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MS Defense

Department of Atmospheric Science

Thurs., July 11, 2:10 pm, EN6085

Microphysical impact of cloud seeding on wintertime orographic clouds observed during SNOWIE

Melinda Hatt

University of Wyoming

Abstract

Water supplies in the western U.S. rely heavily on mountain snowpack. Winter orographic cloud seeding is one potential approach to alleviate increasing demands on water supplies by increasing the amount of snowfall in mountain regions. The hypothesis behind glaciogenic cloud seeding is that by introducing an ice nucleating particle, in this case silver iodide (AgI), into a cloud containing supercooled liquid water, ice crystals will form and grow to sizes large enough to fall as precipitation, thus increasing the amount of precipitation fallout. SNOWIE, a recently completed field measurements campaign, provides the first-ever direct, unambiguous observations confirming this chain of events following the introduction of AgI into a supercooled orographic cloud. The analysis presented here investigates four cases during SNOWIE and analyzes in-situ measurements taken within seeded and non-seeded regions of cloud to evaluate the microphysical impact of cloud seeding, and examines the evolution of these characteristics within seeded plumes. A simple advection model is developed and used to predict the location of AgI plumes downwind of the release location. The model output is overlaid on flight tracks from the University of Wyoming King Air (UWKA) to determine when the aircraft likely passed through seeded plumes. Each of the cases presented show indication of seeding impact from radar reflectivity measurements by the Wyoming Cloud Radar and in-situ measurements from instruments on board the UWKA. An increase 10 L-1 or more of ice crystal concentration, or 0.1 g/m3 of ice water content, was measured within regions where the AgI plumes were expected to be located based on the advection model compared to the natural cloud, giving indication that ice was produced as a result of cloud seeding. Many of these regions also displayed an increase in ice content by roughly 0.1 g/m3 over the 40- to 90-minute observation period, along with an increase in particle size, suggesting that these ice crystals grew in size after their initial formation. In many instances, decreasing liquid water measurements to near zero imply that the liquid within these regions was depleted by the growing ice crystals through riming and deposition. Radar measurements showing an increase in reflectivity near the surface in these regions over time indicate that these large ice crystals fell toward the surface. These observations together imply that the ice crystals that were formed as a result of cloud seeding grew to sizes large enough to fall as snow.

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