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<br />.. <br /> <br />1998 articles and papers: <br /> <br />,." <br /> <br />8.27. Holroyd, E. w., and A. B. Super, 1998: Experiments with pulsed seeding by AgI and liquid <br />propane in slightly supercooled wi'!ter orographic clouds over Utah's Wasatch Plateau. J. Weather <br />Modification, 30, 51-76. <br /> <br />ABSTRACT <br /> <br />A series of orographic cloud seeding experiments were conducted on Utah's Wasatch Plateau during the <br />1994-95 and 1995-96 winters. Their purpose was to permit physical assessment of the effect of both <br />silver iodide and liquid propane seeding, particularly at only slightly supercooled temperatures. Seeding <br />materials were released in I-hour and half-hour pulses from a location well up the plateau's windward <br />slope. The terrain often channeled the seeding plumes to an observation site, or target, located at a canyon <br />. head on the Plateau top's windward edge. Snow particles were detected at the target with a vane-mounted <br />-2D-C optical array probe whose strobing speed was governed by a heated anemometer. AgI nuclei were <br />detected there by an NCAR ice nucleus counter to confirm the presence and successful targeting of <br />seeding materials. <br /> <br />The experiments were carried out in conditions both favorable and unfavorable for expected effectiveness <br />of the seeding agents. There was no evidence for the effectiveness of AgI plumes injected into clouds at a <br />release site temperature warmer than about -3 oC and target temperature .of about -5 oC. Increases in ice <br />particle concentrations and precipitation rates were observable for AgI seeding with target air <br />temperatures colder than -6 oC. <br /> <br />Liquid propane was released into air at temperatures of -0.4 to -3.4 oC, resulting in about 10 ice particles <br />L-1 at the target with only one release nozzle and about 20 L-1 for two nozzles. The difference in ice <br />particle concentrations formed by adding the second nozzle was statistically detectable at a 6 percent level <br />or better, depending on the measurement type. While a temperature dependence of the liquid propane was <br />not observable over such a small temperature range, the exp-eriments have shown that liquid propane is an <br />effective seeding agent for slightly supercooled clouds at temperatures where AgI is ineffective. <br /> <br />Seeding plume detection appeared to be limited during periods of abundant natural snowfall. The larger <br />natural crystals appeared to compete for the available supercooled liquid water and may have removed <br />many of the seeded embryos by aggregation. <br /> <br />SUMMARY AND CONCLUSIONS <br /> <br />. <br /> <br />The previous results on the first winter of experiments (Super and Holroyd, 1997) stressed case studies, <br />particularly for those examples with an obvious seeding effect. This paper examines the entire set more <br />from an operational point of view, searching for consistency of increases< in concentrations and <br />precipi~tion rates over all experiments. A natural data set was created to provide an i,ndication of natural <br />variability . <br /> <br />The seeded data set was partitioned into a set of "reject" cases which should appear natural, the AgI cases <br />which should have limited effects at warm temperatures, and the propane cases. The "reject" cases indeed <br />appeared similar to the natural data set. Both the AgI and propane cases showed increases in ice particle <br />concentrations and precipitation rates as measured by the,2D-C for particles up to 0.8 rom sizes. All three <br />partitions showed the same relationships (not presented) and scatter as in Figure 3 between ice particle <br /> <br />81 <br /> <br />