Laserfiche WebLink
<br />concentrations 600 m above the Plateau were typically more than an order of magnitude less than those <br />measured On top of the Plateau, both at the fixed canyon head site and by an instrumented vehicle driven <br />along the Plateau top's west edge. Some of the lowest aircraft passes failed to detect any AgI during these <br />experiments. On some other experimental days the aircraft failed to detect any AgI even though many <br />passes were made while abundant AgI concentrations (at -20 oc) were being monitored'on the Plateau. <br />The considerable body of aircraft observations from many experimental days has shown that AgI is rarely <br />transported as high as 1,000 m above the Plateau, and then only in weak concentrations. <br /> <br />Another important factor to be considered is the rate at which AgI activates ice particles. The discussion <br />above presents the most optimistic case in which calculations of effective AgI concentrations assume total <br />nucleation by the aerosol. However, CSU Isothermal Cloud Chamber AgI generator calibrations are done <br />over extended periods, often tens of minutes, to allow aerosol the time necessary to nucleate. NCAR <br />counters are operated at very high cloud droplet concentrations to enhance nucleation during their limited <br />cloud chamber residence time. But orographic clouds have limited droplet concentrations. <br /> <br />DeMott et al. (1995) show that the AgI aerosol produced by the NA WC generator and the AgI-NH4I- <br />acetone-water solution operates by contact nucleation, a slow process. They calculated that, for a constant <br />temperature, only about 7 percent of the potential yield would be realized during a 20 minute transit of <br />this AgI aerosol through a natural cloud of 100 droplets cm-3. Simple calculations oftransport times <br />within SL W cloud over the Plateau show that AgI will be exposed to liquid cloud on the order of <br />20 minutes. Therefore, prior calculations of INC effectiveness, based on NCAR counter measurements <br />and the CSU generator calibration, are probably overestimated by more than an order of magnitude. The <br />sooner ice particles are formed within SL W cloud, the greater their probability of growing to snowflake <br />sizes and settling to the surface before being transported beyond the mountain barrier. <br /> <br />These INC observations indicate a limited "window of opportunity" f()feffective AgI seeding since <br />measurements have shown little evidence of significant ice particle enhancement in cloud warmer than <br />-9 oC. This finding is in agreement with the Tushar Mountains observations presented by Sassen and <br />Zhao (1993). Both data sets demonstrate the difficulty of effective ground-based AgI seeding with mildly <br />supercooled orographic clouds typical of Utah's mountains. Seedable opportunities are limited to the <br />colder "tail" of the distribution of SL W temperatures. <br /> <br />Further evidence on this point is provided by the Bridger Range Experiment conducted in the colder <br />climate of southwestern Montana. Statistical analysis of that experiment by Super and Heimbach (1983), <br />later supported by aircraft microphysical observations (Super and Heimbach 1988), strongly suggested <br />that AgI seeding from high altitude sites was effective only when ridge top (equivalent to Plateau top) <br />temperatures were colder than -9 oC. About half the Bridger Range periods were that cold. But if one <br />assumes similar vertical transport of AgI over the Bridger Range and the Plateau, as aircraft observations <br />have indicated, a much smaller fraction of Utah storm periods would be seedable with AgI. This <br />comparison further indicates that effective ground-based AgI seeding in Utah is limited to a fraction of the <br />time that SLW is available. Moreover, at least a portion of the apparent success of the BridgerRange <br />seeding may have been due to the frequent in-cloud operation of the AgI generators, with instantaneous <br />ice particle production caused by the supersaturated conditions very near the gener~tors (Finnigan and <br />Pitter 1988). It is known the AgI generators produce abundant quantities of water while consuming <br />propane fuel and the acetone in which the AgI is mixed. This can lead to local condensation-freezing as <br />soon as the AgI aerosol is exposed to the supersaturated but supercooled cloud. This rapid ice nucleation <br />mechanism 'will not occur with valley-released AgI. ' <br /> <br />14 <br />