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<br />. . <br /> <br />.- <br /> <br />greater dispersion which would reduce the concentrations over the target, and greater difficulty in <br />horizontal targeting. <br /> <br />8.26. Super, A. B., and E. W. Holroyd, 1997: Some physical evidence of AgI and liquid propane <br />seeding effects on Utah's Wasatch Plateau. J. Weather Modification, 29, 8-32. <br /> <br />ABSTRACT <br /> <br />A series of cloud seeding experiments were conducted on Utah's Wasatch Plateau during the winter of <br />1994-95. Their purpose was to permit physical assessment of the effects of both silver iodide and liquid <br />propane seeding, particularly at only slightly supercooled temperatures. Seeding materials were released <br />in I-hour pulses from a location well up the plateau's windward slope. The terrain often channeled the <br />seeding plumes to an observing site, or target, located at a canyon head on the plateau top's upwind edge. <br />Snow particles were detected at the target With a vane-mounted 2D-C optical array probe whose strobing <br />speed was governed by an anemometer. AgI nuclei were detected there by an NCAR ice nucleus counter <br />to confirm the presence and successful targeting of seeding materials. <br /> <br />Seeding with AgI under cold conditions produced obvious large increases in ice particle concentrations <br />and measurable increases in precipitation at the ground during one experiment. Seeding with AgI under <br />only slight supercooled conditions, in which the contact~freezing mechanism is not expected to be <br />effective, typically produced a negligible ice particle supplement. A forced condensation-freezing <br />mechanism may have been operable during two experiments, producing detectable enhancements in the <br />ice particle concentrations, but further verification is needed. Liquid propane seeding produced <br />measurable increases in ice particle concentrations in some experiments, showing it to be an alternative to <br />AgI at only slightly supercooled temperatures. <br /> <br />SUMMARY AND RECOMMENDATIONS <br /> <br />Some of the seeding experiments produced obvious changes in microphysical characteristics and snowfall <br />during periods with very limited natural snowfall. Others produced none, as expected, because of intense <br />natural snowfalls, relatively warm cloud temperatures, drier conditions, or poor targeting of the materials. <br />Natural variability masked some of the results or created misleading control intervals. Though only a few <br />of the experiments were relatively free of such complications, this operational style de.sign with a variety <br />of cloud environments was intentional. It provided a range of con~itions likely to be encountered in <br />operational seeding programs. Furthermore, the stress on physical, rather than statistical (see Heimbach <br />and Super, 1996) verification of the experiments provides a basis for seein~ what happened with each <br />experiment, even if the natural variability was a complicating factor. <br /> <br />, . <br /> <br />One of the case study experiments (December 15) used an AgI release wi\ile cloud temperatures were <br />relatively cold, near -7.8 at the HAS and -10.4 oC at the TAR. The resulting seeded ~C and snowfall was <br />estimated at 140 L-~ and 1.0 rrim h-I respectively, 4.2 km east-northeast of the HAS at the TAR. This <br />experiment provides convincing evidence that AgI seeding can produce significant snowfall under certain <br />conditions. <br /> <br />'. <br /> <br />A second case study experiment (March 5) released liquid propane at HAS cloud and TAR temperatures <br />of -2.1 oC and -4.1 oC, respectively. Propane seeding was estimated to have enhanced IPC by about <br /> <br />79 <br /> <br />