Statistical simulation of the angular distribution of reflected solar radiation in optically anisotropic ice-crystal clouds: Horizontally oriented particles

The results of statistical modeling of the angular characteristics of reflected solar radiation in the atmosphere containing a layer of ice crystal clouds with anisotropy caused by the orientation of particles in space are presented. Numerical experiments are carried out using (1) original algorithm of the Monte Carlo method for calculating characteristics of unpolarized solar radiation taking into account the multiple scattering, reflection from the underlying surface, and attenuation by aerosol particles and due to molecular scattering and (2) optical model of anisotropic medium, composed of horizontally oriented hexagonal plates or Parry columns (visible range), developed previously in Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences, Tomsk, Russia. We discuss the patterns of formation of reflected solar radiation without taking into account the aerosol-molecular component of the atmosphere and surface reflection; these simulation results make it possible to identify the specific features of radiative transfer, caused exclusively by the effects of anisotropy of ice-crystal clouds. The factors that determine the influence of the surface reflection and aerosol attenuation on halo phenomena are described. It is shown that, as the surface albedo increases, the near-isotropic radiation field is formed outside the main halo lines and the intensity of different halo lines decreases down to the level when these lines or their components become almost invisible against the background of the contribution formed by surface-reflected radiation. Our radiation code is to be used for a research into the optical phenomena, observed from the Earth's surface and space, and, in particular, for an additional study of scattering by oriented ice crystals near the specular reflection direction (solar glint).