The Origin of Ice in Mountain Cap Clouds

William A. Cooper University of Wyoming, Laramie 82071

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Gabor Vali University of Wyoming, Laramie 82071

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Abstract

Ice crystal development in relatively simple layer clouds was studied using airborne instrumentation. The patterns in the development of ice in those clouds suggest that the ice originates in association with the initial condensation process, near the upwind edge of the cloud. Since continued ice production does not occur beyond that region, the ice development can be attributed to nucleation. There is no evidence for secondary ice generation. Either condensation-freezing or contact nucleation could account for the observed nucleation process, but special properties are required for the nuclei in either case. Ice crystal concentrations show a clear temperature trend, as expected for a nucleation process.

Abstract

Ice crystal development in relatively simple layer clouds was studied using airborne instrumentation. The patterns in the development of ice in those clouds suggest that the ice originates in association with the initial condensation process, near the upwind edge of the cloud. Since continued ice production does not occur beyond that region, the ice development can be attributed to nucleation. There is no evidence for secondary ice generation. Either condensation-freezing or contact nucleation could account for the observed nucleation process, but special properties are required for the nuclei in either case. Ice crystal concentrations show a clear temperature trend, as expected for a nucleation process.

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