Seminar

Abstract

Quantifying the mass growth of ice crystals from the vapor phase is a challenging problem, especially at temperatures below about -30C. At low temperatures, there are indications that surface processes substantially reduce crystal vapor growth rates. The strength of surface kinetic resistance is often represented by the deposition coefficient, which is defined as the ratio of vapor molecules that incorporate into the crystal and contribute to mass growth to the total number of vapor molecules that impact the surface. Relatively few laboratory studies of the deposition coefficient have been done at temperatures below -30C, but those that have either produce high deposition coefficients (above 0.1) or low values (below 0.01). Measurements of ice crystal mass growth in our levitation diffusion chamber produce low values of the deposition coefficient (~ 0.01) when an artificial nucleating agent is used whereas a range of deposition coefficients (~ 0.001 to 1) are produced when crystals are formed from pure water. Our measurements also suggest that growth can become substantially inhibited when the crystal growth temperature is changed. The consequences of our results for parameterizing ice crystal growth in numerical models will also be discussed.

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Jerry Harrington is a Professor of Meteorology in the Department of Meteorology and Atmospheric Science at Penn State University.