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<br />were primarily used for the CRBPP but some higher remote-controlled generators were also used <br />(Elliott et al. ibid.). Cooper and Marwitz (ibid.) did not suggest any specifics as to how a better <br />seeding generator network might be designed. <br /> <br />Rangno and Hobbs (1980), in their criticism of published CRBPP results by Elliott et al. (1978), <br />noted that, "Three separate airborne sampling programs in Colorado, two of which occurred in <br />support ofCRBPP and the other in support of the Park Range Project in northwest Colorado, all <br />found negligible vertical dispersion under stable conditions." They were referring to the same two <br />aircraft sampling programs just mentioned, and results from the report by Rhea et al. (1969). <br /> <br />Rangno and Hobbs (1993), in one of their several criticisms of the Climax, Colorado, <br />experiments, upon which the CRBPP design was largely based, presented convincing evidence that <br />valley-released AgI was often transported away from rather than toward the Climax target area. <br /> <br />Super (1994) summarized the results of monitoring valley-released AgI during 12 storm days on <br />the Wasatch Plateau of central Utah. He questioned the operational program's effectiveness because <br />of the relatively warm SL W temperatures and resulting low concentrations of effective AgI ice <br />nuclei found in the SL W clouds. Another concern with the long-term Utah operational seeding <br />program, also contributing to low concentrations of effective AgI ice nuclei, was the typical 16 km <br />(10 mile) spacing between generators. Technical personnel of the operational seeding company <br />stated that about 4-5 km (2.5 to 3.1 mile) spacing would be required in order to achieve AgI plume <br />overlap (Griffith et al. 1992; Griffith 1996). The expected consequence of the significantly larger <br />16 km spacing would be wide un seeded gaps between instantaneous AgI plumes passing over the <br />mountain barriers which must reduce seeding effectiveness. <br /> <br />Six cases studies of valley-based AgI seeding over the Wasatch Plateau were described by Super <br />(1995). These were the only 1991 field season sampling periods when AgI released from eight <br />valley generators was detected at aircraft sampling altitudes over the Plateau. The spacing of these <br />generators was closer than usual, about 5-10 km (3.1 to 6.2 mile) in support of the research program <br />(Griffith et al. 1992). Silver iodide concentrations, observed at -20oC in the acoustical ice nucleus <br />counter cloud chambers, were about an order of magnitude less at lowest aircraft sampling levels <br />about 2000 ft above the Plateau top than observed atop the Plateau by an instrumented van. The <br />AgI was seldom transported up to 3300 ft above the plateau top. Aircraft cloud physics aircraft <br />sensors failed to indicate any ice particle concentration (lPC) increases during three of the six <br />experiments, when sampling level temperatures were warmer than -90C. Evidence existed that <br />seeding caused IPC increases during at least two of the three colder cloud experiments. However, <br />any associated increases in snowfall were minor, even during an experiment with heavy aircraft <br />icing in cloud cold enough for plentiful nucleation by AgI. The observational evidence from the <br />1991 field season indicated the valley-based operational seeding was usually ineffective. <br /> <br />Heimbach and Hall (1994) applied the three-dimensional, time dependent numerical model of <br />Clark and associates of the National Center for Atmospheric Research to a case study over the <br />Wasatch Plateau. Model results were in good agreement with a comprehensive set of surface and <br />aircraft observations. Both showed considerable pooling of valley-released AgI with limited <br />transport over the Plateau. The model indicated that higher elevation generator sites would have <br />been superior to the valley releases. <br /> <br />Another modeling case study over the Wasatch Plateau, reported by Heimbach et al. (1997), <br />suggested that gravity waves could transport valley-released AgI through a surface-based inversion <br />under some conditions. While the plateau top was reached by abundant AgI, measured <br /> <br />19 <br />