Investigations of the Targeting of Ground-Released Silver Iodide in Utah Part I: Ground Observations of Silver-in-Snow and Ice Nuclei

Abstract

During the winter of 1989-90, the Utah Division of Water Resources, the National Oceanic and Atmospheric Administration, and the Bureau of Reclamation cooperated in a limited sampling project to investigate the transport and dispersion of silver iodide (AgI) cloud-seeding aerosol over two target areas in the mountains of Utah. Seeding was done using the ground-based AgI generator network of the Utah operational cloud seeding program. Transport and dispersion over the Wasatch Range and Wasatch Plateau were evaluated using a silver-in-snow sampling technique and the real-time detection of AgI aerosol or sulfur hexafluoride gas. This report contains an extensive review of past silver-in-snow results from several different regions as a basis for comparison with the current study. The 1989-90 Utah results indicated that a low percentage (< 15 percent) of bulk snow samples from 10 mountain target area locations had silver (Ag) concentrations above background values for periods when seeding had been conducted. Consistently poor targeting and/or low seeding generator output could explain the general lack of detectable Ag in the two Utah target areas. Seeding generator output also forms the basis for estimates of average ice particle masses required to achieve the greater than 11 percent snowfall increase reported from statistical analyses of the Utah operational program. These estimates are based on very optimistic assumptions (perfect targeting, 100 percent nucleation, 100 percent fallout, etc.). They indicate that snowfall enhancements of 10 percent or greater are unlikely with the current AgI seeding rate of 6 grams per hour used in Utah. Real-time detection of ground-released AgI showed that seeding material was routinely transported up a particular canyon when releases were made near the bottom of the canyon. Concentrations of AgI at the up-canyon observation site (adjusted for nucleation activity at -10°C) were, however, estimated to average only about one ice nucleus per liter. This relatively low concentration of active AgI nuclei offers a partial explanation of the observed low percentage of silver-in-snow above background sampling sites above the a canyons where AgI was released.