Polarized SHDOM

The Spherical Harmonics Discrete Ordinate Method model (SHDOM) for three-dimensional atmospheric radiative transfer has been modified for polarization. The real generalized spherical harmonics basis presented in Doicu et al. 2013 (JQSRT) has been implemented. Two new polarized reflecting surface models have been added: Fresnel reflection from a Gaussian distribution of wave slopes for ocean (Mishchenko and Travis 1997), and unpolarized modified RPV combined with Fresnel reflection from randomly oriented microfacets for land (Diner et al. 2012). The SHDOM upgrade has been coded efficiently with, for example, special subroutines for unpolarized (Nstokes=1) radiative transfer and the ability to choose 3 or 4 Stokes parameters for polarized transfer. If the BDRF reflecting surfaces are uniform, then reflection matrices are precomputed at the discrete ordinates, saving considerably computation. The multiple processor option for running SHDOM on large problems has been extended to polarization and to the visualization image output option. Polarized SHDOM has been extensively tested by comparison to SHDOM with a different polarization basis, with MYSTIC (Emde et al. 2010), and with the old unpolarized SHDOM.

The public distribution of polarized SHDOM contains companion programs that prepare unpolarized or polarized optical property files for SHDOM. Programs are included to generate scattering tables for spherical particles with a Mie code and for spheroidal or cylindrical particles with a T-matrix code (Mishchenko and Travis 1998). The distribution also contains documentation and example scripts modified for the polarization upgrade. Example calculations will be shown illustrating the effect on polarized radiances in clear pixels due to 3D radiative transfer from nearby fair weather cumulus clouds.