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Real-time detection of ground-released AgI showed that seeding material was routinely transported up a particular canyon when releases were made near the bottom of the canyon. Concentrations'of AgI at the up-canyon observations site (adjusted for nucleation activity at -10 OC) were, however, estimated to average only about one ice nucleus per liter. This relatively low concentration of active AgI nuclei offers a partial explanation of the observed low percentage of silver- in-snow above background at sampling sites above the canyons where AgI was released. eo SUMMARY Silver-in snow analyses and detection of ice nuclei during the UtahINOAA winter field program in 1989- 90 have produced some insights into the transport and dispersion of AgI aerosols over two of Utah's operational cloud seeding target areas. Because of the lack of accompanying detailed meteorological and cloud microphysical data, a complete explanation of the results found during this limited field effort will require further study. The results of the silver-in snow investigations at sites in the Wasatch Range and Wasatch Plateau revealed that background (nonseeded) Ag concentrations were similar to those found by Long (1984) in the Tushar Mountains of southern Utah. In addition, as Long found in Ag samples from 1983, there was little evidence of enhanced Ag concentration in snow samples from seeded periods. Possible explanations for the low frequency of above-background Ag are: I) consistently poor targeting of the AgI occurred from the predominantly valley-based generators to the high altitude target areas located east (presumably downwind) ofthe generators, 2) the 6 g h-I AgI output of the Utah operational generators was not sufficient to produce enhanced Ag concentrations, even if targeting was good. ,A comparison with theresults of the Bridger Range experiments, where seeded snow samples consistently contained enhanced Ag and where good targeting existed, supports the explanation that generator output could account for the different Ag concentration in seeded samples. The mean monthly Bridger AgI generator outp!!t exceeded Utah generator output by a factor of 20, while the Ag concentration in seeded snow samples in the Montana experiment exceeded Utah samples by factors of 10-30. However, given the relatively short periods represented by Utah samples (1-14 days compared to Bridger season,al data), one would expect a larger percentage of "hits" than was observed in the Utah target areas. The low.frequency of the above-background Ag concentrations brings into questions whether routine targeting was accomplished. Part II of this paper will address this question in more detail as will the more comprehensive transport and dispersion studies performed in the 1991 UtahINOAA program, yet to be fully analyzed. The question of seeding generator output was also addressed in the context of 11.4 percent precipitation increases for the Utah operational program suggested by statistical arialysis of precipitation data (Griffith et al. 1991). A number of optimistic assumptions were employed such as a generator effectiveness of 5 x 1013 ice nuclei per gram of AgI for the SA W zone, perfect targeting by the valley-based generators, nucleation by all potentially active AgI particles, and growth and fallout of all nucleated ice crystals onto the targets. Using these assumptions, an estimate was made of the average ice particle mass required to produce the reported precipitations increases. The average computed masses of 0.07 to 0.23 mg were much larger than typical average masses of natural snow particles documented in several locations in the western United States. It is possible (but believed unlikely) that generated ice nucleus effectiveness in . . 42 - -:.......::.,~;;,,;~~_;:~~;.~c j