Observations and Model Simulation of AgI Seeding within a Winter Storm over Utah’s Wasatch Plateau

Authors

  • Edmond W Holroyd, III Bureau of Reclamation Denver, CO
  • James A Heimbach University of North Carolina at Asheville Asheville, NC
  • Arlin B Super Bureau of Reclamation Denver, CO

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 SF6 tracer gas and a drifting frame of reference. Seeded and nonseeded periods and domains were defined using the derived plume hisstory. 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. Application of the Clark mesoscale numerical model suggested the case was characterized by weakand shallow clouds principally driven by orographic influences with little buoyant contribution. The simulated tracer and cloud patterns was 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.

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Scientific Papers