Formation of Atmospheric Halos by Hexagonal Ice Crystals

Ice clouds consist almost exclusively of non-spherical ice crystals with various shapes (i.e., habits) and sizes. Hexagonal crystals (i.e., columns and plates) represent the building blocks of the most common ice crystal habits. Previous studies have shown that atmospheric halos begin to form when the size parameter (ratio between size of particle and wavelength of incident light) of ice crystals increases to between 80 and 100 depending on the assumed geometry of the crystal, method used to solve the radiative transfer equations, and the wavelength of incident light. It is important to identify the threshold size at which atmospheric halos emerge because it determines the applicability of the conventional geometric optic method on the calculations of scattering properties of small particles. The halo display is an intrinsic optical feature in the geometric optic regime, which is not shown in the resonant regime.

This study extends previous efforts by investigating how variations in the sizes and aspect ratios (ratio between crystal length and width) of ice crystals affect the size parameter at which haloes first emerge. High-resolution images of ice crystals obtained from aircraft probes during field projects are used to define the range in aspect ratio that different sized ice crystals can have. Then, the single-scattering properties of hexagonal crystals with sizes of 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 µm and aspect ratios of 0.25, 0.5, 1.0, 2.0, and 4.0 at a wavelength ë of 0.55 µm are calculated using the discrete dipole approximation. From these accurate and large simulations, the threshold size at which 22 and 46 degree halos form is determined. Further, the impact of the aspect ratio of hexagonal crystals on the halo formation is also quantified. The applicability of these calculations to realistic ice clouds is discussed in context of the range of shapes and high variability in aspect ratios and sizes of hexagonal crystals observed, and in terms of the impact of the imperfect shapes and inhomogeneity of hexagonal crystals that can sometimes prevent the formation of atmospheric haloes. -->014-->