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8.14. Holroyd, E. w., J. A. Heimbach and A. B. Super, 1995: Observations and model simulation of AgI seeding within a winter storm over Utah's Wasatch Plateau. J. Weather Modification, 27, 36-56. . - ABSTRACT Observations from a cloud seeding experiment conducted over the Wasatch Plateau of central Utah were analyzed for treatment effect and were modeled. The day was characterized by weak surface winds, . light snowfall and weak convection embedded in a thin orographic cloud during the final stages of a storm. Silver iodide was released from a generator well up the windward (west) slope of the Plateau. Seeded periods and locations were defined using measurements of co-released SF 6 tracer gas and a drifting frame of reference. Seeded and non seeded periods and domains were defined using the derived plume history. A strong seeding signal was found in the occurrence of ice particles, both on the Plateau top and at aircraft. levels. Calculations based on van~mounted 2D-C probe observations along the Plateau top's west edge indicated increased snowfall rates in the silver iodide plume, primarily due to aggregates. While some -precipitation gauge observations farther downwind suggested possible increased snowfall, the evidence was not definitive and any seeding-caused snowfall was quite limited in amount. Applications of the Clark mesoscale numerical model suggested the case was characterized by weak and shallow clouds principally driven by orographic influences with little buoyant contribution. The simulated tracer and cloud patterns were associated with orographic lifting and gravity waves. The model correctly predicted plume transport from the release point over the target area, but at a slower rate than indicated by field measurements. The plume core was predicted to be transported over the Radar-Radiometer site and somewhat south of the Target site to a height of about 1 km above the Plateau in good agreement with observations. SUMMARY AND CONCLUSIONS The 21 February 1994 case was analyzed for plume characteristics including seeding responses for a single AgI plume co-rel~ased with SF6 tracer gas well up the windward side of the Wasatch Plateau. The day was characterized by weak winds primarily from the southwest with weak convection enabling vertical transport of AgI to flight levels. The exposure of sites and mobile sampling platforms to the plume was estimated using a frame of reference which moved with the average wind. This enabled spatial and temporal coordination of multiple plume encounters, and allowed a uniform objective definition of nonseeded periods and locations. For the sampling aircraft, nonseeded portions of a flight path were 24 s flight intervals 12 s on either side of the co-released SF6 plume. The nonseeded zones for the instrumented van were 2 km distances 1 km outside the SF 6 plume. The plume was estimated to have been over particular ground sites for 4.25 h and to have generally moved from 235 degrees with a short period having a more southerly component. The angular plume width over the west flight track was estimated to have been 15 degrees with a brief wider period associated with a wind shift. Vertical plume transport to the Plateau top and to flight levels was rapid in the presence of limited horizontal winds speeds. High concentrations of ice particles were associated with measured and predicted plumes at locations ,sampled by the van and aircraft. Ice particle concentrations and precipitation rates were enhanced, compared to nonseeded zones, by factors of about 10 for upwind highway van measurements, 40 for west track aircraft measurements, and variable amounts for east track aircraft passes. Ice water contents were always greater in the plume than in the non seeded zones. Particle trajectories and concentration patterns indicated that most growth and precipitation occurreq near the west track, where there was the most SL W. 58