10.6 Estimates of Variable Deposition Coefficients and Ice Crystal Growth Transitions Inferred from Laboratory Measurements of Vapor-Grown Ice Crystals

Wednesday, 11 July 2018: 2:45 PM
Regency D (Hyatt Regency Vancouver)
Gwenore Pokrifka, Pennsylvania State University, University Park, PA; and M. Hanson, J. Y. Harrington, and A. M. Moyle

Prior measurements of the formation and vapor growth of small ice crystals at temperatures below -30 °C produce estimated deposition coefficients that are either very low (< 0.01) or are high (> 0.2). In this study, we present measurements of the mass growth from the vapor of ice particles formed both homogeneously and heterogeneously in a levitation diffusion chamber at temperatures between -43.4 and -40.2 °C. The data are analyzed using a diffusion-kinetics model to predict the particle deposition coefficients, and it is found that the model cannot replicate some of the measured growth time-series. Thus, two new analysis methods are developed that expose growth features that are masked by a standard time-series analysis. The mass growth of the heterogeneously formed ice crystals can be fit with a classical, ledge growth model and a deposition coefficient that varies weakly with size. The deposition coefficient is predicable and ranges from 0.005 to 0.2. The mass growth of only half of the homogeneously frozen ice crystals can be fit with a ledge growth model, with deposition coefficients ranging from 0.018 to 0.5. The mass growth of the other half of the homogeneously frozen ice crystals cannot be fit with a classical, ledge growth model. These particles require a growth transition from an efficient state of growth to an inefficient state, and a rapid decrease in the deposition coefficient with size. These results are suggestive of a surface transition occurring on the homogeneously frozen particles during growth. A simple growth model is proposed for these particles and it is shown that the model produces an accurate fit to the measured particle growth rates. Since growth transitions cause the deposition coefficient to change rapidly, they may be responsible, in part, for discrepancies in past deposition coefficients estimated from the mass growth rates of small ice crystals.
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