长波）辐射。从长期来看，吸收和释放的速率大致相等，地球-大气系统可以说与太阳处于平衡或辐射平衡状态。任何能改变这种平衡状态的过程都有可能改变我们的天气和气候。大气气溶胶是可以在大气中自然发现的小液体或固体颗粒（例如，风吹起的灰尘），但也可以由各种人类活动产生（例如，汽车尾气或燃烧植被）。它们很重要，因为它们可以改变地球的反射率（或反照率），从而改变气候系统可用的短波能量的数量。根据它们的组成和大小，它们也可以吸收短波和长波辐射。最近有研究表明，在海洋上空，矿物粉尘（比如从撒哈拉沙漠吹来的）不仅对大气顶部的能量平衡有很大影响，而且对表面的能量平衡也有很大影响，使表面温度降低。不幸的是，诸如粉尘量和粉尘粒度等数量在空间和时间上变化很大，这使得气候科学家很难非常确定地估计粉尘对能量平衡的总体影响。此外，由于水和植被或沙漠的表面性质不同，海洋上的能量平衡变化将与陆地上的变化大不相同。北非是撒哈拉沙漠的所在地，这是地球上最重要的沙漠沙尘源。Meteosat-8卫星是一个空间平台，放置在赤道上空的地球静止轨道上，靠近格林威治子午线。Meteosat-8上有两种新仪器，一种是地球同步辐射预算（GERB）实验，旨在高精度测量地球的能量平衡，另一种是旋转增强可见光和红外成像仪（SEVIRI），它以几种不同的波长观察地球，因此能够提供有关水蒸气、云和气溶胶等数量的信息。由于这颗卫星是地球同步卫星，GERB和SEVIRI在一天中的任何时候都能看到地球的同一部分，它的位置意味着它们处于观察非洲大陆北部的理想位置，因此可以非常详细地监测沙尘爆发。在这个项目中，我们打算利用GERB和SEVIRI在空间和时间上连续观察非洲的事实，以便对沙漠尘埃对大气顶部和地面能量平衡的影响进行一项新的调查。该项目的时机特别好，因为它恰逢同一地区的一项重大现场活动。在这项活动中进行的局部或点测量将为我们提供所需的信息，以检查我们用于从SEVIRI获取尘埃和表面通量信息的方法是否正确。一旦我们确信这些方法有效，我们将能够将它们应用于完整的卫星数据记录，以便获得沙漠尘埃对非洲西北部和热带大西洋上空地球辐射平衡影响的独特记录。这些信息将大大提高我们对尘埃-辐射相互作用的理解，并可用于改进短期天气预报和我们对未来气候变化的预测。

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.