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<br />must exceed at least 10 per liter for even detectable snowfall increases at the surface (Super and <br />Boo, 1988; Super 1994; Holroyd and Super, 1998). Higher concentrations are required for <br />moderate seeded snowfall enhancement. For example, Super and Holroyd (1997) presented clear <br />physical evidence of an AgI-seeded snowfall increase of 0.04 inches per hour with an associated <br />seeded ice crystal concentration of about 140 per liter. Median hourly snowfall rates in Colorado <br />mountains are typicaily about halfth.at rnte. <br /> <br />It is likely that some operational seeding projects have rarely achieved in-cloud AgI <br />effective ice nuclei concentrations of even 10 per liter over large cloud volumes (e.g., Super <br />[1999a] and several investigations cited therein). Given that ground-released AgI plumes are <br />usually concentrated in the lowest 2,000 ft above mountain barriers, it has been shown at a number <br />of western locations that seeded SL W layers are frequently too wann for significant AgI nucleation <br />with otherwise suitable conditions for seeding. The frequency will vary with barrier elevation and <br />local climate, but even as far north as southwestern Montana only half the storm periods had crest <br />line temperatures as cold as -90C during the Bridger Range Experiment (Super and Heimbach <br />1983). Seeding did not appear to be effective with wanner temperatures. <br /> <br />Reynolds (1996) used mountain-top icing rate meters in the northern Sierra Nevada to show <br />that temperatures were wanner than -4OC about 80 percent of the time when SL W was present. <br />Temperatures near plume tops some 2,000 ft higher would be near -80C. The high frequency of <br />only slightly supercooled liquid water clouds in California led Reynolds (1996) to turn away from <br />AgI seeding in favor of experimental propane seeding. Plume tracking and other measurements <br />over the Grand Mesa of Colorado were used by Holroyd et al. (1988) to estimate that 1/3 to 1/2 of <br />winter storms were not cold enough for adequate nucleation rates with AgI. Similar findings were <br />reported by Super (1994) for the Wasatch Plateau of central Utah. Plateau-top temperatures were <br />colder than -4OC less than 25 percent of the hours with significant vertically-integrated SL W <br />observed with a microwave radiometer. Sassen and Zhao (1993) reported a high frequency of <br />mildly SWL cloud over the Tushar Mountains of southern Utah. They concluded that only a <br />limited temperature "window" existed that had AgI cloud seeding potential. In swnmary, there is <br />considerabl~ evidence that a large fraction of the warmer, wetter SL W orographic clouds in the <br />western states cannot be effectively seeded by ground-released AgI. <br /> <br />1.4 Silver-in-Snow Analyses <br /> <br />The silver-in-snow analyses presented in this report represent an important first step in <br />providing direct physical evidence for the evaluation of the DWB seeding-program effectiveness, <br />independent from any statistical suggestions. It is important to recognize that finding silver in <br />snow concentrations above background nonseeded levels does not prove that AgI seeding <br />increased the snowfall, or even that AgI created new ice crystals. Brownian motion, electrostatic <br />attraction and other scavenging mechanisms are known to result in capture of AgI aerosol by <br />natural ice crystals and snowflakes. The ice and AgI particles fall to the surface, thereby <br />enhancing silver-in-snow concentrations in the snowfall. Even direct deposition of tiny AgI <br />particles can provide some silver enhancement. However, failure to find enhanced silver levels <br />in an expected larget area indicates that proper AgI targeting was not achieved. Further, failure <br /> <br />8 <br /> <br /> <br />