ABSTRACT UNCERTAINTIES IN RADIATIVE TRANSFER COMPUTATIONS – CONSEQUENCES ON THE MERIS LEVEL-2 PRODUCTS –

Operational MERIS level-2 processing uses auxiliary data generated by two radiative transfer tools. These two codes simulate upwelling radiances using different approaches based on the matrix-operator method (FUB), the discrete ordinate method and the successive orders technique (LISE). Intervalidation of these two radiative transfer tools was performed in order to implement them in the MERIS level-2 processing. The parameterization of the water vapor absorption defined for each of these two codes leads to a well agreement not only for the MERIS bands with residual absorption but also in the MERIS band centred at 900 nm which is used for the water vapor retrieval. As for the strong oxygen absorption at the 760.625 nm MERIS wavelength, its parameterization varies between the two codes. Nevertheless, the systematic biases in the two codes will be removed thanks to the use of a differential method between two MERIS adjacent bands. For the oxygen absorption at 760.625 nm, a more exhaustive study needs to be achieved. An extensive exercise was also conducted for cases without gaseous absorption. The scattering processes both by the molecules and the aerosols were retrieved within few tenths of a percent. Nevertheless, some substantial discrepancies occurred if the polarization is not taken into account mainly in the Rayleigh scattering computations. Over land, errors on the aerosol optical thickness reach up to 30 percent in some geometries as observed in the SeaWiFS images. Over ocean, the impact of the code inaccuracy in the retrieval of water-leaving radiances is large, up to 50 % in relativ