Single scattering parameters of randomly oriented snow particles at microwave frequencies

Abstract

To develop a generally applicable fast and accurate parameterization method for computations of single scattering parameters at microwave frequencies requires a thorough knowledge of how the ice particle shape affects the scattering parameters. This study computes single scattering parameters (scattering cross sections (Csca), absorption cross sections (Cabs), and asymmetry factors) of various nonspherical snow particles using the discrete dipole approximation (DDA) method and the T‐matrix method to examine the sensitivity of scattering parameters to snow particles at frequencies between 95 GHz and 340 GHz. Results show that Csca/πreff2, Cabs/πreff2, and asymmetry factors of complex particles at a fixed size parameter x = 2πreff/λ do not depend on the specific particle shapes when x is less than about 2.5. Here λ is the wavelength and reff is the radius of equal‐volume ice spheres. The Mie theory may be used to compute the single scattering parameters of randomly oriented snow particles if radius of equal‐volume ice spheres reff is known over this range. On the other hand, when x > 2.5, scattering parameters of nonspherical particles are sensitive to the particle shapes because they are in an anomalous diffraction regime. In this regime, particles have a smaller projected area for a given volume so that the “unfavorable” interference effect grows, resulting in smaller minimum values of scattering cross sections and asymmetry factors. Single scattering parameters averaged over a Gamma size distribution show that scattering coefficients are sensitive to shapes and that differences are larger than 10% when πDm/λ; the size parameter of the median mass diameter (Dm) is greater than 1. Single scattering albedo values do not show significant differences over most size parameter ranges considered in this study. Asymmetry factors are sensitive to particle shapes when πDm/λ is greater than 2.