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 surface using characteristics of SEVIRI. We will present the retrieval of the surface contribution and define the surface reflectance map of reference. Knowing the reflectance of the surface for each pixel and observation time, we will correct measurements for Rayleigh scattering. Using simple assumptions, aerosols optical thickness at a visible wavelength will be retrieved. The results will be compared with some AERONET data from stations available in the area of interest. Finally, we will go back to the assumptions we made in the retrieval scheme to explain the weakness of the method. 6.1.3 Reference Surface Reflectance Map The signal measured by the instruments is a mix of different contribution of the Earth-Atmosphere system: surface, molecules, aerosols, gases and clouds. Cloud Masking First of all, cloudy pixels have to be removed for aerosol retrieval. Because of the short time we had to develop our methodology we used a simple cloud mask. It is based on threshold tests on the reflectance at 670 nm, 865nm and 1.6µm and brightness temperature at 11µm (see Appendix C). To avoid pixels containing cloud border, we also remove neighbouring pixels. Finally, the cloud mask we used does not detect cloud shadows, which produce pixels with lower reflectances than the surface reflectance alone. Rayleigh correction Once the cloud masking is done, the contribution of the Rayleigh scattering at 670nm is removed from the signal using classical method adapted to the spectral properties of the channel. The remaining quantity (R ray_corrected ) is then the contribution of the surface and the aerosols. Surface contribution The surface reflectance map is built from the 3D data set of the Rayleigh corrected reflectances corresponding to the cloud-free cases. For each time slot (from 5:00 UTC till 19:00 UTC, every 15 minutes) we have, at maximum, a set of 14 values of the Rayleigh corrected pixel reflectances. The three dimensions of the data set are the pixel longitude, latitude and the date (1 to 14 July). The retrieval method assumes that the minimum value corresponds to the zero aerosol case, or at least, the case with the least aerosol contamination. In another words, the minimum is the value of the surface reflectance (R surf_ref ) seen in the correct geometry. The value obtained is assumed to correspond with the reflectance for a lambertian surface as simulated in radiative transfer - 18 -
SAF on Climate Monitoring Visiting Scientists Report Doc. No: 1.0 Issue : 1.0 Date : 4 October 2006 calculations. This assumption is mainly true when neglecting interaction between surface and molecules and for viewing geometries far from the “hot spot” of the vegetation (Knapp et al., 2002). The errors due to this assumption will be much lower for geostationary satellites because the bulk of observations are geometrically far from the “hot spot” (Knapp et al., 2005). The reference map of surface reflectance is generated for each observation time by applying above described method to all the image pixels. Finally, Fig. 7 summarises in a diagram the method to retrieve the reference surface reflectance map. Fig. 8 shows for every hour the reference surface reflectance map for the period 1-14 th July. The reference area covers almost all the continental areas. Noise appears for low solar elevations in the morning. This can be due to the cloud masking method, which is not robust for such sun-satellite geometries. Around 13:00 UTC some high reflectances appear in middle-Europe but there are large doubts about the accuracy of cloud detection in this area (lots of partly cloudy situations and scattered clouds). Fig. 9 gives the time series of the reference maps of surface reflectance for several locations: the Flandres in Belgium, the Landes forest in South-West of France, the Andalusia in the South of Spain and the north of Algeria. Temporal variation of the surface reflectance seems reasonable with regards to vegetation reflectance and biomass models used in MERIS Look-up-tables, i.e., variation of the surface reflectance with the scattering angle is similar to that used in MERIS LUT. The erroneous values of the surface reflectance for the Flandres area early in the morning is due to clouds, which are not detected by our cloud mask. Independently of such erroneous values, the curves are affected by a noise of small amplitude. The retrieval of ground reflectances is altered by differences in the residual aerosol loading within the composite period and differences in the observation date that corresponded to the minimum surface reflectance at the observation time. In the near future, one can find a solution to minimize the noise that is based on the following facts: • The surface BRDF varies smoothly with the solar angles along the day. One is allowed to apply a fit to the surface BRDF versus time. • A high reflectance value once or two times during the day indicates either a thin cloud or high aerosol load that was not detected by cloud masking or because no cloud free observations occurred during the composite time period at that particular time and locations. Those points should be discarded. In the data sample we used for this study, less than 0.01 % of the pixels were detected cloudy during the entire 14 days period. - 19 -
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Aerosol retrievals from METEOSAT-8 - CM SAF

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