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Processing of geostationary satellite observations for earth radiation budget studies

maandag, 8 december, 2008 - 15:30
Campus: Brussels Humanities, Sciences & Engineering campus
auditorium P. Janssens
Nicolas Clerbaux

Accurate measurements are needed to improve our understanding of the Earth Radiation Budget (ERB). Despite continuous efforts to improve the observation systems, models remain necessary to convert the raw measurements in a form usable by the scientific community. These models concern the spectral, the angular, the spatial, and the temporal properties of the radiation leaving the Earth at the top of the atmosphere. The geostationary orbit allows to resolve the full diurnal cycle and consequently there is no need for any temporal modeling.

This is the main motivation to include the Geostationary Earth Radiation Budget (GERB) instrument on the Meteosat Second Generation satellites. However, using the geostationary orbit the spectral, angular, and spatial models are still needed. In this work, we address these modelings in the case of the GERB project.

Assumptions about the spectral signature of the observed scene are necessary to compensate the telescope and detector spectral responses. This is especially important for geostationary observations as the distance implies the use of a powerful telescope. We describe the method used to unfilter the GERB shortwave (SW) and longwave (LW) measurements. Another spectral modeling problem that we address is the narrowband-to-broadband techniques for the SEVIRI imager. These broadband estimates are useful to model spatially the repartition of the energy within the large GERB footprints. This allows to compensate for the point spread function of the instrument, to enhance the spatial resolution, and to produce the "GERB-like" products. Finally, angular modeling of the radiation field is needed to convert the directional radiation measurement in hemispheric flux. This step is very important for geostationary observation as a point of the Earth is always observed from the same direction.

We discuss the rationale of what is implemented for the Edition-1 GERB data processing. These modeling steps should be done and validated carefully.

Indeed, any model error is likely to introduce biases in the GERB products. The errors that these models introduce in the final GERB products are theoretically quantified using radiative transfer computations. Further high-level validations are given by comparison of the GERB and CERES
radiances and fluxes for the SW and LW radiations. Recommendations for the future Edition--2 of the GERB processing are made in the present document and a summary is given in the conclusions.