WMOD00388 - Laserfiche WebLink

Home Browse Search

22 ., . .1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . of all samples fi'om 7 different primary target stations contained silver above background levels, considered to be 6 parts per trillion by mass (ppt). Percentages of samples with above background silver concentrations ranged from 39 to 100%, but only 2 stations had less than 73 %. Average silver levels per sampling site ranged from 14 to 118 ppt with an overall average value of 61 ppt and median of29 ppt. As expected, concentrations were lower at the 4 secondary target stations, ranging from 10 to 14 ppt. These primary targets values are all well above the natural background and are impressive results. Two primary target stations, both near 10,000 ft, were used to examine the two different inert tracers released for ground-based and aircraft seeding, respectively. One is exposed to southwesterly flows and the chemical tracer analyses showed it was primarily targeted by ground releases. It had the second highest silver level averaging 118 ppt. The other site was sheltered by ridges from the predominant southwesterly flow and showed no evidence of ground-based seeding. All the silver there was from aircraft seeding with an average silver concentration of 22 ppt. It should not be concluded that valley or foothill AgI seeding always fails, even though published physical and chemical evidence supporting such seeding is weak. Several articles cited in this report discussed frequent stable layers which would trap valley-released AgI. But such layers were not always present. The discussion of near-valley winds in Appendix C suggests that a cross- barrier component with adequate speed to support mechanical forcing up the Wasatch Plateau's windward slope was sometimes present, but during less than 30% of the hours with significant SL W over the Plateau. General lack of detection of adequate concentrations of ice nuclei above the Plateau, effective at SL W cloud temperatures, may not have been primarily caused by trapping inversions as suggested for a number of other mountain ranges. As discussed in Appendix C, the proximity oftwo parallel ranges may have enhanced gravity waves which sometimes helped AgI transport and dispersion over the Plateau. The main factors causing the limited IN concentrations observed may have been a combination of the wide spacing between generators, relatively low AgI consumption rates of8 g h-1 (20-30 g h-1 are common), and use of an AgI solution which provided limited effective IN at mildly supercooled cloud temperatures. Since such low-level manual seeding has considerable economic advantages, it is recommended that further testing be done with especially foothill and canyon-mouth AgI generators, but with the following improvements. Generators should be spaced no more than 2.5 miles apart, AgI outputs should be increased to about 30 g h-1, and an improved AgI solution should be used that is as effective and fast-acting as possible while still being practical for field use. Finally, the generator IN effectiveness per gram of AgI should be as high as possible. Unfortunately, the Colorado State University Cloud Simulation Laboratory is no longer available for testing particular generator configurations and AgI solutions. Consequently, reference to their most recent reports would have to provide guidance in these areas. 6c. Seeding from High Elevation Locations A large body of observations exists which demonstrates that seeding plumes released from high elevation sites are routinely transported over downwind mountain crestlines when the wind direction in that layer has a component perpendicular to the crestlines. That is, transport and dispersion are routine when the airflow is forced up and over the windward slope of the barrier. Mountain surface and aircraft observations have included sampling the AgI itself with acoustical counters, seeded ice crystals produced by AgI or propane, and fast-response tracer gas detection, usually with SF6. Published articles which support this assertion, briefly reviewed in Appendix B, include Super (1974), Holroyd et al. (1988), Super and Heimbach (1988), Holroyd et al. r 195), Super and Heimbach (2005a), and references in the next paragraph.