Laserfiche WebLink
<br />valley floor and foothill winds were typically calm or very light during mountain snowfall. Like the <br />Park Range, the Bridger Range has a broad upwind valley for prevailing winds. <br /> <br />Consequently, initial attempts to use foothill generators were abandoned, and the randomized <br />Bridger Range Experiment (Super and Heimbach 1983) deployed manual AgI generators about 2/3 <br />of the way up the windward slope. These were maintained by field technicians who lived in small <br />shelters while frequently checking the generators. They also optically tracked pilot balloons when <br />visibility permitted. This was an expensive seeding approach compared to use offoothill <br />generators, as all major supplies and equipment were lifted in by helicopter. It had been intended <br />to use remote-controlled AgI generators at these sites, but they proved unreliable. Winter access <br />was limited to arduous climbs to the seeding sites. But the result was that clouds were routinely <br />seeded as shown by Super (1974), Super and Heimbach (1983), Heimbach and Super (1988) and <br />Super and Heimbach (1988). Even to the present day, few other experiments or operational projects <br />have provided such strong evidence that orographic clouds were routinely seeded. The general lack <br />of such documentation is considered a major weakness in several past and ongoing winter seeding <br />programs, leaving open the very real possibility that clouds were, and are, often inadequately seeded <br />or not seeded at all. Physical evidence always trumps suggestions from statistical analyses, <br />especially when the latter may be subject to bias. <br /> <br />Marwitz (1980) presented results based on 3 months of aircraft sampling and other observations <br />made over the San Juan Mountains of southwest Colorado. That investigation was during the final <br />1974/75 winter of the five winter Colorado River Basin Pilot Project (CRBPP), described in detail <br />by Elliott et al. (1978). They noted that seeding was done with 20 manually-operated AgI <br />generators and 13 remote-controlled units, "--- placed in valleys and on hills 10-30 km (6-19 miles) <br />south and west of the target area." The study reported by Marwitz (ibid.) was prompted by results <br />of aircraft and other sampling made the prior winter by the University of Washington Cloud <br />Physics Group under the leadership of Professor Peter Hobbs. Marwitz (ibid.) cited their 1975 <br />internal report, no longer available to the authors of this report, as follows, "Hobbs et al. also <br />studied the transport of ice nuclei from the ground-based seeding generators. They found that the <br />ice nuclei did not reach cloud level under stable conditions, and entered the cloud too close to the <br />target area under marginally unstable conditions to increase snowfall over Wolf Creek Pass. These <br />unexpected and important results led to our additional field investigation." Clearly the University of <br />Washington study raised grave concerns about ground-based seeding of the San Juan Mountains, <br />even during marginally unstable conditions. <br /> <br />The investigation reported by Marwitz (1980) used aircraft sampling, rawinsondes released at 3 <br />hr intervals, and finer scale data from 12 storm passages. He concluded that, "The typical storm <br />was seen to evolve through four stages: stable, neutral, unstable and dissipation. During the stable <br />stage, much of the flow below mountain top level is blocked and diverted toward the west. During <br />the neutral stage, the storm is deep - typically extending throughout much of the troposphere. <br />During the unstable stage, a zone of horizontal convergence appears to form near the surface at the <br />base of the mountain on the upwind side of the mountain and a convective cloud line is often <br />present over this convergence zone. Subsidence at mountain top height causes dissipation." <br /> <br />A companion paper to Marwitz (ibid.), by Copper and Marwitz (1980), indicated that seeding <br />potential might exist over the San Juans during some storm phases. They suggested that the <br />unstable storm stage was most suitable for seeding as it had the highest liquid water contents and <br />regions with relatively low natural ice concentrations. These regions were associated with <br />convection. But AgI was detected during only 3 of 10 sampled storm periods indicating the <br />ground-based seeding approach used by the CRBPP was not very effective. And detection does not <br />necessarily mean that a sufficient concentration of AgI ice nuclei were present. Valley generators <br /> <br />18 <br />