A High Altitude Ground-Based Cloud Seeing Experiment Conducted in Southern Utah

Arlen W Huggins, Ken Sassen


A wintertime ground-based cloud seeding experiment conducted as part of the 1989 Utah/NOAA cooperative weather modification program is described. The results from one experiment on 3 February 1989 are presented. Meteorological conditions led to the development of orographic clouds over the Tushar Mtns of southern Utah which appeared to be nearly ideal for seeding operations. Radiometrically measured liquid water was abundant in the vicinity of seeding generators and the water appeared to be sufficiently supercooled to enable nucleation by silver iodide. The experiment entailed pulsed releases of silver iodide from high altitude generators located on upwind ridges of the Tushar Mtns. A Ka-band radar, aspirated PMS 2D-C probe, and manual microphysics observations were used to monitor precipitation 10-13 km downwind of the seeding generators. Snow samples were also collected periodically and analyzed for silver content. The overall results were disconcerting in that two estimated periods of effect showed no enhanced silver content, and no clear microphysical or radar seeding signatures due to large background variability produced by a propagating mesoscale cloud feature, and natural snow characteristics which resembled the expected characteristics due to seeding. A third period of effect had apparent microphysical and radar signatures, but also lacked the presence of silver in the snow. Targeting of the single downwind ground target apparently failed in this case due to inadequate wind documentation in the cloud layer, remote-generator malfunction, or fallout of the seeded plume upwind of the target.

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