Characterizing Single-Scattering Properties of Snow Aggregate Particles Integrated over Size Distributions in the Microwave Spectrum

Approximately 7000 snow aggregate particles have been synthesized, using a heuristic aggregation algorithm, from 9 realistic snowflake habits simulated using the now famous Snowfake ice crystal growth model. These particles exhibit mass-dimension relations consistent with those derived from observations. In addition, ranging from 0.1 to 3.5 mm in liquid-equivalent diameter, the sizes of these particle cover ranges wide enough for assemblies of realistic particle size distributions. The single-scattering properties, such as scattering/absorption/extinction/backscatter cross sections, single-scattering albedo, asymmetry factor, as well as the scattering matrix, are obtained for each aggregate particle using the discrete-dipole approximation (DDA) code DDSCAT at 13 microwave frequencies, ranging from 10 to 190 GHz. Preliminary radiative transfer calculations show that the single-scattering properties so obtained yield much more reasonable brightness temperatures than those derived from "fluffy sphere" Mie approximations. However, in order to achieve better retrievals involving these complex particles, we need to be able to characterize their single-scattering with only a few parameters. In this study, we present such an attempt using a pair of generalized effective radii, expressed as ratios of particle volume to particle surface area and to orientation-averaged particle cross section, in addition to mass content. It is shown that these effective radii are indeed effective in characterizing the PSD-integrated single-scattering properties of these complex particles. Pristine ice crystals simulated using the "Snowfake" ice crystal growth mode (3rd row from top) and example aggregates generated using the corresponding pristine particles (bottom 3 rows, i.e. 4th to 6th rows from top).