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<br />2010
<br />
<br />JOURNAL OF CLIMATE AND APPLIED METEOROLOGY:
<br />
<br />VOLUME 22
<br />
<br />Suggested increases were also found on the lee slope
<br />and broad valley beyond the intended target area. These
<br />tended to be with Ridge temperatures colder than about
<br />-120C. There was also a suggestion that seeding may
<br />have decreased downwind precipitation for 700 mb
<br />temperatures near the -10 to -140C range.
<br />Independent snow course data were examined for
<br />the two courses on the lee slope of the Main Ridge,
<br />located between the intended target and the seeding
<br />generators. These data show unusually high snowpack
<br />water equivalent values for the 1970-71 and 1971-72
<br />seeded winters. Values for these winters depart from
<br />the values predicted by control snow courses by - 17
<br />and 25% respectively.
<br />The apparent increases in precipitation associated
<br />with seeding during the colder storms are believed to
<br />be consistent with current physical understanding,
<br />which admittedly still has serious limitations. The air-
<br />flow and AgI plume tracing results from the BRE were
<br />reviewed. These are consistent with the hypothesis that
<br />the seeding agent generally was transported up and
<br />over the Main Ridge and toward the intended target,
<br />with sufficient diffusion for wide-area coverage by the
<br />AgI plumes. Also, the strong uplift above the windward
<br />slope of the Main Ridge should have produced con-
<br />siderable condensate in the zone through which the
<br />AgI was transported. Further, simulation-laboratory
<br />calibrations ofthe seeding generators used suggest that
<br />artificial nucleation should have been very limited until
<br />Ridge temperatures were colder than about -90C.
<br />In the opinion of the authors, the results presented
<br />herein are encouraging enough to justify further field
<br />efforts in the Bridger Range. However, a confirmatory
<br />statistical experiment is not advocated at this time.
<br />Rather, it is recommended that a limited program of
<br />airborne measurements, primarily downwind of the
<br />southern generator site, is the most appropriate next
<br />step. Several plume tracing flights have shown that the
<br />southern generator site is capable of routinely pro-
<br />ducing AgI plumes that are transported over both the
<br />Main Ridge and Bangtail Ridge target area, with tops
<br />about 600 m above the highest point in the latter ridge.
<br />Preliminary contacts with the Federal Aviation Ad-
<br />ministration have suggested that it should be practical
<br />to make north-south aircraft passes above the Bangtail
<br />Ridge during storms, while within 300 m of the highest
<br />terrain. This would place the aircraft well into the
<br />seeding plume. An NCAR ice nucleus counter and
<br />repeated flight passes in opposite directions would de-
<br />lineate the seeding plume boundaries. Simultaneous
<br />measurements ofliquid water content and ice particle
<br />concentrations and size spectra should yield marked
<br />differences between in-plume observations and those
<br />made crosswind of the plume in nonseeded clouds.
<br />Analysis of such data, collected from several storms,
<br />should yield physical evidence that tends to either con-
<br />firm or reject the suggestions of the statistical analysis
<br />
<br />presented. If the airborne physical observations ap-
<br />peared in accord with the statistical suggestions, a con-
<br />firmatory statistical experiment with a strong physical
<br />component would appear to be justified.
<br />
<br />Acknowledgments. Many individuals contributed to
<br />the Bridger Range Experiment. Among the staff at
<br />Montana State University, Tony Grainger, Jack
<br />McPartland ~nd Bob Yaw made extraordinary efforts
<br />to carry out the research program with limited re-
<br />sources. Others with a significant role included Jim
<br />Edie and Val Mitchell. Professors Lewis Grant and
<br />Paul Mielke of Colorado State University made sub-
<br />stantial contributions to the development and design
<br />of the BRE. Gerhard Langer of the National Center
<br />for Atmospheric Research devoted much effort, espe-
<br />cially in the airborne tracing of AgI plumes. Finally,
<br />Dr. Archie Kahan and Staff of the Bureau of Recla-
<br />mation are to be acknowledged for' their support and
<br />assistance. The program was chiefly funded under
<br />Contract 14-06-D-6798, Bureau of Reclamation, De-
<br />partment of Interior. '
<br />
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