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Last modified
7/28/2009 2:40:35 PM
Creation date
4/24/2008 2:52:55 PM
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Weather Modification
Title
The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
Date
12/1/1991
Weather Modification - Doc Type
Report
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<br />3. REVIEW OF RELEVANT LITERATURE AND WEATHER OBSERVATIONS <br /> <br />As noted above. both experimental and operational winter cloud seeding have been conducted in Utah. <br />Experimental seeding refers to projects with the primary aim of increasing knowledge rather than water. <br />The purpose of such projects is to continue to improve existing technology. On the other hand, <br />operational projects apply existing technology for the purpose of enhancing precipitation and streamflow. <br />However, research can be "piggy-backed" on operational programs to evaluate their effectiveness and to <br />suggest means of improving them. The Utah/NOAA cooperative research program is an excellent example <br />of this approach. Analyses of both operational and experimental seeding programs have provided a <br />substantial amount of applicable information for the current study. <br /> <br />3.1 Statistical Evidence <br /> <br />Published statistical analyses of past Utah seeding programs will be examined realizing that such analyses <br />are frequently inconclusive, especially when applied to operational projects, The difficulties of <br />nonrandomized operational program evaluation are well known; for example, see Dennis (1980). Earlier <br />experimental programs that were randomized often had serious design flaws in light oftoday's knowledge. <br />In some cases the complexity of transport and dispersion was underestimated and there is serious do~bt <br />that the clouds were routinely seeded with sufficient concentrations of ice nuclei. The same flaws likely <br />. have often existed in operational programs. <br /> <br />A number of statistical analyses have been reported from Utah, mostly dealing with operational seeding <br />projects. An exception is the evaluation by Hill (1979) which described two randomized winter seeding <br />experiments in the northern Wasatch Mountains near Logan. The first seeding experiment, conducted <br />during the winters of 1969-70, 1970-71, and 1971-72, used aircraft seeding with pyrotechnic flares <br />containing AgI. Three mountain-top AgI generators were operated for the second experiment during the <br />winters of 1973-74, 1974-75, and 1975-76. Briefly, the results of these experiments were mixed. The <br />aerial seeding experiment showed no statistically significant increases, More encouraging but still not <br />significant results were found for the ground based seeding program. <br /> <br />A major problem with the design of the northern Wasatch experiments was the use of 500 mb <br />(approximately 18,000 ft m.s.!.) temperatures and rawinsonde-estimated cloud-top temperatures as <br />indicators of cloud seedability. This was a common approach at the time before routine observations of <br />cloud SL W were practical, It was believed that clouds with warm tops were more likely to be seedable <br />than cold-topped clouds (Grant and Elliott, 1974) since the latter should have more abundant natural ice <br />nuclei (both artificial and natural ice nuclei are very temperature dependant but the former are more <br />effective at wanner temperatures), An attempt to resolve this problem was to use aircraft icing reports <br />as indicative of cloud seedability (this approach was later expanded upon by Hill, 1982a). When <br />precipitation data were stratified by this parameter and the results of both the aerial and ground seeding <br />programs were combined, seasonal increases of about 15 percent were suggested. It appeared that most <br />of this suggested increase was derived from approximately one-sixth of the winter storms. However, the <br />tentative nature of these suggestions was stressed along with the need for verification (Hill, 1979). <br /> <br />A second major problem with the experimental designs of the two northern Wasatch programs likely was <br />inadequate dispersion of the seeding material. Crosswind spacing between the three mountain-top <br />generators was about 11 and 18 mi, respectively, Based on observations of AgI plumes from other high <br />altitude generators (Holroyd et al" 1988; Super and Heimbach, 1988), it seems unlikely that most of the <br />cloud volume upwind and over the Wasatch target area contained AgI with that much distance between <br />release points. <br /> <br />7 <br />
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