Light Scattering by Single Natural Ice Crystals

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.

Optical properties of pristine ice crystals in mid-latitude cirrus clouds: a case study during CIRCLE-2 experiment

“…This size appears to delineate the lower limit of ice crystal size responsible (at visible wavelengths) for halo formation according to earlier laboratory studies (Sassen and Liou, 1979;Barkey et al, 2002) and theoretical results (Mishchenko and Macke, 1999). In situ observations at the South Pole station (Shcherbakov et al, 2006a;Lawson et al, 2006) also reported very well-marked 22 • halo peaks with pristine ice crystals no larger than 100 µm. Therefore, the reported ice crystal sizes do not appear to be critical for the halo/no-halo interpretation.…”

“…Defaults in crystal geometry, roughness of the ice surface or imperfect internal structure do hamper the formation of halo (see among others Baran and Labonnote, 2007). The model from Shcherbakov et al (2006a) has been employed to compute the angular scattering patterns by using measured crystal shape and size and by considering both roughness and internal structure. In this model, the surface roughness assumes the Weibull statistics and the air bubble density is hypothesized to represent the internal heterogeneities of the crystal.…”

“…Surprisingly, for one case, the Polar Nephelometer data do not reveal any signature of 22 • (and 46 • ) halos although for the second case, well-marked 22 • halo features are observed in a few cloud portions along with prevalent planar-plate ice crystals. The interpretation of these results is discussed in the light of a careful analysis of the ice particle morphologies and by using a theoretical model of light scattering (Shcherbakov et al, 2006a). Implications for realistic modelling of scattering properties of cirrus clouds are given.…”

PHIPS-HALO: the airborne particle habit imaging and polar scattering probe – Part 2: Characterization and first results

“…Crystal (b) induces scattering intensities measured in the side and backward directions that exceed those of the less structured crystal (a) by up to 1 order of magnitude. It is acknowledged, however, that detailed light scattering simulations, like in the work of Shcherbakov et al (2006), are necessary to unambiguously prove that the observed differences can be attributed to differences in the ice crystal complexity.…”

“…Angular light scattering function measurements in midlatitude cirrus clouds as well as Arctic mid-level ice and mixed-phase clouds by the Polar Nephelometer (PN) (Gayet et al, 1997) revealed high back reflection (expressed by a low asymmetry parameter g), indicating ice crystals with significant structural complexity (Gayet et al, 2004;Jourdan et al, 2010). However, these light scattering measurements, which are averages over thousands of ice particles, cannot easily be related to the ice microphysics even if supporting measurements are available (Shcherbakov et al, 2006;Jourdan et al, 2010). Therefore, the question that fundamentally defines the angular light scat measurements and a different approach is required.…”