Effects of Surface Roughness with Two Scales on Light Scattering by Hexagonal Ice Crystals Large Compared to the Wavelength: DDA Results

The effect of ice crystal surface roughness on light scattering by ice crystals which are large compared to the wavelength was studied, in particular changes to the 2D scattering patterns, azimuthally averaged phase functions, degree of linear polarisation patterns and asymmetry parameters for a range of orientations and roughness scales. It was found that roughness has an effect on light scattering by hexagonal prisms, particularly when the roughness features are of comparable size to the wavelength. The roughness model that has the most effect on light scattering takes account of more than one roughness scale. Rough geometry was implemented by a Gaussian roughness method that took roughness parameters derived from sand grains, which have been reported to be suitable proxies for rough ice crystals. Light scattering data for these geometries was computed using the ADDA discrete dipole approximation method.