A computational method for riming ice crystals with varying habits

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Description

Cloud and climate models require accurate estimates of phase-partitioning among ice, liquid, and vapor to predict precipitation processes, which help determine cloud lifetime and net cloud radiative forcing. Theoretical and computational advances have led to better vapor depositional growth of ice in cloud models, but improvements are needed in the treatment of aggregation and riming.

In a single particle model, we grow ice crystals by vapor deposition and allow different habits to develop, which are approximated by spheroids with a reduced density. A theoretical method is developed and tested in which ice crystals are allowed to collect rime mass, which changes the ice crystal mass, aspect ratio, and density. Evolving ice crystals in the method removes the need for different ice categories like pristine ice and graupel and allows ice crystals to evolve in a mechanistic way depending on the environmental conditions.

Model output is compared to wind tunnel data, and the results compare well where significant mass increase due to riming occurs for ice crystals grown isometrically at liquid water contents less than 0.5 gm-3. Ice crystals with more extreme aspect ratios do not collect significant rime mass at liquid water contents less than 0.5 gm-3. This riming method will be parameterized for use in bulk microphysical models.