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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />t <br />t <br />. <br />t <br />t <br />t <br />t <br />. <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />. <br />t <br />t <br /> <br />Similar poor targeting was reported for two large target areas in the Sierra Nevada by Warburton et <br />ai. (1995). No more than 20 percent of the precipitation falling in the Truckee- Tahoe target of the <br />central Sierra Nevada during seeding over several winters contained "seeding silver" greater than <br />the natural background value. Similar results were found for all wind directions in the Lake <br />Almanor region of northern California. While 42 percent of samples were above background for <br />westerly winds, the corresponding value for southerly winds was only 8 percent. Moreover, AgI <br />concentrations above background were found in presumed control areas with southerly flow, <br />evidence of mistargeting. <br /> <br />Not all seeding programs have resulted in poor targeting. For example, seasonal samples were <br />obtained during the Bridger Range Experiment (Super and Heimbach 1983) from snow pits dug <br />prior to snowmelt. These samples were spaced over the expected target area, a secondary ridge <br />about 5 to 11 miles downwind of the seeding generators. Silver levels ranged from 30 to 100 <br />ppt, well above the 10 ppt detection limits. About 40 to 70 percent of the total silver emitted by <br />the project's ground-based seeding generators during hours having snowfall in the target was <br />contained in the seasonal target snowpack above 6,000 ft elevation. A later, one-winter <br />operational seeding program with "piggy-backed" research provided further evidence of <br />significantly enhanced silver levels in the target snowfall (Heimbach and Super 1988). These <br />trace chemistry findings supported other reported physical and statistical results reported by <br />Super and Heimbach (1983, 1988). A key feature of the Bridger Range seeding was the use of <br />high altitude AgI generators, located over 2/3 of the way from the upwind valley floor to the <br />crest line. Generators were manually operated and their functionality frequently verified so that <br />their reliability was high. Aircraft plume tracing reported by Super (1974) for near-stonn <br />conditions showed routine targeting with these generators, later verified by in-cloud sampling <br />reported by Super and Heimbach (1988). <br /> <br />Several short-tenn physical experiments have been conducted with high altitude generators or <br />propane dispensers. All have shown routine transport and dispersion of the seeding agent and <br />seeded ice crystals over mountain crest lines. For examples, see Holroyd et ai. (1988) for the <br />Grand Mesa of Colorado, and Super (1995) for the Wasatch Plateau of Utah. Additional plume <br />tracing cases studies have been published for both locations, including favorable comparisons de y <br />with a sophisticated numerical model (Clark Model) in Utah (e.g., Heimbach et ai. 1997, 1998). Ch', <br />Another important finding from these studies was that ground-released AgI plumes remained ~ J....(J <br />relatively close to the mountainous terrain, at least in the absence of convection. Plumes often <br />ascended no more than about 1500 feet above the typical higher terrain. Seeding plumes in low <br />concentrations were sometimes found 2000 feet above mountain barriers, but were seldom <br />transported as high as 3000 feet above the mountains. Horizontal plume widths of about 15 <br />degrees were typical in all locations studied. Fortunately, mountain-induced SL W is usually <br />confined to the same approximate altitude range over windward slopes and crest lines, so high <br />altitude seeding releases will be transported into the SL W zone. In summary, there is <br />considerable evidence that seeding agents released from high altitude sites up windward slopes <br />usually reach the SL W zone, necessary for seeding to be effective. <br /> <br />Most of the research results cited in the above were from winter orographic cloud seeding <br />programs designed to increase snowfall. The operational programs being conducted in Santa <br />Barbara County are somewhat of a hybrid situation; the goal is typically to increase precipitation <br />Weather Damage Modification Program 25 <br />