Rapid computation of the optical properties of hexagonal columns using complex angular momentum theory

Asymptotic approximations to the Mie absorption and extinction efficiency factors have previously been derived from complex-angular-momentum theory. In this paper adjustments are made to the complex-angular-momentum absorption efficiency, based on the equivalent volume-to-area sphere, so that the asymptotic approximation can be applied to other particle shapes. Results are presented for randomly oriented finite-hexagonal ice columns in the infrared, comparing adjusted complex-angular-momentum theory to absorption solutions obtained from ray-tracing and Discretised-Mie-Formalism. The adjusted complex-angular-momentum absorption efficiencies and single scattering albedos are within 3% of the ray-tracing results. In the resonance region the adjusted complex-angular-momentum absorption efficiency is within 6% of the Discretised-Mie-Formalism results. The advantages of adjusted complex-angular-momentum theory are its speed compared to Discretised-Mie-Formalism and the ability to directly compare terms with geometric optics. The study of adjusted complex-angular-momentum suggests that phenomena often referred to as "tunnelling" are dependent on morphology as well as refractive index and size parameter. The dependence on morphology can be used to estimate the absorption efficiency at other infrared wavelengths. The analysis is also used to show the limits over which the anomalous diffraction approximation, often used in remote sensing retrievals of ice cloud properties, is a Valid assumption. (C) 1999 Elsevier Science Ltd. All rights reserved.