WMOD00528 - Laserfiche WebLink

Home Browse Search

The third highest ratio, 1.19, was from the 1988-89 season when only two counties were se€:ded. Such a result would not be expected and it seems likely that causes other than seeding resulted in this large difference between the target and control precipitation. The large departure during the 1988-1989 season raises questions about the stability of the target-control relationship, similar to those raised by Hill (1982b). Several of the control gauges used in the Griffith et al. (1991) evaluation differ from those used by two of the same authors 10 years earlier (Thompson and Griffith, 1981). For example, three Nevada gauges were used in the 1981 analysis vs. five in 1991, and five Utah gauges were used in 1981 vs. two in 1991. These discrepancies also reduce the degree of confidence that can be placed on the results of the most recent statistical analysis, In summary, the statistical results reviewed above can not be considered conclusive, but some suggest that average seasonal snowpack increases of about 10 percent may be occurring with the Utah operational program. Similar suggested increases have resulted from analyses of some operational programs and experiments in the Rocky Mountain region. The lO-percent value is consistent with the most recent American Meteorological Society policy statement review of the Western United States quoted in section 3.4, 3.2 Physical Evidence A considerable body of physical evidence relevant to the question of cloud seeding feasibility has been obtained in Utah and nearby States. The Utah State University program obtained a large body of physical evidence, primarily during the 1970's and early 1980's. For example, Hill (1980b) developed seeding criteria based on measurements of SL W, precipitation, cloud top temperature and vertical air motion. Hill and Woffinden (1980) developed a balloon-borne instrument ("cloudsonde") for measuring vertical profiles of SLW. Observations with this device were used by Hill (1986) to show that, "upward vertical motion and supercooled liquid water are found most often in a vertical zone from about 500 m below the barrier crest to about 2500 m above with a median height of between 500 and 1000 m above the crest." The existence of most SL W at low levels above the windward slope and crest of the barrier also was documented by Hobbs (1975a) for the Cascade Mountains of Washington, by Heggli et al. (1983) and Heggli and Rauber (1988) for the Sierra Nevada, and has been shown over different ranges in Colorado (e.g., Holroyd and Super, 1984). Knowing that most SLW tends to be in this region is of considerable importance when attempting to target the SL W with AgI. Since most of the SL W is near the barrier surface, produced by the maximum vertical motions of forced airflow over the mountain (at least in the absence of convection), it is more likely that ground-based seeding can succeed in providing ice nuclei to the SL W. On the other hand, cloud temperatures near the barrier sometimes will be too wann for significant nucleation by Agl. Typical generator outputs and plume dispersion will result in low concentrations of effective ice nuclei at temperature wanner than about -8 oC, using conventional solutions and seeding rates. Field research has been conducted in Utah under the Utah/NOAA Cooperative Program early in each of the years 1981, 1983, 1985, 1987, 1989, 1990, and 1991. Field efforts were between mid-January and mid-March and lasted from 6 to 8 weeks. All but the 1990 and 1991 field projects were carried out in the Tushar Mountains of southern Utah near Beaver. The limited 1990 obselVational program and more extensive 1991 field project were cOl1:ducted on the Wasatch Plateau of central Utah near FailView. Portions of both experimental areas drain into the Sevier River system so the observations have direct relevance to this feasibility study. 9