SINGLE SCATTERING PROPERTIES OF ATMOSPHERIC ICE CRYSTALS

Simulations of scattering and polarization properties for randomly ori ented polyhedral ice crystals are presented based on the geometric opt ics and the far-field diffraction approximation. Particle shapes range from various hexagonal symmetric particles to highly complex shaped d eterministic and random fractals. All calculations are performed at a wavelength of 0.55 mu m. Hexagonal symmetric particles show several na rrow scattering peaks besides the well known 22 degrees and 46 degrees 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 ang le of about 156 degrees, which may allow a polarimetric distinction be tween these two crystal types. The effects of particle size are studie d by applying observationally derived aspect-ratio parameterizations t o the individual particle types. Differences in the asymmetry paramete r versus size relations for column-like particle types are basically c aused by different aspect-ratio parameterizations rather than by the d ifferent types of columns. Thus, solid hexagonal columns appear repres entative for all column-like ice crystals. The dependency on crystal t ype is much stronger for plate-like particles. Increasing distortion o f the crystal shapes leads to a considerable smoothing of the scatteri ng signature. Scattering by complex-shaped particles is discussed for deterministic and randomized triadic Koch-fractals. The scattering sig nature for disordered fractals converges with increasing distortion an d may be regarded as characteristic for complex-shaped ice crystals.