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<br />OcrOBER 1988 <br /> <br />ARLIN B. SUPER AND JAMES A. HEIMBACH, JR. <br /> <br />1163 <br /> <br />TABLE 3. Summary of AgI plume measurements at 2.7 km with supporting information. <br /> <br /> 2.7 km Plume <br /> Upwind Sounding BRTA <br /> Mean <br /> 2.2km 3.0km 2.7 km 2.7-3.0 km concentration Direction of <br /> dir./sp. dir./sp. dir./sp. all Width (nuclei L -I) transport <br /> -!: (K km-') <br /> Date (deg/m S-I) (degfm S-I) (deg/m S-I) az (km) (at - 20oC) (deg) <br /> 10 Jan 255/16 305/1 289/6 3.2 6.0 333 282 <br /> 6.2 231 279 <br /> 15 Jan (a.m.) 305/17 315/13 298/15 1.7 8.2 57 287 <br />r 15 Jan (p.m.) 290/10 285/14 299/13 2.0 4.8 154 284 <br /> 5.0 134 286 <br /> 4.8 88** 285 <br /> 4.8 30** 286 <br /> 19 Jan 275/14 300/15 291/1 23.5 0.3* 20,000* 295 <br /> 5.0 1256 294 <br /> 12.5 28** 271 <br /> 28 Jan (a.m.) 240/1 300/1 268/8 1.7 1.8 136 277 <br /> 2.0 275 265 <br /> 28 Jan (p.m.) 250/1 275/10 277/10 1.4 6.9 110 277 <br /> 7.2 128 270 <br /> 4.7 185 273 <br /> <br />* Doubtful due to plume meandering. <br />** Decreasing due to generator shut down. <br /> <br />~ <br /> <br />7. Summary and conclusions <br />Several aircraft sampling missions were conducted <br />during January 1985 in clouds seeded with AgI over <br />the Bridger Mountain Range of Montana. The purpose <br />of these flights was to make low-level crosswind ob- <br />servations of ice nuclei, ice crystals and supercooled <br />liquid water (SL W) over the target area of a prior cloud <br />seeding experiment, the Bridger Range Experiment <br />(BRE ). Earlier analyses of the BRE have provided <br />strong statistical suggestions that AgI seeding some- <br />times markedly enhanced snowfall. If these suggestions <br />are valid, it was reasoned that the in-cloud aircraft <br />sampling should yield physical evidence that 1) the AgI <br />was actually transported over the intended target area <br />during storm events; 2) SL W sometimes existed in the <br />region through which the AgI was transported; 3) the <br />AgI interacted with the SL W to produce additional ice <br />particles; and 4) detected enhancements in ice particle <br />concentration (IPC) should also show evidence of in- <br />creased snowfall rates. <br />Although the January 1985 observational period was <br />abnormally dry, six successful in-cloud missions were <br />flown on four days. In each case, AgI was released from <br />the southern seeding site used during the BRE, which <br />was located at 2.2 km elevation well up the windward <br />(west) slope of the Main Ridge. On each mission, the <br />AgI was consistently detected over the Bangtail Ridge <br />Target Area (BRTA) about 17 km east of the seeding <br />site. <br /> <br />The estimated AgI plume widths were usually in the <br />range of 5 to 8 km and maximum plume tops ranged <br />from 2.7 to 3.0 km. The atmosphere was absolutely <br />stable in each case, as evidenced by the fact that () e <br />increased with height. Absolute stability is typical of <br />winter storms over the Bridger Range. Mechanical <br />mixing is believed to have produced the observed dis- <br />perSIOn. <br />Also typical was the mean W to WNW flow over <br />the range during each mission, which transported the <br />AgI over the southern portion of the BR T A. This was <br />the region of suggested snowfall increases due to seeding <br />during the 1969-70 winter, when only the southern <br />seeding site was used (see Fig. 4 of SH). <br />The finding that AgI targeting can be successful is <br />quite important. While many winter orographic pro- <br />jects have used ground-based seeding, there is surpris- <br />ingly little direct evidence that the seeding material <br />was routinely transported into the intended clouds. In <br />fact, evidence from several projects seems to show that <br />this was not the case. Failure to use adequately high <br />elevation sites so that clouds were actually seeded may <br />be responsible for the inconclusive results of a number <br />of past experiments. <br />Supercooled liquid water contents varied widely <br />among the six aircraft missions. Moderate and wide- <br />spread SL W contents were found on the two missions <br />of 15 Jan. Liquid water was found upwind of the <br />Bridger Range and well above the BRTA on 19 Jan. <br />