Experimental and Computational Investigation into Light Scattering by Atmospheric Ice Crystals

Abstract

An investigation was carried out into light scattering by Gaussian rough ice crystals. Gaussian rough crystal geometries were generated using roughness parameters derived from mineral dust grains, which have been reported to be suitable proxies for rough ice crystals. Light scattering data for these geometries was computed using the discrete dipole approximation (DDA) method. Phase functions, 2D scattering patterns, degree of linear polarisation patterns and asymmetry parameters were computed for smooth, moderately rough and highly rough crystals with a variety of orientations and size parameters. A sodium fluorosilicate ice analogue crystal with three partially roughened prism facets was created using focused ion beam (FIB) milling and 2D scattering patterns were collected from it using the small ice detector (SID) 3 cloud probe. It was found that roughness reduces features in the phase function compared to scattering by smooth hexagonal prisms, particularly when the roughness features were horizontally much larger than the wavelength. However, the most effective roughness model also takes account of horizontal features whose size is closer to that of the wavelength. Horizontal features smaller than the wavelength have very little effect.