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<br />3. Summary of Findings Regarding Seeding Methods <br /> <br />I) Aircraft seeding of Colorado orographic clouds is considered impractical for most if not all <br />mountain barriers for several reasons. These include the high expense of 24/7 operations, <br />especially when considering the need to fill most of the SL W volume with seeding material. <br />Previous observations have shown that the along-the-barrier length which can be effectively <br />seeding with a single aircraft is limited to perhaps 20-25 miles (Deshler et al. 1990). Lines of <br />seeding material or resulting ice crystals have slow rates of horizontal and vertical dispersion <br />upwind of mountain barriers. Consequently, it is difficult to fill most of the SL W cloud volume <br />with seeding crystals without an excessive number of high performance aircraft. Safety issues <br />related to flying near mountains in known icing conditions, especially during hours of darkness, <br />and conflicts with airspace availability near high traffic air routes also create problems for aircraft <br />seeding. However, there may be a limited number of mountain barriers where aircraft seeding <br />might be worth considering in more detail. Such mountains would need to be relatively isolated <br />so aircraft could safely descend below the freezing level when airframe icing becomes excessive, <br />or where aircraft could remain well upwind in regions with only limited exposure to icing. <br />Moreover, the value of water augmentation would need to be unusually high to justify this <br />expensive option. <br /> <br />2) The large majority of operational seeding projects in the mountains of the western United <br />States have used valley or, sometimes, foothill ground-based AgI generators which can readily be <br />accessed and manually turned on and off. This is certainly the least expensive seeding approach. <br />However, it has serious problems with routinely achieving transport and dispersion of sufficient <br />concentrations of effective ice nuclei into SL W cloud regions. A number of investigations have <br />shown high frequencies of ground-based inversions which trap the AgI near the valley surface <br />and/or barrier jets which carry seeding material parallel to the barrier rather than over it. <br /> <br />A number of aircraft and mountain top observations of AgI from valley seeding have indicated <br />AgI concentrations too low for effective seeding. Moreover, silver-in-snow analysis of target <br />area snowfalls have generally shown that silver concentrations were not increased over natural <br />background levels, implying mistargeting and/or insufficient AgI for effective seeding. These <br />results would, of course, be expected when the AgI is trapped within a stable atmosphere near the <br />valley floor. Even when AgI was detected at SL W cloud levels, concentrations of effective ice <br />nuclei were low (e.g., Super 1995). This unsatisfactory result was likely partially due to wide <br />crosswind spacing between AgI generators and low generator outputs as well as frequent trapping <br />within a stable lower atmosphere. A recent examination of earlier data from the Wasatch Plateau <br />and upwind Sanpete Valley in central Utah found that hours with significant Plateau-top SL W <br />infrequently had surface winds which were suitably organized to mechanically force valley-based <br />seeding material up the Plateau's windward slope. As discussed in Appendix C, less than 30% of <br />such hours had valley winds with a significant upslope component. <br /> <br />An examination of radiosonde (weather balloon) data taken from just upwind of the Plateau, <br />also discussed in Appendix C, suggests that low-level stability there may not be as serious a <br />problem for valley seeding as found by other cited investigations. This may be because the <br />Valley lies between two nearby parallel ranges, with the San Pitch Mountains to the west, a <br />configuration which could enhance mechanical mixing. During stable conditions, the existence <br />of parallel barriers is conducive to establishment of transient gravity waves which can transport a <br />pool oflow-Ievel AgI to high altitudes as suggested by Heimbach and Hall (1994), Heimbach et <br />al. (1997) and Reinking et al. (2000). Gravity waves can also cause subsidence and sublimation <br /> <br />7 <br />