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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. difficult in the Sierra Nevada. Deshler and Reynolds (1990) presented a case study in which the
effects of aerial seeding were followed for over 90 min and 100 km. These direct detection
physical experiments have been encouraging. However, too few have been conducted to
demonstrate how often storm conditions permit the seeding hypothesis to operate, or how much
additional snowfall might result from routine seeding. Moreover, the technology exists to design
improved direct detection experiments to better determine appropriate seeding rates and
approaches for given cloud conditions.

Improvements in numerical modeling of winter orographic clouds have significantly aided the
understanding of airllow and microphysical processes (Young, 1974; Cotton et al., 1986;
Bruintjes et al., 1992). Incorporating observations from a particular mountain region into a
numerical model adapted for that region can markedly increase understanding of the key
processes involved and how seeding influences them. Numerical model simulations can be run
with each set of intensive observations to continually improve experimental design, and to test
and improve the model. The use of state-of-the-art models with comprehensive observations
from direct detection seeding experiments is expected to provide the most rapid progress
possible toward demonstrating a scientifically sound weather modification technology.

3. GENERAL APPROACH TO THE PROPOSED PROGRAM

This section presents cloud seeding fundamentals and the general hypothesis by which seeding
is expected to enhance snowfall. An overview of the proposed experimental program is
presented, and the results of the experimental site selection process are described (and
presented in detail in app. A).

3.1 Fundamentals of Winter Orographic Storms and Cloud Seeding

Winter storms over the Basin's mountains vary from simple orographic to orographically
enhanced. As described by Rangno (1986), orographic storms are "cloud systems that form
solely as the result of air rising over terrain, which are seen as quasi-stationary clouds of
variable coverage on satellite imagery." Rangno refers to orographically enhanced storms as
"long-lived cloud systems associated with fronts and troughs that are trackable on satellite
imagery prior to impinging on mountain barriers." Most significant precipitation-producing
storms are orographically enhanced where the uplift caused by the mountains produces
condensate in the lowest kilometer or so above the terrain during passage of synoptic
(large-scale) storm systems.

The degree to which the barrier-enhanced SLW is converted to snowfall partially depends on
the natural ice particle concentration, which, in turn, is related to cloud depth. Shallow clouds
with wann top temperatures (above about -20 OC) generally appear to be the most suitable for
seeding (e.g., Grant and Elliott, 1974; Dennis, 1980) because they tend to have limited ice
crystal concentrations. Thick clouds with cold tops produce abundant ice crystals that may
settle through and largely convert the orographically enhanced SLW to snowfall. However,
wide variability in ice crystal concentration exists at a given temperature in some clouds
because of variations in available ice nuclei and secondary crystal production processes (Hallett
and Mossop, 1974). Sometimes, even a high concentration of natural ice particles cannot
convert all of the SLW to snowfall because of strong cross~barrier winds which produce
abundant liquid water.