Parameterizations of single scattering properties of frozen hydrometeors at millimeter-wave frequencies

Physical approaches in microwave remote sensing to measure nonspherical frozen hydrometeors are frequently parameterized as spherical particles, assuming either dielectric mixing approximation or equivalent spheres, to make Mie-theory applicable. However, the applicability of those simplified approximations in millimeter-wave radar and radiometric remote sensing of frozen hydrometeors need to be evaluated. To seek a parameterization to represent the electromagnetic (EM) properties of frozen hydrometeors at millimeter-wave frequencies, this study analyzes the Discrete Dipole Approximation (DDA) method calculated single scattering from nonspherical snow crystals at millimeter-wave frequencies (95, 140, 183, 220, and 340 GHz). Sizes, shapes, density, and refractive indices for five snow crystal models (hexagonal column, hexagonal plate, sector plate, planar rosette:, and spatial rosette) employed in this study are described in Kim et al. (2004). The results shown in this study assume that all particles are randomly oriented. By comparing with the DDA calculated single scattering properties, this study evaluates fast parameterization methods to represent the EM properties of snow at millimeter-wave frequencies. Dielectric mixing theory, which is widely used by the microwave remote sensing community, and the equivalent sphere approach, which is commonly used by infrared (IR) and ultraviolet (UV) remote sensing community are considered. In addition, equivalent cylinders and equivalent ellipsoids are also tested to examine how much the single scattering properties depend on the detailed shapes of ice crystals.