WMOD00413 - Laserfiche WebLink

Home Browse Search

temperatures are cold enough for nucleation with AgI. Propane will be used during warmer periods with SLW present. The effectiveness of both seeding agents as a function of cloud temperature and other variables will be investigated during the direct detection phase. The adequacy of T&D of seeding agents and resulting ice crystals from particular sites also will be verified during the direct detection phase. Recently developed AgI solutions will be tested prior to the statistical/physical phase along with the AgI-NH4I-acetone solution used in several past experiments and operational programs. As discussed by Super and Heimbach (1983), the Bridger Range Experiment provided little evidence that seeding was effective at crestline temperatures wanner than about -9 oC with the AgI-NH4I-acetone agent, presumably because ice nucleus activity was too low. On the other hand, Finnegan and Pitter (1988) note that high altitude releases of AgI may cause rapid formation of high concentrations of ice crystals when temperatures are below -6 oC and at least ice saturation exists. Such embryonic crystals may grow essentially from the time they leave the AgI generators as they are carried up the mountain slope into the maximum SLW zone. Further work is needed to establish the wann temperature limit for AgI effectiveness in orographic (not laboratory) clouds. Silver iodide is temperature dependent, with ice nuclei effectiveness increasing about one order of magnitude per 4 oC decrease in temperature. About half of all winter hours with SLW present may be too warm for ground-based AgI seeding with AgI-NH4I-acetone at both proposed locations. Holroyd et al. (1988) presented observations over the Grand Mesa that support this supposition. Moreover, observations from southem Utah, discussed by Sassen and Zhao (1992), showed the wanner hours contained the majority of SLW. This finding was recently confirmed on the Wasatch Plateau (Huggins et al., 1992). Therefore, using an AgI solution with higher effectiveness at wanner temperatures would be desirable. Some solutions appear promising in cloud chamber simulation studies (e.g., DeMott et al., 1983), but should be tested in actual cloud conditions. Moreover, operational problems (clogging, excessive corrosiveness) have been reported with some newer "exotic" AgI solutions. Liquid propane seeding was originally developed to dissipate fog over airport runways, and such seeding is used operationally. The technology was recently adapted to seed wann (0 to -50C) winter clouds over the Sierra Nevada (Reynolds, 1989). Because a large fraction of the CREST winter storms will be too wann for ground-based AgI seeding even with improved solutions, propane seeding will be tested during the direct detection phase. Propane seeding will be attempted when the seeding sites are at least at ice saturation so that embryonic ice particles can survive transport to cloud levels. In many cases the seeding sites will be in SLW cloud so that propane seeding should result in immediate and rapid ice particle growth downwind of the release points, as the crystals are transported up and over the barrier. The main drawback of propane seeding is that the gas expansion must take place where the atmosphere is quite moist to create ice crystals that will continue to grow and not sublimate. That process requires dispensers to be located far up the windward mountainside, near or above cloud base, which limits time and distance available for crystal dispersion and growth. Silver iodide can be released farther upwind, well below cloud base, provided the airflow carries the material into the clouds. Ice nucleation takes place when the AgI reaches a cold enough region of SLW cloud. Greater dispersion can be expected with AgI released farther upwind, so AgI generators can be more widely separated than propane dispensers. Moreover, the increased 22 I I: I I I: I I I I I I I I I I I I I I