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

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. A recently completed field measurements campaign, SNOWIE, provides the first-ever direct, unambiguous observations confirming this chain of events following the introduction of AgI into a supercooled orographic cloud. Two cases were examined and highlighted in recent publications by French et al. (2018) and Tessendorf et al. (2018).

This work extends those earlier studies by expanding the number of cases examined (only two cases were presented in the earlier work). Here, the microphysical evolution within AgI seeding plumes during SNOWIE is explored. 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), an aircraft specially equipped with instruments to document thermodynamic and microphysical parameters in cloud. Measurements from an airborne, W-band radar (Wyoming Cloud Radar; WCR) mounted on the UWKA provide additional evidence for identifying regions within seeding plumes whose microphysical characteristics have been affected by the presence of AgI. A detailed examination of the microphysical characteristics within seeding plumes is presented, including droplet concentration, ice and liquid water content, and the size, concentration, and habit of precipitating hydrometeors. A comparison of these characteristics is made in regions before, during, and after the UWKA enters the seeded plume to demonstrate how the seeding material affects these properties.

French, J. R., and co-authors, 2018: Precipitation from Orographic Cloud Seeding. Proc. Natl. Acad. Sci., 115 (6), 1168-1173

Tessendorf, S. A., and co-authors, 2018: A Transformational Approach to Winter Orographic Weather Modification Research: The SNOWIE Project. Bull. Am. Meteorol. Soc. In Press