Single scattering parameters of randomly oriented snow particles at microwave frequencies

[ 1] 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 (C-sca), absorption cross sections (C-abs), 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 C-sca/pi r(eff)(2), C-abs/pi r(eff)(2), and asymmetry factors of complex particles at a fixed size parameter x = 2 pi r(eff)/lambda do not depend on the specific particle shapes when x is less than about 2.5. Here lambda is the wavelength and r(eff) 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 pi D-m/lambda; the size parameter of the median mass diameter (D-m) 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 pi D-m/lambda is greater than 2.