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 alpha when surface attachment kinetics limit growth and when ice crystal shape is predicted. Parametric models can be used to calculate alpha for faceted growth as long as characteristic supersaturation values are known. However, previously published measurements of are limited to temperatures higher than -40 degrees C. Estimates of at temperatures between -40 degrees and -70 degrees C are provided here through reanalysis of vapor growth data. The estimated follow the same functional temperature dependence as data taken at higher temperatures. Polynomial fits to are used as inputs to a parameterization of alpha 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 degrees and -20 degrees C can be modeled by ledge nucleation for larger (hundreds of micrometers) crystals; however, comparisons with free-fall chamber data at -7 degrees C suggest that dislocation growth may be required to model the vapor growth of small crystals (similar to 20 mu m) 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 degrees 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.