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<br />infonned choices about when, where and how to seed winter clouds while attempting to optimize water <br />resources. <br /> <br />6.2 Specific Experimental Area and Experience Gained <br /> <br />The importance of logistical considerations in the success of physical experiments cannot be <br />overemphasized. TIle Wasatch Plateau east of Fairview and Mount Pleasant is most attractive for seeding <br />experiments because all-weather highways provide ready access to the top of the plateau. As shown on <br />figure 6-1, the highway which runs south of the DOT snowplow shed follows the upwind edge of the <br />plateau for 5 miles. This offers an almost unique opportunity for surrace sampling through seeding plumes <br />released on the windward slope; for example, from the high altitude seeding site shown of figure 6-1. <br />Both truck-mounted and stationary instruments can measure the presence of the seeding material or a co- <br />released tracer gas to detennine which portion of the highway is in seeded cloud and which portions are <br />in neighboring nonseeded cloud. The latter regions serve as "controls", documenting natural ice particle <br />concentrations, sizes, types and snowfall rates against which the seeded ("target") zone can be compared. <br />Simultaneously, similar observations can be made above the plateau by an aircraft flying to within 1,000 ft <br />of the higher terrain. The ability to rapidly monitor both seeded and nearby natural cloud with aircraft <br />and truck could provide very convincing infonnation on the specific effects of seeding. Moreover, such <br />infonnation can be collected through many stonns so the response of different cloud conditions can be <br />studied. <br /> <br />Silver iodide (and sometimes SFJ was released from the high altitude seeding site shown on figure 6-1 <br />on several occasions during the 1991 field season. This seeding site was about 1,000 ft below the plateau <br />top. The plume from this site was routinely detected with truck-mounted instruments driven back and <br />forth along the 5 miles of that parallel the west edge of the plateau (hereafter called "upwind highway"). <br />The same plume often was detected by the aircraft sampling above the windward and lee edges of the <br />plateau. Moreover, SLW frequently existed during these plume sampling missions. Thus, there is no <br />doubt that a seeding plume can routinely be transported and detected on and above the plateau during <br />prevailing westerly flow. Near-surrace winds were from the southwest for many hours prior to each <br />frontal passage, transporting the high altitude plume approximately across the middle of the upwind <br />highway segment. <br /> <br />Early in the 1991 season, prior to an accident that eliminated use of a truck-mounted Particle Measuring <br />Systems 2D-C particle imaging probe for the rest of the field program, limited sampling of ice crystals <br />was accomplished. The 2D-C probe was vane-mounted so it pointed into the resultant wind and ice <br />particles flowed between the sampling anns in the nonnal manner. Sampling was too limited for <br />conclusive results but the concept of observing both AgI and ice particles from a moving truck was <br />proven. Similar sampling by an aircraft flying crosswind through a ground-released AgI plume provided <br />convincing evidence of order of magnitude IPC increases and doubling of precipitation rates (Super and <br />Heimbach, 1988). The combination of truck sampling ofice particles on the plateau and aircraft sampling <br />above could provide irrefutable evidence of seeding effects. <br /> <br />The relatively flat terrain on top the plateau allows for good off-road travel by snowmobile or skies. It <br />is therefore practical to operate instrumentation such as precipitation gauges all along the plateau top. <br />Several gauges were maintained over the plateau during the 1991 experimental period. <br /> <br />48 <br />