WMOD00413

• Title: Validation of Precipitation Management by Seeding Winter Orographic Clouds in the Colorado River Basin
• Date: 9/1/1993
• Weather Modification - Doc Type: Report

Larger areas will be targeted during the follow-on statistical experiments. These areas will be
specified as the statistical design is developed. Observations from the direct detection phase,
and numerical modeling results, are needed to specify the exact target areas for the statistical
experiments on the Grand Mesa and Wasatch Plateau, respectively. At this time, the target
areas are anticipated to be in the range of 500 to 1000 km2.

Limiting target size will increase association with nearby crosswind and upwind control
precipitation gauges, discussed further in section 5, which reduces the experimental duration
needed for significant results. A very large target area is not needed to demonstrate cloud
seeding effectiveness. However, the specified area must be routinely targeted with appropriate
concentrations of seeded ice particles.

Both the Grand Mesa and the Wasatch Plateau are reasonably typical of high terrain in the
Basin. Both barriers have broad along4he-wind dimensions. Consequently, with typical winds,
ice crystals formed by seeding over the windward slopes should have sufficient time (distance)
to grow and settle to the surface before reaching the lee subsidence zone where cloud droplet
evaporation and ice crystal sublimation take place. Both barriers have moderate on top relief,
permitting low-level aircraft sampling during storms. Both areas have all-weather highways for
access to high elevations, and are amenable to oversnow traveL Operational and experimental
seeding programs have been conducted on each barrier in the recent past. However, the
proposed experimental programs are much more comprehensive than the limited past
experiments. Both mountain areas have a high frequency of snowfall and SL W occurrence as
discussed in detail in appendix B.

4. DIRECT DETECTION EXPERIMENTAL PHASE

Section 4 discusses the CREST direct detection phase, including expected seeding impacts and
suspension criteria, experimental scheduling, the role of numerical modeling, and how seeding
will be conducted. The observ~tions to be made during direct detection experiments, along with
their purposes, are discussed in detail. Procedures for a typical experiment are described.

4.1 Seeding Impacts and Suspension Criteria

Environmental compliance activities will be finalized for each experimental area prior to any
seeding. The procedures involved in compliance are briefly discussed in section 8, and described
in detail in appendix D. An environmental monitoring plan will be developed and implemented
prior to seeding.

Suspension criteria will be developed prior to seeding. Seeding will be suspended during periods
of well-above normal snowpack, serious avalanche danger, and possibly other periods when
additional snowfall may be undesirable. Although the direct detection seeding experiments are
not expected to enhance the snowpack to any significant degree, it would be prudent to avoid
the perception that seeding may have contributed to problem situations.

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