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What is the Role of Ice in Summer Rain-Showers?

Roscoe R. Braham Jr.Cloud Physics Laboratory, The University of Chicago

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Abstract

Recent observations indicate that ice pellets and snow pellets are present in most convective clouds in the Central United States by the time these clouds reach top temperatures of −10C. The attendant circumstances raise the question of whether the ice plays an active role in rain development in these clouds or whether its presence is purely incidental. The ice pellets are usually preceded by the development of liquid precipitation particles large enough to produce rain by coalescence with cloud droplets. The pellet concentrations are not related to ground-level ice nuclei concentrations. Apparently the pellets form as a result of freezing of the drops, contrary to most laboratory studies of droplet freezing. Observations can be brought into harmony by invoking the droplet splintering measurements of Mason and Maybank. The presence of numerous small ice particles in these clouds at temperatures warmer than −10C casts doubt upon the value of seeding with ice nuclei for rain inducement.

Abstract

Recent observations indicate that ice pellets and snow pellets are present in most convective clouds in the Central United States by the time these clouds reach top temperatures of −10C. The attendant circumstances raise the question of whether the ice plays an active role in rain development in these clouds or whether its presence is purely incidental. The ice pellets are usually preceded by the development of liquid precipitation particles large enough to produce rain by coalescence with cloud droplets. The pellet concentrations are not related to ground-level ice nuclei concentrations. Apparently the pellets form as a result of freezing of the drops, contrary to most laboratory studies of droplet freezing. Observations can be brought into harmony by invoking the droplet splintering measurements of Mason and Maybank. The presence of numerous small ice particles in these clouds at temperatures warmer than −10C casts doubt upon the value of seeding with ice nuclei for rain inducement.

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