Parametrization of the effect of surface reflection on spectral infrared radiance measurements. Application to IASI

The recent launch of the Advanced Infrared Sounder (AIRS) on board EOS-Aqua and the scheduled launch of the Infrared Atmospheric Sounder Interferometer (IASI) on board the Meteorological Operational Satellite (METOP) in 2005 open interesting perspectives for remote sensing applications. Owing to their enhanced spectral resolution and sensitivity, this new generation of high-resolution infrared vertical sounders is first aimed at improving the vertical resolution of temperature and water vapor profile retrievals needed by the weather forecasting community. Another important possible use of these instruments, in the context of the study of global warming, is to permit the retrieval of the concentrations of greenhouse gases like , etc. In order to reach these two main objectives, improvement in the modeling of the radiative transfer is therefore necessary. One of the points which still needs some improvements is the contribution of the downward radiation reflected by the surface back to the satellite which is often improperly accounted for in radiative transfer calculation to save computer time. In this article, we show how it is possible to simplify the problem through the computation of a spectrally dependent “effective” emissivity for which a simple parametrization is proposed, while preserving the accuracy of the results.