Single Scattering Properties of Atmospheric Ice Crystals

Abstract

Simulations of scattering and polarization properties for randomly oriented polyhedral ice crystals are presented based on the geometric optics and the far-field diffraction approximation. Particle shapes range from various hexagonal symmetric particles to highly complex shaped deterministic and random fractals. All calculations are performed at a wavelength of 0.55 µm. Hexagonal symmetric particles show several narrow scattering peaks besides the well known 22° and 46° halos. Column-like ice crystals provide neutral points (NP) at larger scattering angles than plate-like ice crystals. The ranges of NPs for column-like and plate-like crystals are separated at a scattering angle of about 156°, which may allow a polarimetric distinction between these two crystal types. The effects of particle size are studied by applying observationally derived aspect-ratio parameterizations to the individual particle types. Differences in the asymmetry parameter versus size relations for column-like particle types are basically caused by different aspect-ratio parameterizations rather than by the different types of columns. Thus, solid hexagonal columns appear representative for all column-like ice crystals. The dependency on crystal type is much stronger for plate-like particles. Increasing distortion of the crystal shapes leads to a considerable smoothing of the scattering signature. Scattering by complex-shaped particles is discussed for deterministic and randomized triadic Koch-fractals. The scattering signature for disordered fractals converges with increasing distortion and may be regarded as characteristic for complex-shaped ice crystals.