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<br />While thin cloud top SL W layers may contribute to natural ice crystal formation, they are not <br />considered an important moisture source for seeding. In addition to the problems cited, seeding <br />cloud top SL W would require aircraft releases. That requirement makes routine seeding <br />impractical over the rugged and extensive Colorado mountains. As discussed in detail by Super <br />(1999a), now available as Appendix A of Medina (2000), aircraft seeding has several drawbacks. <br />Briefly, such seeding is quite expensive, especially if operations are to be undertaken on a 24/7 <br />basis needed to maximize seeding effectiveness. Flying in winter orographic cloud can be <br />dangerous if moderate or heavy icing exists, that is, when SL W is abundant at flight levels. Pilots <br />are especially reluctant to fly near mountains at night, even with twin-engine aircraft equipped for <br />flight into known icing conditions. Experience with expensive twin jet-prop engine research <br />aircraft has demonstrated they cannot remain in moderate much less severe icing for very long. <br />Operational seeding aircraft are usually less capable of remaining in an icing environment. <br />Should loss of an engine or a forced landing be required, darkness is an obvious additional <br />danger, especially over rugged terrain. <br /> <br />Besides the high expense and safety issues of airborne seeding, there are serious problems <br />attempting to fill a significant portion of the SL W cloud with seeding material or seeded ice <br />crystals. These have been discussed in detail by Hill (1980) and Deshler et al. (1990) among <br />others. Deshler et al. (ibid.) used aircraft and ground-based microphysical observations over <br />California's Sierra Nevada to conclude that, "Achieving fairly continuous coverage along the <br />direction of seed line advection requires seed lines to be no longer than 37 km (23 mi), yet treatable <br />cloud may extend for hundreds of kilometers along the barrier." Based on their finding it would <br />require many aircraft to effectively seed Colorado's major ranges during large-scale storm passages. <br />Another major problem for aircraft seeding in several areas is conflicts over available airspace, <br />especially if seeding is desired near major air traffic routes where preference will be given to <br />commercial or military flights. There are locations where aircraft seeding may be practical, such <br />as some of California's coastal ranges, where aircraft can descend below the freezing level to shed <br />rime ice buildup. But, for all the reasons given, aircraft seeding is not considered practical for <br />Colorado mountains. Consequently, it will not be considered further in this report. <br /> <br />The large majority of winter orographic cloud seeding programs have used ground-based AgI <br />generators. These are most frequently located in mountain valleys and are typically manually <br />controlled. Sometimes generators are sited on somewhat higher foothills and/or canyon mouth <br />locations. Remote-controlled AgI generators are used less commonly, because of increased cost <br />and operational complexity, although some California and Nevada projects have used them <br />exclusively. It will be shown that, in general, higher elevation seeding releases are more likely to <br />be successful in providing seeding agent to orographic SL W cloud. In fact, if generators are <br />located higher than about midway from a valley floor to a crestline, AgI transport into the <br />primary SL W zone has been shown to be routine within the appropriate range of upslope wind <br />directions. <br /> <br />Further discussion of ground-based seeding will be separated into two categories. The first <br />and most common will be called "valley" releases, considered to include any releases up to and <br />including foothill and canyon entrance locations. Such releases typically use manually operated <br />AgI generators with ready access to the seeding sites, usually by road. The second category will <br />be referred to as "high-altitude" releases meaning any seeding higher than midway from the <br />valley floor to typical crestline elevations. Such sites are significantly more difficult to access, <br />requiring travel by skies, snowshoes, snowmobile or helicopter. Remote-controlled AgI <br />generators or propane dispensers are used at high altitude seeding sites. Only limited attention <br />will be given to releases in the elevation interval between canyon entrances and midway up the <br />windward slope. Plume tracking information from such locations is limited and results have <br /> <br />16 <br />