Use of an Ice Particle Aggregate Simulator to extract dimensional properties of ice-ice aggregates for microphysical parameterization

Description

Aggregation is the collection of individual crystals (monomers) and is influenced by the crystal shape (habit). Of particular interest in this study is ice-ice two-monomer aggregation, which will improve microphysical parameterizations through more precise aggregate characteristics and in turn improve 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 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 aggregate properties are extracted from two primary habits (plates and columns) to determine the influence of ice crystal habit on aggregation. It was found that both plates and columns aggregate to less extreme aspect ratios but extreme columnar monomers become less extreme at a much greater rate than extreme plates. Newly formed aggregate properties from statistical analyses are to be directly implemented into the Adaptive Habit Model (AHM), a bulk microphysical scheme which evolves ice particle habit through vapor deposition, crucial in realistically evolving cloud ice mass distribution and in representing the collection process. It is important to note that the methodology presented avoids traditional thresholding of axis lengths due to aggregation and provides a seamless transition between monomer crystals and aggregates in models.