Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide over the Rocky Mountains. Part II: Observations over the Bridger Range, Montana

During January 1985 six aircraft sampling flights were made in cloud over the target area of an earlier randomized exploratory cloud seeding experiment in the Bridger Range, Montana. One of the two diver iodide (AgI) generator sites used in the earlier experiment was operated well up the wed (windward) slope of the north-south oriented Main Ridge, Crosswind aircraft sampling was done to within 300 m above the secondary ridge target about 17 km downwind of the AgI generator. The AgI plume was detected over the target area on each of the six missions and was generally 5–8 km wide. Three of the missions detected supercooled liquid water (SLW) in the region of the AgI plume. The ice particle concentration (IPC) averaged about an order of magnitude higher in the seeded zone in these cases, and the estimated precipitation rate was greater, as compared with crosswind control zones. Most seeded ice particles were small hexagonal plates, appropriate for the prevailing temperatures and moisture conditions. The AgI generator was deliberately turned off in one of the experiments. and the seeding effects decreased with time beginning about one hour later. The other three missions sampled negligible SLW in the seeded region over the target area. Observations did not indicate detectable changes in ice particle concentrations, sizes or habits. The results of this series of physical experiments are in agreement with statistical suggestions from the earlier randomized experiment. It appears that seeding the stable orographic clouds over the Bridger Range sometimes caused marked increases in IPC, presumably leading to more surface snowfall. The physical observations indicate that enhanced IPC was largely dependent upon the availability of SLW when temperatures were cold enough for AgI nucleation.