Microphysical Evolution of Seeded Wintertime Orographic Clouds

Glaciogenic seeding with silver iodide in supercooled orographic clouds relies on the hypothesis that the seeding material will freeze some of the liquid droplets, allowing these newly formed ice crystals to grow through vapor deposition, riming, and aggregation until they are large enough to fall to the ground as snow. Here we examine airborne in-situ measurements within seeding plumes to investigate the evolution of hydrometeors in seeded orographic clouds. The data were collected during the 2017 Seeded and Natural Orographic Wintertime clouds – the Idaho Experiment (SNOWIE). We look in detail at one case for which the University the Wyoming King Air Research aircraft made seven passes through two seeding plumes as they advected over a mountain range during a 75-minute period. Outside of the plumes and prior to their first appearance, clouds at flight level were composed entirely of cloud liquid water droplets a few 10’s of microns in diameter. Within the plumes and early in their evolution, condensed water mass was split between liquid and ice. The ice crystals quickly grew to several hundred microns in diameter. Examination of particle images from within the plumes suggests hydrometeor growth is first dominated by riming, and later, after the liquid is consumed, by deposition. Ice crystals with diameters up to an exceeding 1 mm were observed throughout much of the plumes’ evolution. The persistence of such large ice crystals within 500 m of cloud top also suggests that ice crystals continued to nucleate throughout the entire observation period.