Laboratory Study of the Influence of Nucleation and Chemistry on Ice Particle Habit and Growth Rates

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Despite past efforts, a number of fundamental aspects of initial ice crystal formation (termed "nucleation") in clouds remain uncertain. Previous experiments have produced scattered results partly because the nucleation mode was not well controlled and the methods used introduced artifacts. These investigators plan to eliminate such problems by developing a new two-capillary instrument in which a single crystal nucleates and grows at the tip of an ultra-thin pure-silica glass capillary within a small growth chamber that can be isolated from the vapor source. The presence of two well-separated crystals held under identical isothermal conditions will allow precise observation of the deposition and sublimation processes. Advantages of this approach over previous methods (e.g., diffusion chambers, substrate chambers, and electrodynamic balances) include greater control over the thermodynamic conditions and the ability to characterize potential crystal-crystal and crystal-substrate interactions. Experiments in the new twin-capillary chamber will be made for a range of air pressures and temperatures, including conditions relevant to cirrus formation, as well as for multiple nucleation modes. By nucleating the initial crystal in different ways, these investigators expect to determine how much of the previously observed scatter in crystal habits arose from variations in nucleation mode.The intellectual merit of this research rests on development of more precise, artifact-free data describing crystal shape ("habit") development and deposition coefficient functions as a function of environmental conditions. These results will allow improved evaluation of current crystal growth models. Broader impacts will accrue through training of a graduate student in techniques of new-instrument design and data analysis and through dissemination of results via multidisciplinary conferences and scientific journals. Ultimately these findings will contribute to improved representation of cirrus clouds in the earth's climate system. Additional outreach will include web-based distribution of high-quality video microscopy depicting crystal growth and sublimation.