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

Asymptotic approximations to the Mie absorption and extinction efficiency f actors have previously been derived from complex-angular-momentum theory. I n this paper adjustments are made to the complex-angular-momentum absorptio n efficiency, based on the equivalent volume-to-area sphere, so that the as ymptotic approximation can be applied to other particle shapes. Results are presented for randomly oriented finite-hexagonal ice columns in the infrar ed, comparing adjusted complex-angular-momentum theory to absorption soluti ons obtained from ray-tracing and Discretised-Mie-Formalism. The adjusted c omplex-angular-momentum absorption efficiencies and single scattering albed os are within 3% of the ray-tracing results. In the resonance region the ad justed complex-angular-momentum absorption efficiency is within 6% of the D iscretised-Mie-Formalism results. The advantages of adjusted complex-angula r-momentum theory are its speed compared to Discretised-Mie-Formalism and t he ability to directly compare terms with geometric optics. The study of ad justed complex-angular-momentum suggests that phenomena often referred to a s "tunnelling" are dependent on morphology as well as refractive index and size parameter. The dependence on morphology can be used to estimate the ab sorption efficiency at other infrared wavelengths. The analysis is also use d to show the limits over which the anomalous diffraction approximation, of ten used in remote sensing retrievals of ice cloud properties, is a Valid a ssumption. (C) 1999 Elsevier Science Ltd. All rights reserved.