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

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surveillance radar on the Mesa. Some costs would be incurred in obtaining the WSR-88D
observations, but a significant net savings would result. This savings would be partially offset,
however, by higher travel costs on the Mesa caused by more oversnow travel. WSR.88D units
scheduled for Utah will be too distant to serve a similar function for the Wasatch Plateau.

The third noteworthy difference is that an upwind barrier, the San Pitch Mountains, li~s
parallel to the Wasatch Plateau at about 30 km distance (about 1.5 h travel time with typical
winds). Seeding the Plateau from near the top of the San Pitch Mountains may be feasible, and
that possibility will be tested during the direct detection phase. Seeding from the upwind
barrier would provide considerably more time for AgI dispersion, and more time for ice crystal
growth and fallout, two significant advantages.

The Grand Mesa also has an upwind barrier for southwest through west flow: the
Uncompahgre Plateau. However, the Plateau is about 80 km upwind, so targeting would be
expected to be difficult, especially because of the more complex terrain. The Mesa has an
east-west orientation and the Uncompahgre Plateau lies from southeast-to-northwest. Such far
upwind seeding during the direct detection phase on the Mesa is not presently recommended.
However, numerical model simulations should be examined to further evaluate this possibility.

4.4 Numerical Modeling

Work will commence during the CREST's first year on adaptation of an existing
time-dependent, three-dimensional numerical model to the specific experimental areas. This
model likely will be the nonhydrostatic anelastic numerical model developed at the NCAR
(National Center for Atmospheric Research) by Dr. T. Clark and associates (Clark, 1977;
Bruintjes et al., 1992). The sophisticated model will be used to examine key physical processes
in winter storms and precipitation. Processes of particular interest incJlude the
three-dimensional airflow and associated T&D of ground-released seeding agents, and growth
and fallout of precipitation particles. The model will be used for in-depth case studies aimed at
improving understanding of the physics involved in natural storms, and the perturbations
caused by seeding. Field observations and model predictions will be continually compared to
test the model and indicate where improvements are needed.

Numerical modeling runs using the field observations will help assess which atmospheric
conditions are suitable for cloud seeding and which are not. This process will provide valuable
input to the statistical experiment design effort.

A simplified version of the model will be used in real-time to decide which seeding release
points to use during individual experiments. The availability of affordable, powerful <computer
workstations will allow use of sophisticated models for targeting decisions. The State of Arizona
weather modification program successfully tested field use of the Clark model during early
1992.

4.5 Seeding Sites and Agents

The proposed program will exclusively use ground-based seeding because of the relatively high
costs and logistic (airframe icing) and volume filling (limited dispersion) problems involved with
aircraft seeding of winter orographic clouds. Remote-controlled AgI generators and propane
dispensers will be operated at high altitude seeding sites. Silver iodide will be used when SLW

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