Light Scattering by Single Natural Ice Crystals

Abstract

Abstract During the South Pole Ice Crystal Experiment, angular scattering intensities (ASIs) of single ice crystals formed in natural conditions were measured for the first time with the polar nephelometer instrument. The microphysical properties of the ice crystals were simultaneously obtained with a cloud particle imager. The observations of the scattering properties of numerous ice crystals reveal high variability of the ASIs in terms of magnitude and distribution over scattering angles. To interpret observed ASI features, lookup tables were computed with a modified ray tracing code, which takes into account the optical geometry of the polar nephelometer. The numerical simulations consider a wide range of input parameters for the description of the ice crystal properties (particle orientation, aspect ratio, surface roughness, and internal inclusions). A new model of surface roughness, which assumes the Weibull statistics, was proposed. The simulations reproduce the overwhelming majority of the observed ASIs features and trace very well the quasi-specular reflection from crystal facets. The discrepancies observed between the model and the experimental data correspond to the rays, which pass through the ice crystal and are scattered toward the backward angles. This feature may be attributed to the internal structure of the ice crystals that should be considered in modeling refinements.