Remote sensing of clouds using linearly and circularly polarized laser beams: techniques to compute signal polarization

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Polarization parameters of the radiation field scattered by clouds are needed for developing remote sensing methods for microphysical clouds properties. Using polarization, investigators may, firstly, upgrade information capacity of signals and, secondly, develop polarization-based methods to discriminate a useful signal. The very first calculations to find information content of visible radiation polarization scattered by a cloud were performed at the beginning of the 1970s (see Hansen, 1971; Hovenier, 1971; Kattawar and Plass, 1972). Hansen with the aid of the adding-doubling method and Kattawar and Plass on the base of the Monte Carlo method evaluated multiple-scattered parameters of polarization as a supplementary to the radiance of radiation reflected and transmitted by clouds. Their interest was focused on the problem of passive remote sounding of cloud parameters. For the problem of sunlight reflection, the aforementioned calculations have shown that the polarization degree is more sensitive to cloud microstructure than radiance. This is connected with the fact that the peculiarities of the angular distribution of polarization are far less subjected to smoothing upon multiple scattering than those of radiance. This underlies the techniques of studying microphysical properties of clouds and ice clouds composition and of distinguishing ice clouds from water-droplet clouds from the measured degree of polarization (for example, Hansen and Hovenier, 1974; Breon and Goloub, 1998; Chepfer et al., 1998;Kokhanovsky and Weichert, 2002;Mishchenko et al., 2002, 2006; Kokhanovsky, 2003; Liou et al., 2000;Sun et al., 2006;Goloub et al., 2000).