Ice Nucleus Concentrations at −20C During Convective Storms

G. A. Isaac McGill University, Montreal, Quebec, Canada

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R. H. Douglas McGill University, Montreal, Quebec, Canada

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Abstract

Increases in surface ice nucleus concentrations at −20C (by a factor of 10 or higher) have been measured during precipitation from hailing and non-hailing convective storms; these increases are associated with the storm downdrafts. The intensity of the ice nucleus concentration fluctuations and the concentration at −20C are similar in both Alberta and Quebec storms. One of the seven convective storms for which measurements are presented was seeded with AgI released from an aircraft; the seeding material was subsequently detected at the surface.

If the observed higher ice nucleus concentration in the downdraft mixes with a storm updraft of 10 m sec−1, simple calculations indicate that no dramatic change would occur in the ice content of the updraft: to produce 5 gm m−3 of ice by −20C, in such an updraft, 105 times the normal background concentration of ice nuclei would be required.

Abstract

Increases in surface ice nucleus concentrations at −20C (by a factor of 10 or higher) have been measured during precipitation from hailing and non-hailing convective storms; these increases are associated with the storm downdrafts. The intensity of the ice nucleus concentration fluctuations and the concentration at −20C are similar in both Alberta and Quebec storms. One of the seven convective storms for which measurements are presented was seeded with AgI released from an aircraft; the seeding material was subsequently detected at the surface.

If the observed higher ice nucleus concentration in the downdraft mixes with a storm updraft of 10 m sec−1, simple calculations indicate that no dramatic change would occur in the ice content of the updraft: to produce 5 gm m−3 of ice by −20C, in such an updraft, 105 times the normal background concentration of ice nuclei would be required.

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