Aerosol retrievals from METEOSAT-8 - CM SAF

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SAF on Climate Monitoring Visiting Scientists Report Doc. No: 1.0 Issue : 1.0 Date : 4 October 2006 sampling of the surface BRDF and/or the aerosol phase function Beside the above-mentioned new aspects that are especially dedicated to MSG/SEVIRI, it is also possible to use more classical methods. First, for MERIS a method is used that links the vegetation index empirically to surface reflectance. The lack of a blue channel on SEVIRI will reduce the vegetation index mostly to an NDVI, which is not independent of atmospheric turbidity. Thus NDVI is not a robust index for the prediction of the surface reflectance if it is computed from a single observation. However, a spectral index built with multiple views, and using information from the blue part of the spectrum of the High Resolution Visible (HRV) channel should not be discarded. Nevertheless, the use of the HRV channel is not trivial and studies are needed to investigate methods to separate the reflectances in blue from the HRV channel reflectances. Second, the MODIS like approach that uses visible surface reflectance at 670 nm from the channel at 1600 nm (King et al. 1992). This approach has been described in an early stage of the operational algorithm for the LandSAF. To our knowledge no application of this method on real MSG data has been published. Finally, in particular conditions where one can assume that the aerosol model and loading vary little in time, it is possible to use consecutive observations with different sun zenith angles and, according to the reciprocity principle, retrieve aerosol properties. This approach follows the principles AATSR or MISR algorithms that are designed for multiple view angles. Note that the paper devoted to Meteosat by Pinty et al., (2000) describes a method to derive surface albedo by decoupling surface and aerosol contribution from a set of images acquired during the day, following a method close to the MISR algorithm. However, the main constraint of this method is that the geophysical system shall not vary much during the day, which contradicts with the wish to use MSG as a tool to measure the rapidly changing aerosol fields. - 16 -
SAF on Climate Monitoring Visiting Scientists Report Doc. No: 1.0 Issue : 1.0 Date : 4 October 2006 6 MSG/SEVIRI methodology 6.1.1 Introduction This chapter presents the preliminary results of MSG/SEVIRI retrievals of aerosol properties over land. We decided to demonstrate the potential MSG/SEVIRI for aerosol retrievals by developing a simple algorithm to retrieve the AOT from SEVIRI. We focused on the aerosol optical thickness (AOT), which is a key parameter for measuring the aerosol loading in the atmosphere. Most of the assumptions used in the algorithm were made to support the development of an aerosol product from MSG/SEVIRI in a short time. From this prototype, a robust algorithm can easily be obtained after implementation of radiative transfer calculations. In short, the AOT is retrieved from the reflectances at one wavelength (i.e. 670nm) in two steps, using a method similar to the one Knapp et al. (2005) developed for GOES-8. The first step is the creation of a 3D images data set (for a period of 15 days), from which we built a mosaic (reflectance surface maps of reference) where the darkest pixel of the time period is chosen as the “clear sky” condition. In the second step these surface reflectance map are used to retrieve AOT for each images. 6.1.2 Method description The method is based on the unique capabilities of the MSG/SEVIRI measurements. We take advantage the high temporal resolution (15 minutes) and fixed viewing geometry of SEVIRI. We can reasonably assume that for given acquisition time (i.e. 8/00 UTC) within a period of 15 days: • The solar zenith angle and the relative azimuth angle do not vary significantly. So for one pixel, the viewing geometry is stable. For example, at 12:00 UTC and over the fifteen first day of July 2005, the solar zenith angle varies of a value between 0.5° and 1.6° depending on the Earth's locations. The variation in azimuth angle is between -1.2 and 4.5°. • The surface properties do not change (except for particular and exceptional cases such as floods, fires and snowfall). Measurements that are made every day for the same pixel at the same time can then be easily compared. After screening the clouds and assuming no change in surface reflectivity, the differences in the measurements come from differences in the state of the atmosphere or changes in aerosols properties. In a first step we will explain how we get ride of the surface by monitoring the - 17 -
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Aerosol retrievals from METEOSAT-8 - CM SAF

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