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The study was initially commissioned by EUMETSAT as an Invitation To Tender. The University of Lille 1 and Free University of Berlin won the ITT. HYGEOS participated as a sub-contractor.

Among the EUMETSAT various activities related to EPS-SG, the timely and efficient processing of the data down-linked from the orbiting platforms is of outstanding importance to the primary users. To this end, it is necessary to simulate realistic data from the observation missions to be embarked upon for EPS-SG. This study concerns the optical missions METimage and 3MI and the generation of appropriate test data to support the ground processor development. Given the EPS-SG orbit and instrument viewing geometries, top of the atmosphere (TOA) radiances for the full range of representative atmospheric and surface conditions are generated for each instrument. The simulated TOA radiances are generated at level 1b, equivalent to the calibrated and geolocated measurements made by each instrument. 



3MI synthetic level 1b data in the 865 nm channel with total radiance (left) and polarized radiance (right). These observations were simulated using data from the 12/09/2007. Isolines of scattering angles have been added every 10 degrees allowing the easy identification of the backscatter direction (BS on figures, 180 degrees) and the cloud bow (CB on figures) produced by spherical droplets in liquid clouds around 140 degrees of scattering angle. The glitter (G on figures) is very bright in polarized radiance.




For this purpose full orbit propagation are performed based on 3 test orbits derived from EPS orbital parameters. Observation geometries (sensor reference frame) for 3MI and METimage are simulated based on instrument sampling characteristics. Geolocation and sampling geometries are used as input to radiative transfer simulator in which surface and atmosphere (clouds, aerosols, gas) are realistically described based on ancillary information obtained for dates and time of required simulation (among which AVHRR products for clouds, MACC reanalysis for aerosols, ECMWF reanalysis for atmospheric state, MODIS BRDF parameters for land properties). The simulated TOA radiances are generated at level 1b, equivalent to the calibrated and geolocated measurements. 

HYGEOS is in charge to develop and run the radiance simulator. This simulator starting point is a set of ancillary data (e.g. cloud optical depth, effective radius and altitude, aerosol optical depth and type, surface BRDF parameters...) and a set of sun and view geometries. These are provided as geolocated 1b grid. The simulator then (i) read these input for each pixel, (ii) run the radiative transfer solving (using the ARTDECO pacakge) and (iii) write the result into a corresponding level 1b grid. The parralelization of that computation is necessary due to the high CPU demand of radiative transfer solvers in absorbing and scattering atmosphere.




METimage synthetic data RGB. Left panel is a composite of 670 nm (red), 555 nm (green) and 443 nm (blue). The Right panel is a composite of 2250 nm (red), 1630 nm (green) and 865 nm (blue).