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1152 -, I JOURNAL OF APPLIED METEOROLOGY VOLUME 27 Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide over the Rocky Mountains. Part II: Observations over the Bridger Range, Montana ARLIN B. SUPER U.S Bureau of Reclamation, Denver, Colorado JAMES A. HEIMBACH, JR.... Montana State University, Bozeman, Montana (Manuscript received 6 April 1987, in final form 14 March 1988) I ~ ABSTRACT 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 silver iodide (AgI) generator sites used in the earlier experiment was operated well up the west (windward) slope of the north-south oriented Main Ridge. Crosswind aircraft sampling was done to within 300 m above the secondary ridge target area 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 (SL W) in the region of the AgI plume. The ice particle concentration (lPC) 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 SL W 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 SL W when temperatures were cold enough for AgI nucleation. 1. Introduction The Bridger Range Experiment (BRE) was a ran- domized exploratory single-area cloud seeding exper- iment conducted in southwestern Montana during the winters of 1969-72. Super and Heimbach (1983; here- after SH) presented strong statistical suggestions from the BRE that seeding with silver iodide (AgI) released well up the west slope of the Main Ridge sometimes increased the downwind snowfall. This was supported by transport and dispersion investigations and airflow studies. The latter suggested that supercooled liquid water (SLW) should often have been produced near * Present Affiliation: University of North Carolina at Asheville, Asheville, North Carolina. Corresponding author address: Dr. Arlin B. Super, Bureau ofRec- lamation, Code D-3720, Denver Federal Center, Denver, CO 80225. @ 1988 American Meteorological Society the windward slopes of the Main Ridge during oro- graphic storms with westerly flow. SH also indicated that some of the SLW should have been nucleated by the AgI when Main Ridge crest tem- peratures were lower than about -90C, and that re- sulting ice particles should have been transported to- ward the Bangtail Ridge Target Area (BR T A) located between 10 and 20 km east of the seeding sites. Further exploratory analysis by Super (1986) sug- gested that the AgI seeding had little or no effect on BR T A snowfall during most periods, but caused very marked increases during a small proportion of the storm events. Shallow storms with warm cloud top temperatures appeared to be particularly favorable seeding candidates, but some deep storm systems with cold tops also had suggested precipitation increases. No evidence was found of decreased snowfall due to seeding. Although the post hoc statistical analysis of the BRE was very encouraging, SH recommended caution in its interpretation because of its exploratory nature and the limited physical observations obtained during the