WMOD00312 (2) - Laserfiche WebLink

Home Browse Search

1. In the presence of water or ice saturation, application of a glaciogenic seeding agent will produce small, compact ice crystals over the temperature range -2 to -15 oc within 10 min. Additional liquid water is necessary to produce crystal riming. If SLW is limited, a mechanism such as aggregation will be necessary to bring the seeded crystals to the surface before they pass over the crest of the barrier. 2. Because of low SLW contents, and their relatively warm supercooled temperature, expected precipitation rate increases will be smalL Actual observations show rate increases caused by aerial seeding of 0.1 to 0.6 mm h-1. 3. The treatment effect for AgI is larger than CO2 because of the continuous nucleating ability of AgI. CO2 is only capable of producing ice crystals at the point of release. Reynolds (1988) estimated one 37-km-Iong CO2 seedlint:! might produce 20 acre-ft of additional precipitation; an AgI line might produce as much as 40 acre-ft, given appropriate cloud conditions. 3.3.2 Ground Seeding Experiments During the final year of the SCPP, the effectiveness of ground-based AgI generators in seeding winter cloud systems over the ARB was investigated. This investigation was considered important because studies had shown that most of the SLW was concentrated in the lowest 1 km above the barrier. Therefore, the SLW might best be treated by high elevation releas'es of seeding agents. Reynolds et aL (1989) report the results of this study. A network of 24 ground-based AgI generators was operated in a coordinated fashion during the SCPP field season, November 3, 1986, through January 9, 1987. These dispensers were. made up of those operated by the DRI (Desert Research Institute) for the State of Nevada, SMUD (Sacramento Municipal Utilities District), PG&E (Pacific Gas and Electric) operated for the Mokuleme basin, and a network of six manual generators installed especially for this program. Figure 3.6 shows the positioning of the generators about the target area. All generators, except PG&E, burned a 3 pct by weight solution of AgI-ammonium iodide- ammonium perchlorate in acetone. The ammonium perchlorate additive was used because laboratory studies had shown it increased AgI's nucleating efficiency at temperatures near - 5 oc by several orders of magnitude (DeMott et a1. (1983). PG&E did not use the perchlorate additive. This solution was consumed at arate sufficient to yield 30 g of AgI per hour. The method used to evaluate seeding was a periodic collection of snow samples during the 2-mo program at 15 separate sites within the target area. The samples were then analyzed in the laboratory for the presence of silver. Although the observance of silver does not prove seeding effects because the silver could have been scavenged, it does imply that directional targeting was occurring. In addition to these ground-based samples of silver, aerial sampling of silver was performed using an ice nucleus counter onboard the project research aircraft. The counter provided indications about the vertical and horizontal transport of ground-released seeding material. A total of 18 randomized experiments was conducted during the 2-mo field experiment; 12 were seeded and 6 were left as controls. Statistical evaluation of precipitation for seeding effects for this small sample was not performed. Qualitative evaluation based on meteorological conditions and silver analysis however, can infer if seeding could have produced additional snowfall. 13