Quantitative Computed Contribution of Water Raman Scattering Effects on Ocean Colour
von Bismarck, Jonas; Kritten, Lena; Preusker, Rene; Fischer, Jürgen

Many studies in the past have suggested that Raman scattering of the solar light-field due to energy absorption by vibrational modes of water molecules can contributes significantly to the signals observed by ocean remote sensing satellites. While radiative transfer models that qualitatively approximate the effect of water Raman scattering on water leaving radiances have been available for a while, a new version of the radiative transfer code MOMO, which enables the spectraly accurate and fully angle resolved inclusion of inelastic scattering sources, allows very detailed quantitative analyzes of the effect on the light field in the ocean-atmosphere system. The Code allows to flexibly compute higher orders of inelastic scattering, if this precision is required. Furthermore, the newest code version allows to incorporate the effects of rotational Raman scattering in the atmosphere, which mainly has an impact on remote sensing applications that require a very high spectral resolution. We have used the model for sensitivity studies reagarding the impact of Raman scattering on different applications, of which some shall be introduced here.
Within the framework of the ESA support to science project waterradiance, we have performed a sensitivity study regarding the effect of water Raman scattering on water leaving radiances and top of the atmosphere radiances in the spectral bands of the MERIS/OLCI instrument using this new code version. A simple bio-optical model was used to simulate cases with different chlorophyll concentrations representing a variety of Case-1 water types. Furthermore, the effect of sea water temperature and salinity on the fraction of Raman scattered radiation was determined for each simulated case.
The incorporation of a more complex multi-parametric bio-optical model, provided by Roland Doerffer and developed within the ESA funded support to science project Coastcolour, enabled even more detailed studies of varying water body types. Among others, we examined the remaining influence of water Raman scattering in Sun glint contaminated areas on satellite measurements, and studied the effect of (computation time saving) azimuthal averaging of the Raman scattering effect on the accuracy of the computed water-leaving radiances. Furthermore, within the framework of the SIOCS (Sensor Independent Ocean Color Service, funded by the German Federal Ministry of Economics and Technology and the DLR) project, we are using the new MOMO-Raman code to generate a highly resolved Lookup table of water-leaving and top-of-atmosphere radiances, with a 1nm resolution in the visible and SWIR, under variation of the bio-optical parameters.