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Application of Ice Nucleation Kinetics in Orographic Clouds

Rochelle R. BlumensteinDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Robert M. RauberDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Lewis O. GrantDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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William G. FinneganDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Abstract

Ice nucleation by silver iodide-sodium iodide aerosol particles has been characterized in the Colorado State University isothermal cloud chamber using the techniques of chemical kinetics. Two separate mechanisms of condensation-freezing ice nucleation have been observed. One mechanism occurs at water saturation and is a characteristically slow process, with a half-life of the order of 10–30 min. The other mechanism occurs when the environment is supersaturated with respect to liquid water. This mechanism is characteristically fast, requires less than a minute for completion, and results in a higher yield of ice crystals than the slow mechanism.

The mechanism, rate and yield data obtained in the laboratory investigations are applied to an orographic cloud particle trajectory model to assess the ice nucleation characteristics of silver iodide-sodium iodide aerosol particles in the temporal and spatial scale of an orographic cloud. The importance of nucleation mechanism, rate and yield are investigated to determine the control these parameters have on the extent and location of ice nucleation within the cloud and the effect on precipitation distribution. In certain conditional ice crystal production was found to be prolonged over time and space. Resulting precipitation occurred over large areas. In other conditions, ice nucleation occurred primarily within a zone of a few kilometers. Precipitation was then found to occur in a more restricted area. The mechanism and rates of nucleation therefore can affect the targeting and analysis of seeding effects in weather modification experiments.

Abstract

Ice nucleation by silver iodide-sodium iodide aerosol particles has been characterized in the Colorado State University isothermal cloud chamber using the techniques of chemical kinetics. Two separate mechanisms of condensation-freezing ice nucleation have been observed. One mechanism occurs at water saturation and is a characteristically slow process, with a half-life of the order of 10–30 min. The other mechanism occurs when the environment is supersaturated with respect to liquid water. This mechanism is characteristically fast, requires less than a minute for completion, and results in a higher yield of ice crystals than the slow mechanism.

The mechanism, rate and yield data obtained in the laboratory investigations are applied to an orographic cloud particle trajectory model to assess the ice nucleation characteristics of silver iodide-sodium iodide aerosol particles in the temporal and spatial scale of an orographic cloud. The importance of nucleation mechanism, rate and yield are investigated to determine the control these parameters have on the extent and location of ice nucleation within the cloud and the effect on precipitation distribution. In certain conditional ice crystal production was found to be prolonged over time and space. Resulting precipitation occurred over large areas. In other conditions, ice nucleation occurred primarily within a zone of a few kilometers. Precipitation was then found to occur in a more restricted area. The mechanism and rates of nucleation therefore can affect the targeting and analysis of seeding effects in weather modification experiments.

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