Extended Abstract (2.8M)P1.30
Laboratory measured ice crystal capacitances and mass dimensional relations
Matthew Bailey, DRI, Reno, NV; and J. Hallett
Capacitance theories of ice crystal growth continue to receive attention even though they are unsupported by laboratory or in situ measurements. Recently the details of atmospheric ice crystal habits have been clarified, and they reveal that the symmetric shapes addressed in current capacitance growth models constitute a few percent of ice crystals under nearly all atmospheric conditions, with irregularly shaped polycrystals and distorted hexagonal plates and columns dominating. Mass growth rates have been carefully measured and used to calculate the capacitances of both simple hexagonal and complex polycrystalline habits for temperatures from -20 C to -70 C under simulated in situ conditions of ice supersaturation and vapor diffusivity (pressure). The most stringent test of the capacitance theory is provided by the analysis of individual isolated crystals grown at low ice supersaturation. In these and nearly all other cases of simple hexagonal shapes with which theoretical capacitance calculations can be compared, theoretical values are much higher than the measured capacitances, and only rarely come close to the theoretical prediction. Real ice crystals typically exhibit a high degree of imperfections and defects, factors that result in non-uniform variable vapor densities over irregularly growing surfaces, resulting in a resistance to vapor diffusion growth. Additionally, mass dimensional relations have been determined for the same crystals and growth conditions. While these show agreement with some past and recent estimates, they also show disagreement in many cases. Variability in measured mass dimensional relations show that crystals with the same habit and maximum dimension, but grown at different temperatures and growth rates, can have significantly different masses, i.e. effective crystal density is dependent on temperature, vapor diffusivity, and ice supersaturation.
Poster Session 1, Cloud Physics Poster Session 1
Monday, 28 June 2010, 5:30 PM-8:30 PM, Exhibit Hall
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