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1160 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 over the entire state and beyond, with some cloud top temperature below -50oC near the Bridger Range. Snow began in the early morning and diminished dur- ing the afternoon. Throughout the morning, skies were obscured over the seeding site. Ice crystal clouds over the BR T A permitted intermittent visual ground contact from the aircraft. Due to air traffic restrictions, the morning climbout sounding reached only the 3.7 km level (-190C). Par- ticle concentrations recorded by the 260X probe varied from a few to 20 L -1 up to 3.7 km. No icing rate data were available at the Crest Observatory on this day due to an equipment malfunction. The AgI generator suf- fered from slag formation in the combustion chamber throughout the mission, so AgI output was reduced by an unknown amount. North-south passes over the BRTA detected no SL W at the 3.6 and 3.0 km levels, and the IPC generally varied from 2 to 10 L -1 with peaks of 15 L -I . Mean SL W amounts detected within 5 km of the SSL on the first six passes at 2.7 km altitude were zero, while four subsequent passes averaged 0.01-0.02 g m - 3. Narrow, transitory peak values of 0.1 g m -3 were re- corded near the SSL (over the highest Bangtail Ridge terrain) during the latter passes. Observed AgI counts at 2.7 km altitude were lower than usual, probably due to the generator difficulties, and totals ranged from 12 to 83 on the ten passes. A single pass at 3.0 km altitude recorded only 5 counts. Plume position was consistent with the observed westerly flow. The seeded zone could not be defined by enhanced IPC as on 10 and 15 Jan, but a central seeded zone was defined by the mean AgI plume edge positions at 2.7 km. It was compared with control zones from 3 to 6 km crosswind. No significant differences in IPC, crystal sizes or habits were apparent, and it was con- cluded that seeding had no significant effect on the region sampled. This is not surprising in view of the essentially all ice crystal cloud that existed over the BRTA. f 28 Jan, p.m. The aircraft climbout sounding observed cloud top at 4.7 km and -270e. Trace SLW amounts were en- countered from 3.6 to 3.8 km and 4.6 to 4.7 km. Ice particle concentrations from the 260X probe ranged from 1 to 10 L -1. No SLW was detected during BRTA passes at 3.4 and 3.0 km altitudes, but IPC varied from 10 to 40 L -1 , and once exceeded 60 L -1 . A weak AgI plume was observed at 3.0 km. During subsequent 2.85 km passes, AgI was detected 2-6 km south of the SSL, and IPC remained highly variable, ranging from 5 to 70 L -1 . Except for a very few transient SL W zones, the cloud consisted of ice crystals. Six passes at the 2.7 km level consistently found the strong AgI plume posi- tioned where expected, considering the observed west- erly wind. Infrequently, small SLW pockets were en- countered but were transitory in the presence of natural IPC from 5 to 50 L-1. No microphysical changes were observed within the seeded cloud over the BRTA. At the levels sampled, the limited SL W was apparently being rapidly depleted by the high natural IPC. One can only speculate on what transpired nearer the windward slopes of the Main Ridge and Bangtail Ridge, where SL W production might have been greater. 5. Summary of physical observations and comparison with earlier statistical analyses The six in-cloud sampling missions are summarized in Table 1, which shows that the atmosphere was ab- solutely stable in each case. As reported by SH, absolute stability was usually observed during the BRE. As will be shown in section 6, flow was W to WNW during the January 1985 experiments, which is also typical of the BRE. The seeding material was routinely observed on all flights at the lowest 2.7 km sampling level over the target area but was rarely detected above the 3.0 km level. This is consistent with earlier VFR plume ob- servations over the Main Ridge. Clearly, high altitude ground-based seeding is capable of providing AgI to clouds over both the Main Ridge and downwind Bang- tail Ridge during westerly flow. Mechanical turbulence is believed to provide the dispersion mechanism in the stable atmosphere. Table 1 shows that each of the six missions had a Crest Observatory temperature of -90C or lower, thereby satisfying one criterion of SH for seeding to be effective. Three of the missions had SL W present in the seeded zone over the BR T A. Obvious seeding sig- natures were found in these cases with considerable enhancement of the IPC and apparent increases in pre- cipitation. Any decreases in SL W that may have re- sulted were masked by spatial and temporal variability in SL W content. The other three cases, with virtually no SL W in the seeded zone, had no apparent micro- physical changes associated with the AgI plumes. This is in agreement with the physical hypothesis discussed by SH and stated in Part I. An abundance of hexagonal plates were found in the seeded zones of 10 and 15 Jan. These were appro- priate for the temperature and moisture regimes ob- served, presuming they were caused by the AgI. Most of these ice crystals were less than 0.6 mm in size, but some grew to the 1.0-1.6 mm range. Estimates of precipitation rates were made using Holroyd's (1987) computerized scheme applied to the 2D-C images, which should be used with caution be- cause of the limitations discussed in Part I. However, the estimates suggest at least a doubling of precipitation due to seeding on 10 and 15 Jan. Such large percentage increases are consistent with the statistical findings of ~ "