On calculating deposition coefficients and aspect-ratio evolution in approximate models of ice crystal vapor growth

Models of ice crystal vapor growth require estimates of the deposition coefficient a when surface attachment kinetics limit growth and when ice crystal shape is predicted. Parametric models can be used to calculate a for faceted growth as long as characteristic supersaturation s_char values are known. However, previously published measurements of s_char are limited to temperatures higher than -40°C. Estimates of s_char at temperatures between -40° and -70°C are provided here through reanalysis of vapor growth data. The estimated s_char follow the same functional temperature dependence as data taken at higher temperatures. Polynomial fits to s_char are used as inputs to a parameterization of a suitable for use in cloud models. Comparisons of the parameterization with wind tunnel data show that growth at liquid saturation and constant temperatures between -3° and -20°C can be modeled by ledge nucleation for larger (hundreds of micrometers) crystals; however, comparisons with free-fall chamber data at -7°C suggest that dislocation growth may be required to model the vapor growth of small crystals (~20 μm) at liquid saturation. The comparisons with free-fall chamber data also show that the parameterization can reproduce the measured pressure dependence of aspect-ratio evolution. Comparisons with a hexagonal growth model indicate that aspect-ratio evolution based on the theory of Chen and Lamb produces unrealistically fast column growth near -7°C that is mitigated if a theory based on faceted growth is used. This result indicates that the growth hypothesis used in habit-evolving microphysical models needs to be revised when deposition coefficients are predicted.