Use of an Ice Particle Aggregate Simulator to Extract Dimensional Properties of Ice-Ice Aggregates for Microphysical Parameterization

Aggregation is the process by which individual ice crystals (monomers) collect and is influenced by the crystal shape (habit). Of particular interest in this study is ice-ice two-monomer aggregation, which is expected to improve microphysical parameterizations through more precise aggregate characteristics and in turn better predict the rate of aggregation and snow development. A systematic way to determine the aspect ratio of the aggregate was developed, which takes into account different falling orientations, overlap of each monomer, and any contact angle that may form through constrained randomization. Distributions with varying sample sizes were used to obtain the most frequent aspect ratio, major axis, and minor axis of aggregated particles with respect to the monomer aspect ratio.

Simulations were completed using an Ice Particle Aggregate Simulator (IPAS), a box model that uses hexagonal prisms to theorize ice crystal aggregation and allows for variation in crystal size, shape, number, and falling orientation. After collection in a theoretical grid space, detailed information is extracted from two primary habits (plates and columns) to determine the aggregate ice crystal properties. It was found that both plates and columns aggregate to less extreme aspect ratios at nearly the same rate. Newly formed aggregate properties derived from statistical analyses are amenable to implementation into more sophisticated bulk microphysical models that are designed to predict and evolve particle properties, which is crucial in realistically evolving cloud ice mass distribution and for representing the collection process. The methodology presented avoids traditional thresholding of axis lengths due to aggregation and provides a seamless transition between monomer crystals and aggregates in models.