Extension of T-matrix to scattering of electromagnetic plane waves by non-axisymmetric dielectric particles: application to hexagonal ice cylinders

Abstract

The calculation of scattering by cirrus particles at intermediate size parameters greater than 20 has not as yet been satisfactorily solved with exact theory. This has made it difficult to represent scattering and absorption of ice clouds in remote sensing retrievals and in modelling radiative forcing. The T-matrix approach applied to ice particles has previously been implemented only for axisymmetric particles, but ice clouds consist of particles which are not axisymmetric. In this paper an implementation of T-matrix which provides exact solutions for scattering and absorption from non-axisymmetric particles is presented. Rigorous tests demonstrate the stability and accuracy of the method. Results for finite hexagonal cylinders are presented. The general T-matrix formulation has major conceptual and practical advantages over other existing methods such as the finite difference time domain (FDTD) and the discrete dipole approximation (DDA). These advantages are due largely to its analytic character, which allows exact fulfillment of the radiation condition. Other advantages are the restriction of calculations to the scatterer's surface, and the exploitation of particle symmetries which considerably simplifies computation. The new T-matrix implementation is tested against existing T-matrix results for the circular cylinder and the cube in terms of differential scattering cross-sections, no differences are found between the T-matrix calculations. Comparisons with a recently improved implementation of FDTD for randomly oriented hexagonal ice columns show good agreement with T-matrix in terms of absorption and extinction efficiencies.