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

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.