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.. .~ 8.20. Heimbach, J. A., and W. D. Hall, 1996: Observations and modeling of va /ley-released silver iodide seeding over the Wasatch Plateau. 13th Conference on Planned and Inadvertent Weather Modification, Atlanta, GA, American Meteorological Society, 31-37. . . ABSTRACT This. paper examines the transport of surface-released silver iodide (AgI) over the Wasatch Plateau of central Utah during inversion conditions. Other case studies of transport have been reported for the Wasatch Plateau. For example, Super and Huggins describe targeting of valley- and canyon-released AgI in an analysis of surface observations (1992a) and aircraft observations (1992b) from the 1989-90 field season. Surface silver-in-snow and real-time ice nucleus (IN) measurements indicated that AgI was. transported up the canyon 'over the Wasatch Plateau but in limited amounts. Co-released sulfur hexafluoride SF 6 was detected by the aircraft for one of five flights. These two papers. highlighted the difficulty in targeting seeding material. Heimbach and Hall (1994) modeled'various release _configurations in a case which had neutral stability and no imbedded convection. They found transport of seeding material over the Plateau also demonstrated that downwind depletion of liquid water (L W) was at least in part due to subsidence warming within a gravity wave pattern. Holroyd et al. (1995) reported on the analysis of the 21 February 1994 experiment which had weak surface winds, light snowfall and weak convection in thin orographic clouds over the Plateau. On this date AgI and SF 6 were co-released from a site part of the way up the windward side of the Plateau. A microphysical seeding signal was associated with the IN plume both at the surface and aloft. Modeling results of transport were reasonably close to observations for this case. Bruintjes et al. (1995) applied the Clark model to three cases from the 1987 Arizona/NOAA Cooperative program. Each of these had a single point-source ofSF6. Results from field observations and the modelled them to conclude that the transport and diffusion of seeding material are dependent upon the flow and stability conditions around the valley release site, and gravity waves stimulated by the upwind terrain. ~ DISCUSSION The transport of AgI released from three valley sites was examined for an experiment conducted on 7 February 1994. Sulfur hexafluoride was also released on this date from the mouth of a canyon located on the windward side of the Plateau. The observations indicated transport of the AgI over the Plateau mostly within a shallow layer in spite of being released in a stable surface layer with weak and sometimes easterly winds. Atthe RRS and TAR site, concentrations of 100 in vI, effective at -20 oC, were co~monly sampled. The temperatures at this sites were approximately -6 oC. The IN effectiveness of the generators and solution used in the valley is 2.5 orders of magnitude less for -6 oC (DeMott et al., 1995), giving an effective concentration of 0.24 IN L-1. There were, however, periods having concentrations of several thousand per liter effective at -20oC suggesting concentrations on the order of 5(L -1. No IN or SF 6 plumes were detected over the W track. The only significant IN plume detected by the aircraft was over the E trackin the vicinity of the TAR site at 3.5 km MSL(minimum IFR elevation). For the acoustic IN detector, indicated spread of an IN encounter is due to actual spread plus a temporal component from mixing within the cloud chamber (Heimbach et al., 1977). For this reason, estimating the peak concentration of IN at a point is risky for aircraft sampling. Adjusting for cloud chamber mixing, the maximum concentration of IN was less than 20 L.1 effective at -20 oC. The temperature at this level was -8 oC implying the maximum concentration of effective IN was less than 1.5 L -1. The IN targeted zones of supercooled L W over at least some portions of the Plateau, though indications are that this was in a very shallow layer . " I. 70