Evaluation of the Rayleigh–Gans approximation for microwave scattering by rimed snowflakes

We have evaluated the applicability of the Rayleigh–Gans approximation (RGA) and self‐similar Rayleigh–Gans approximation (SSRGA) for microwave scattering by rimed snowflakes. This study extends previous findings that showed that, for unrimed snowflakes, the RGA is in good agreement with the discrete dipole approximation (DDA), which we used as a reference method. When riming is introduced, the RGA‐derived scattering properties of individual snowflakes deviate significantly – up to 20–25 dB for the backscattering cross‐section in the W band – from the corresponding DDA results. In contrast, the average scattering properties given by RGA are in good agreement with DDA for all but the most heavily rimed snowflakes: the mean bias in the backscattering cross‐section rarely exceeds 1 dB for light and moderate riming. We also found that an adjustment that accounts for the non‐spherical shapes of ice crystals can help eliminate the small constant bias found in RGA in earlier studies. The SSRGA approximates the RGA results with good accuracy at all degrees of riming, indicating that it, too, can be used with up to moderately rimed snowflakes. We evaluate the usability of the Rayleigh–Gans approximation (RGA), a simple and computationally efficient scattering approximation, for microwave scattering by rimed snowflakes. The RGA compares well with reference calculations for all but the most heavily rimed snowflakes. The self‐similar Rayleigh–Gans approximation (SSRGA), which combines RGA with assumptions from fractal geometry, can also reproduce the RGA results well for rimed snowflakes.