Ice Crystal Production by Mountain Surfaces

David C. Rogers Department of Atmospheric Science, University of Wyoming, Laramie, WY 82071

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Gabor Vali Department of Atmospheric Science, University of Wyoming, Laramie, WY 82071

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Abstract

Evidence is presented for a process of ice crystal generation in supercooled orographic clouds in contact with snow-covered mountain surfaces. Comparisons of the crystal concentrations at the surface with aircraft sampling indicate that the “anomalous” crystals originate at the interface of the cloud with the surfaces. Crystal concentrations at the surface, over the temperature range −5° to −23°C, were found to be roughly 100 times higher than in the main body of the clouds. Occasionally, the effects extends to altitudes as much as 1 km above the ground in the clouds studied, and indications are that even greater depths of clouds might be influenced over extended mountain ranges. The mechanism of ice crystal generation involved has not yet been firmly established; several possibilities are discussed in the paper. The phenomenon can be expected to have significant implications for the characteristics of low-altitude orographic clouds with respect to their propensity to produce precipitation; radiative, chemical and electric properties; and their suitability for cloud seeding.

Abstract

Evidence is presented for a process of ice crystal generation in supercooled orographic clouds in contact with snow-covered mountain surfaces. Comparisons of the crystal concentrations at the surface with aircraft sampling indicate that the “anomalous” crystals originate at the interface of the cloud with the surfaces. Crystal concentrations at the surface, over the temperature range −5° to −23°C, were found to be roughly 100 times higher than in the main body of the clouds. Occasionally, the effects extends to altitudes as much as 1 km above the ground in the clouds studied, and indications are that even greater depths of clouds might be influenced over extended mountain ranges. The mechanism of ice crystal generation involved has not yet been firmly established; several possibilities are discussed in the paper. The phenomenon can be expected to have significant implications for the characteristics of low-altitude orographic clouds with respect to their propensity to produce precipitation; radiative, chemical and electric properties; and their suitability for cloud seeding.

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