Ice Nucleus Concentrations at −20C During Convective Storms

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.