Further Analysis of a Snowpack Augmentation Program using Liquid Propane

Abstract

The California Department of Water Resources, with technical assistance from the Bureau of Reclamation, is continuing to pursue the use of liquid propane as a viable agent for seeding winter clouds. The tracer SF6, sulfur hexafluoride, is used to simulate the transport and dispersion of propane-generated ice crystals. Sulfur hexafluoride was released from two separate high-altitude propane dispenser sites as a proxy for seeded ice crystals. Aircraft measurements of SF6 indicated that at the normal flight altitudes of 2500 m over the valley and 2680 m over the downwind ridge, the aircraft was flying near the top of the plumes. When the aircraft was able to fly below cloud base near the release altitude of 2200 m, substantial SF6 was observed. A portion of the plume was observed to rise to elevations about 500 m above the release altitude. The lower portion of the plume was also observed to descend into the valley some 600 m below the release altitude. Comparison of aircraft measurements with model predictions of plume (Gaussian) horizontal dispersion compare favorably. The model verifies that the vertical dispersion of the plume is limited to only 300 to 500 m above the dispenser altitudes. Gravity waves are shown to have a significant impact on the transport of tracer/seeded crystals across the target area. The aircraft intersected 35 seeded plumes on 4 separate days. Only 11 of the 35 plumes indicated seeding effects. The lack of liquid water was considered the main reason why most plumes did not indicate seeding effects. On February 17, distinct seeding effects were noted on five of six plumes intersected from one of the sites. Apparent dynamic seeding effects were also noted on several of these passes. Precipitation at a gauge directly in line with the aircraft-observed seeding plume showed a factor of five increase in precipitation over surrounding gauges for the hour in which seeding effects would have been expected. Seeding effects were also noted on March 16, a day when the temperature at the dispenser was near or slightly above freezing.