Light scattering by hexagonal columns in the discrete dipole approximation

TL;DR

This paper uses the discrete dipole approximation to model light scattering by hexagonal ice prisms, examining the validity of geometric optics and proposing methods to estimate ice crystal sizes from halo features.

Contribution

It presents a detailed wave optics analysis of ice crystal scattering, including birefringence, and compares results with geometric optics for large crystals.

Findings

Excellent agreement between wave and geometric optics at large size parameters.

Halo broadening occurs for smaller crystals, with spillover into shadow angles.

Methods for retrieving ice crystal sizes from halo characteristics are suggested.

Abstract

Scattering by infinite hexagonal ice prisms is calculated using Maxwell's equations in the discrete dipole approximation for size parameters up to x=400. Birefringence is included in the calculations. Applicability of the geometric optics approximation is investigated. Excellent agreement between wave optics and geometric optics is observed for large size parameter in the outer part of the 22 degree halo feature. For smaller ice crystals halo broadening is predicted, and there is appreciable "spillover" of the halo into shadow scattering angles <22 degrees. Ways to retrieve ice crystal sizes are suggested based on the full width at half-maximum of the halo, the power at <22 deg, and the halo polarization.