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<br />Super (1996) showed another obvious case ofIPC and snowfall enhancement caused by AgI seeding <br />during relatively cold Plateau top temperatures (-10.7 oc). This seeding, on December 15, 1994, <br />produced about 1 mm additional snowfall on the Plateau top's west edge during the hour of seeding. The <br />successful March 5, 1995, propane experiment at a Plateau top temperature of -4.5 oC was again <br />reviewed, and it was shown that AgI seeding soon after the propane seeding was ineffective under similar <br />conditions. <br /> <br />". <br /> <br />Significant documentation of seeding-caused IPC and snowfall resulted from the 1994-95 and 1995- <br />96 winter field programs. This could be expected since these limited, economical experiments were <br />designed for that purpose. Moreover, it was practical to conduct many such experiments with only limited <br />ground equipment and two to three field technicians. The basic design was to release seeding material, <br />Agl or propane, in a brief "pulse" of one-half hour or one hour duration. The release point was on a high, <br />exposed ridge only 4.2 kIn horizontal distance and 315 m below the instrumented "target" site located at <br />the head of a major canyon. Prevailing southwest winds funneled the seeded cloud up the canyon and past <br />the target. Ice particle characteristics and snowfall rates could be examined before, during, and after <br />"plume" passage by the target. <br /> <br />Some experiments provided obvious IPC enhancement, and sometimes snowfall augmentation, when <br />examined on a case study basis. However, seeding effects were not obvious in most of the experiments. <br />Some of the failures to clearly demonstrate IPC increases were caused by cloud temperatures too warm for <br />seeding agent effectiveness, especially when AgI was used. Other failures were due to short-term natural <br />variability in snowfall rates which often masked the seeding-caused ice particles. It is likely that many of t- <br />the seeding experiments created tiny crystals which were swept out by larger natural snowflakes and, <br />therefore, were undetectable by the experimental design. Furthermore, abundant natural snow may have <br />consumed all available SL W, thereby starving the embryonic seeded crystals. This series of experiments <br />showed the difficulties of clearly demonstrating ice particle and snowfall production in the presence of <br />even light natural snow. Orographic clouds are anything but steady-state, and natural snowfall rates often <br />vary considerably over a few tens of minutes or less. <br /> <br />4.3 Statistical Analysis of Pulsed Seeding Experiments <br /> <br />When natural snowfall was nil to very light and seeding potential existed, obvious effects of seeding could <br />be demonstrated (Super and Holroyd .1997). But such conditions occur during a minority of the time that <br />orographic cloud is present. However, statistical analysis provided an overview of all similar experiments <br />conducted during the 1994-95 and 1995-96 winters (Holroyd and Super 1998). There were indications of <br />AgI effectiveness in creating small ice particles for target (Plateau top) temperatures colder than about <br />-6 oC. However, the number of such cases was limited because less than 20 percent of all seeding <br />experiments had target temperatures colder than -7 oC. Stronger evidence existed of propane-caused <br />small ice particles, and even the difference between using one propane nozzle (1994-95 winter) and two <br />(1995-96 winter) was evident. Snowfall increases caused by the ,seeded crystals were limited, as might be <br />expected from the limited distance (4.2 kIn) and travel time (median f7 min) between seeding release and <br />the target. This distance was purposely limited to maximize successful targeting ~s the seeded cloud was <br />funneled up a major canyon. <br /> <br />Perhaps the most important finding of these "pulse seeding" experiments was that propane seeding was <br />effective in producing ice particles even with slightly supercooled cloud at the dispenser site (-0.4 to <br />-3.4 OC). About 10 ice particles L-1 was typical at the target with one propane nozzle releasing about <br />3 gal h-1, and 20 L-1 with two nozzles releasing twice ~hat rate. As argued by Super (1994), a seeded IPC <br />over a target in excess of 10 L-1 has the potential to produce meaningful snowfall. Holroydand Super <br /> <br />18 <br />