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<br />003191 <br /> <br />physical observations of the effects of seeding shallow winter clouds (Deshler et al., 1990). Deep <br />cloud systems appeared to be naturally effective in snowfall production (Marwitz, 1987b). The <br />SCPP used the most extensive array of remote and in situ observations to date for monitoring <br />physical processes following seeding. <br /> <br />The SCPP attempted to document the important links in the chain of physical events from <br />airborne release of seeding material to snowfall on the target site. However, a nwnber of <br />problems were encountered, including frequent high natural concentrations of large cloud <br />droplets and/or ice crystals, both associated with maritime clouds near the Pacific Ocean, which <br />made it difficult to discern those ice particles caused by seeding. The aircraft seeding affected <br />limited cloud volwnes because of slow dispersion rates. Although SLW was shown to be present <br />for many hours in winter cloud systems, it was concentrated at low levels, near the barrier top, <br />where temperatures were usually too warm for AgI seeding (0 to -5 OC). <br /> <br />The logistics of coordinating the several observing platforms caused problems for the SCPP. <br />These problems included frequent airspace conflicts caused by high traffic density and, more <br />important, the inability to fly the observing aircraft at low levels over the rugged terrain. Heggli <br />et al. (1983) stated that "altitude restrictions prevented the aircraft from flying within <br />approximately 1 Ian of the Sierra terrain." As discussed later, most of the SLW over mountain <br />barriers exists in the lowest kilometer above the ground The SCPP study area was not <br />amenable to direct detection seeding experiments. As discussed by Deshler et al. (1990), only 2 <br />of 36 experiments were able to document the complete chain of events following seeding, ending <br />with snowfall at the ground. However, the research aircraft was able to docwnent within cloud <br />seeding effects in 35 pct of the sampled seedlines. <br /> <br />~ <br /> <br />2.3 Opportunity to Advance the Technology <br /> <br />Several factors allow significant improvements to past seeding experiments, including: <br /> <br />. An improved comprehension of the importance of proper targeting of seeding material and <br />how to achieve it. <br />. A better appreciation of the mountain barrier characteristics needed for practical <br />experimentation. <br />. Significant improvements in instrumentation. <br />. Development of nwnerical models able to realistically simulate airflow, cloud, and <br />precipitation processes over complex mountain terrain. <br /> <br />These factors have led to the successful conduct of a nwnber of physical seeding experiments in <br />recent years. <br /> <br />Failure to routinely deliver proper concentrations of seeding materials to SLW regions is now <br />recognized as a serious shortcoming of several past projects, especially when low' altitude <br />ground generators were used (Rangno, 1986; Super, 1990; Super and Huggins, 1992a; Super <br />and Huggins, 1992b). The use of high altitude seeding generators has been shown to be capable <br />of routinely targeting mountain clouds over the Grand Mesa of Colorado (Holroyd et al., 1988), <br />the Bridger Range of Montana (Super and Heimbach, 1988), and the Wasatch Plateau of Utah <br />(unpublished preliminary results from 1991). Using liquid propane as a seeding agent for <br /> <br />8 <br />