On the Single-Scattering Properties of Realistic Snowflakes

Although spheres and spheroids have been used extensively by researchers as convenient models to approximate "snowflakes" when computing their microwave scattering properties, recent research indicates that the scattering properties of more accurately simulated snowflakes are fundamentally different from the simplified models. To resolve this well-recognized discrepancy, a new snowflake aggregation model is developed in this study and the microwave single-scattering properties of the modeled aggregate snowflakes are characterized for use in radiative transfer modeling and remote sensing algorithm development. Three different aggregate snowflake types (rounded, oblate and prolate) are generated by random aggregation of 6-bullet rosettes constrained by size-density relationships derived from previous field observations. Additionally, they are further constrained to empirically determined aspect ratios (ar) and fractal dimensions (df) of aggregate flakes. Due to random generation, aggregates may have the same size or mass, yet different morphology, allowing for a study into how detailed structure influences an individual flake's scattering properties. Single-scattering properties of the aggregates were investigated using discrete dipole approximation (DDA) at 10 frequencies: 10.65, 13.6, 18.7, 23.8, 35.6, 36.5, 89.0, 94.0, 165.5 and 183.31 GHz. All of these frequencies are currently used in instruments (radar and radiometers) aboard satellites involved in the research of atmospheric ice particles. Results from DDA were compared to those of Mie theory for solid and soft spheres (with a density 10% that of solid ice) and to T-matrix results for solid and soft spheroidal cases with ar values of 0.8 and 0.6 dependent on flake type (rounded, oblate or prolate). Analyzing modeling results, it is found that above size parameter 0.75, neither solid nor soft sphere and spheroidal approximations accurately represented the DDA results for all aggregate types. The asymmetry parameter and the normalized scattering and backscattering cross-sections of the aggregate groups fell between the soft and solid spherical and spheroidal approximations. This implies that evaluating snow scattering properties using realistic shapes, such as the aggregates created in this study, is necessary in radiative transfer modeling and remote sensing studies. When examining the dependence of the single-scattering properties on each aggregate's detailed structure, morphology seemed of secondary importance. Using normalized standard deviation as a measure of relative uncertainty, it is found that the relative uncertainty in backscattering arising from the different morphologies caused by random aggregation is typically ~17%, 13% and 14% for individual particles and ~20%, 30% and 30% when integrated over size distributions for rounded, oblate and prolate flakes respectively. Relative uncertainties for other single-scattering parameters are less. These analyses indicate that a scattering database can be created to approximate the single-scattering properties of realistic aggregate flakes. A database of such aggregate flakes has been created based upon the research detailed herein, and made available for public use. In this work, it is found that flakes with similar size parameters can scatter differently. Ongoing research indicates that this is due to outer layer morphology of the flake (i.e. the arms of a dendritic snowflake) rather than any interior properties. When the interior of an aggregate flake is scrambled, the scattering results are nearly the same as the unscrambled interior whereas if the outer layer is altered, scattering results differ. Another interesting trend noted is that randomly oriented flakes with differing ar values have noticeably differing backscatter cross-sections and could have significant implications for future research.