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<br />3. TRANSPORT AND DISPERSION OF GROUND-RELEASE_D <br />SEEDING AGENTS AND TRACER GAS <br /> <br />3.1 Background Information <br /> <br />Once the generalized distribution of SL W is known over a mountain barrier, it is of obvious interest to <br />consider the transport and dispersion of seeding material to determine when, where, and in what <br />concentrations the seeding material creates ice particles within the SL W cloud. Such transport and <br />dispersion investigations were the second major research objective ofthe NOAA/Utah AMP. <br /> <br />The Utah operational seeding program has used AgI for many years, and that seeding material was given <br />priority in the Plateau experiments. Liquid propane seeding, which can create ic~ crystals at higher <br />temperatures than AgI, was also given significant attention, since it was found that much of the SL W was <br />too wann for AgI to be effective. Sulfur hexafluoride tracer gas was often used to simulate seeding <br />material transport and dispersion because it can be detected in small concentrations by fast-response <br />instruments. <br /> <br />Unless otherwise stated, AgI in this report refers to the aerosol produced by burning AgI-NH4I-acetone- <br />water solutions in seeding generators. This solution has been used in the Utah operational seeding <br />program, and it was used during the experiments discussed herein, including use of high altitude <br />generators. This aerosol is known to operate by the contact nucleation mechanism, known to be a <br />relatively slow process in typical orographic clouds that have limited cloud droplet concentrations. <br /> <br />3.2 Field Investigations of Transport and Dispersion <br /> <br />Super and Huggins (1992a, 1992b) considered the targeting of ground-released AgI during the field <br />. program of early 1990 from three different approaches. Silver-in-snow analysis was done with bulk snow <br />samples from 10 sampling sites that should have been affected by the operational seeding program. There <br />was little evidence that snow silver concentrations from seeded periods were greater than from nonseeded <br />periods. These results were similar to earlier findings from the Tushar Mountains, but quite different <br />from some projects in other states where seeding clearly increased snow silver concentrations by about an <br />order of magnitude (e.g., Super and Heimbach 1983). <br /> <br />Real-time ice nucleus sampling was conducted well up Big Cottonwood Canyon while AgI releases were <br />made with two generators in and above the canyon mouth. The AgI was routinely observed at the surface <br />sampling location, which was about 500 m higher in elevation than the highest AgI generator. However, <br />estimated AgI concentrations were small at prevailing SL W cloud temperatures. <br /> <br />11 <br /> <br /> <br />r <br />! <br />I <br />I <br />, .. <br />[ <br />f <br />! <br />~ <br /> <br />The third approach reported by Super and Huggins (1992b) used aircraft sampling of ground-released AgI <br />and SF 6 during near-storm prefrontal conditions. Sampling was during times when VFR (visual flight <br />rules) flight operations were permissible, allowing sampling near the terrain. Four of the five aircraft <br />missions ofthis type showed the AgI and tracer gas were confmed to the lower atmosphere and were not <br />transported over the Plateau. Plumes were found overthe Plateau during part of the fifth mission, but <br />estimated ice nucleus concentrations at prevailing cloud temperatures were quite limited. <br /> <br />Griffith et al. (1992) reported on IFR (instrument flight rules) aircraft sampling during the early 1991 <br />program. Two case studies were selected for detailed discussion. Tracer gas released at the mouth of a <br />