Multiple scattering analysis of airborne POLDER image data over the sea

The authors have made a multiple scattering analysis of the airborne polarization and directionality of Earth reflectances (POLDER) data over the Mediterranean Sea, which was obtained in the Medimar campaign in 1991. Assuming an atmosphere-ocean system with an anisotropic Cox-Munk sea surface, the radiance and linear polarization in the upwelling radiation have been numerically evaluated for several typical aerosol models by using the adding and doubling method, in which multiple scatterings within the atmosphere and, multiple reflections at the atmosphere-ocean interface are fully taken into account. Major results based on this study are summarized as follows, (1) For the first time, the Medimar airborne POLDER data was analyzed from the linear polarization point of view by this study. Several typical aerosol models, namely, the Haze M, water-soluble, oceanic, maritime, continental and Junge-type models were fully tested against both the observed reflectance and linear polarization data at 0.85 μm in the principal plane, as well as in the image form with various viewing angle information. (2) By examining various combinations of aerosol´s complex refractive indices and wind speeds, the authors found that only the water-soluble model and Junge-type models with ν=3.5, 4.0, and 4.5 can satisfy the observed reflectance data, but they also found that these models, satisfying the observed reflectance constraints, fail to satisfy the observed linear polarization features in the backward scattering directions. (3) Any aerosol models deduced from the reflectance data analysis should be tested against the polarization data. This study was the first such attempt and all the major aerosol models with a single-peaked size distribution function were rejected, as an appropriate model over the Mediterranean Sea at the Medimar experiment time because they all failed to satisfy the observed polarization data. (4) More studies are certainly needed to find an appropriate aerosol model which can satisfy both the observed reflectance and polarization features. Other types of aerosol models with a multipeaked size distribution function should be fully examined in the future