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.- ... These Wasatch Plateau experiments had a relatively simple design involving pulsed seeding from an elevated site on the upwind slopes of a major plateau. The local terrain channeled the plume towards the target 4.2 km away at the windward edge of the crest with good probability of actually hitting that instrumented target. Admittedly, observations taken over the middle or downwind portion of the Wasatch Plateau top likely would have demonstrated increased growth of seeded crystals and associated increases in seeded snowfalL However, such a design, while providing a better simulation of operational seeding from high altitude sites, would greatly reduce the odds of proper targeting. The pulsed seeding style, with AgI tags for the liquid propane cases, allowed numerous experiments during most storm periods. Even so, natural variability of snowfall made it challenging to detect the results of seeding. Periods with none to trace natural snow did show obvious seeding effects. Periods with an abundance of snow or strong showers tended to mask the seeding signaL The large natural crystals 1) dominated the precipitation gage measurements, 2) presumably swept out many of the small seeded crystals through aggregation, hereby lessening their delectability by the 2D-C, and 3)' consumed orographic supercooled liquid water (SL W), thereby starving the growing seeded embryos. . The experiments described by Holroyd et al. (1988) showed that Agl released from similar sites on the Grand Mesa were easily detectable at aircraft levels from an abundance of small, growing ice particles. Those experiments had a more complex design (Super, et al., 1988) and were more expensive to carry out in terms of equipment and manpower. Super and Boe (1988) describe some of those experiments during a period of little natural snowfall. Seeding effects were readily observable at aircraft levels, but observations at the surface had variable success in demonstrating the presence of the seeding plume. They suggested that in the "failed" experiments the embryo ice crystals did not have sufficient residence time in the presence of SL W to grow to sizes that would fall to the surface. The Bridger Range experiments described by Super and Heimbach (1988) used aircraft to detect ground-released AgI seeding effects. They were present when SL W was available and absent when it was minimal. It therefore appears that detection of seeding effects is strongly influenced by the amount oftime that seeded crystals spend in the presence of SL W. The competition for that SL W by larger natural snow particles and the sweeping action (aggregation) whereby natural crystals remove seeded embryos from the air mean that there may be little or no seeding affects to detect at the surface if there is an abundance of natural snow. In conclusion, it remains highly desirable to further physically document the effectiveness of winter orographic Cloud seeding under various conditions. However, this goal remains challenging even with sophisticated equipment and abundant manpower. Progress has been made under NOAA's Atmospheric Modification Program. However, it seems that future progress is physically verifying weather modifications effectiveness will depend on other sources of support, at least in the near-term. 8.28. Heimbach, J. A., A. B. Super, and W. D. Hall, 1998: Modeling AgI targeting effectiveness for five generalized weather classes in Utah. J. Weather Modification, 30, 35-50. ABSTRACT . . Rawinsonde observations from the 1991 and 1994 Utah/NOAA field programs were stratified into five classes based on temperature profiles. The classed soundings were used to initialize the Clark mesoscale model to simulate the AgI transport from three operational generator sites in the valley upwind of the Wasatch Plateau in central Utah. The goal was to generalize ranges of conditions which would allow successful targeting of valley-released Agl. Not unexpectedly, the most unstable sounding class produced the best targeting. This class was the coldest of the five, producing more effective ice nuclei from the available Agl because of the temperature dependence of this ice nucleating agent. In general, the modeled '., 83 '. ~'-;