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<br />. . <br /> <br />. . <br /> <br />· Growth times and trajectories of natural and augmented precipitation sized hydrometeors are <br />appropriate to intercept the intended target area. <br /> <br />Target/control evaluations of the effectiveness of the Utah operational cloud seeding program have <br />indicated apparent increases in target area precipitation in the range of 10 to 15 percent (Griffith, et at, <br />1991). Recent application of Monte Carlo techniques to these evaluations indicate the results are <br />significant. <br /> <br />APPLICATION OF PROGRAM <br /> <br />These results support the first of the three conditions listed in the Utah conceptual model (Background). <br />This is an important verification of what was assumed to be true of Utah storms based upon observations <br />made in other geographical areas. <br /> <br />Transport of valley-released silver iodide/SF6 over Utah mountain barriers has been documented. Since <br />the supercooled liquid water is predominately located at low level on the upwind slopes of mountain <br />barriers and the generators are located in valleys upwind of these barriers, the silver iodide nuclei are <br />encountering the preferred supercooled liquid water formation zones. This is also an important result <br />related to the second part of the Utah conceptual model (Background). In some cases valley-released <br />silver iodide/SF 6 is not transported over the mountain barrier. These cases generally occur when there are <br />low level atmospheric inversions. An interesting observation on some cases indicate nuclei "pool" under <br />these conditions which are sometimes subsequently scoured from the valley and transported over the <br />barrier with the passage of a synoptic feature. This may suggest that valley generators should be operated <br />under trapping inversions ahead of the passage of synoptic features. NA WC seeding criteria have <br />typically precluded operations under these conditions. <br /> <br />Location of manually operated ground generators at the mouths of canyons on the windward slopes of <br />target barriers may offer a preferred location for transport of silver iodide nuclei over the barrier when <br />transport from valley locations is ineffective. <br /> <br />The plume spread from ground based releases of silver iodide and SF6 (15 to 250) suggest that generators <br />should be located at a spacing of 4 to 5 km apart upwind of the barrier in order to achieve plume overlap. <br />The spacing currently used on the Utah operational program is on the order of 16 km. <br /> <br />Remotely controlled generators may be effective during periods when valley based generators are not <br />effective. The addition of such generators in high yield, high water value locations would offer an <br />improvement to the current Utah operational program. Such operations are substantially more expensive <br />than valley based networks thus the restriction of such remote generators to high yield/high water value <br />target locations. NA WC has installed manually operated silver iodide generators at higher elevation areas <br />where local resid~nts can be located to operate the units. <br /> <br />, . <br /> <br />The improvement in efficiency of the NA WC manual silver iodide generator, as documented in the CSU <br />tests, is ~ important result. The supercooled liquid water detected in Utah winter storms is frequently in <br />the 0 to -10 0 C range. It is in the -6 to -10 0 C range that the recent CSU tests indicated improved <br />efficiency over earlier tests. <br /> <br />Information from the Utah AMP suggest higher concentrations of seeding material are desirable. A <br />change from a 2% to a 3% by weight mixture of silver iodide in acetone along with the denser spacing of <br />seeding generators would provide a means of increasing these concentrations. <br /> <br />69 <br /> <br />L <br /> <br />