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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. Significant improvements in instrumentation.
. Development of numerical models able to realistically simulate airflow, cloud, and
precipitation processes over complex mountain terrain.

These factors have led to the successful conduct of a number of physical seeding experiments in
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recent years.

Failure to routinely deliver proper concentrations of seeding materials to SLW regions is now
recognized as a serious shortcoming of several past projects, especially when low altitude
ground generators were used (Rangno, 1986; Super, 1990; Super and Huggins, 1992a; Super
and Huggins, 1992b). The use of high altitude seeding generators has been shown to be capable
of routinely targeting mountain clouds over the Grand Mesa of Colorado (Holroyd et al., 1988),
the Bridger Range of Montana (Super and Heimbach, 1988), and the Wasatch Plateau of Utah
(unpublished preliminary results from 1991). Using liquid propane as a seeding agent for
winter orographic cloud temperatures as warm as 0 oC appears feasible' (ReJmolds, 1989;
Reynolds, 1991), so many winter storms too warm to treat with ground-released. AgI may be
seedable with propane.

Greater appreciation now exists of the logistics involved in conducting seeding experiments
over mountainous terrain, especially in regard to the need for low level measurements by
aircraft and remote sensing systems. Experimental areas must be chosen that permit aircraft
observations into the lowest kilometer above the terrain where most SLW is conclentrated and
where ground-released seeding plumes are found. Evidence that most SLW is located near
mountain surfaces includes observations (Hobbs, 1975a; Holroyd and Super, 198-1; Hill, 1986;
Thompson and Super, 1987; Heggli and Rauber, 1988) and results of numericaJl model runs
(Young, 1974; Blumenstein et al., 1987). Experimental areas should also permit reasonable
surface access to the barrier top, where a variety of sensing systems must be operated.

Recent improvements in instrumentation have made it practical to monitor the key physical
processes involved in cloud seeding. For example, microwave radiometers can remotely monitor
SLW above mountain barriers. Two-dimensional laser imaging probes can be used on aircraft
and on the ground to observe and analyze vast numbers of ice particles. Tracer gas and radar
chaff can be released and tracked to monitor positions of seeding plumes and resulting ice
crystals. Doppler radars can provide wind fields over mountain barriers. Wind-profiling radars
using the RASS (radio acoustic sounding system) technique can continuously monitor vertical
profiles of wind and virtual temperature. Automatic weather stations can provide surface
measurements of wind, temperature, and moisture over mountain barriers. Chemical analysis
methods are sensitive enough to detect silver in snow from AgI seeding.

Reynolds (1988), in a review of winter snowpack augmentation, showed that a consistent
relationship is emerging between physical studies and statistical results. For example,
Reclamation and other scientists have provided convincing evidence that the physical seeding
hypothesis was correct in a limited number of experiments in recent years. Super and
Heiplbach (1988) confirmed microphysical changes in seeded clouds over the Bridger Range
that presumably increased snowfall (no surface observations were made in the limited 4-week
study). Super and Boe (1988b) showed evidence of precipitation changes at aircraft sampling
levels and on the surface during a 2-mo study period over the Grand Mesa, Colorado. Deshler et
al. (1990) demonstrated seeding-induced microphysical changes at aircraft levels in about 35
pct of their experiments, but, as previously noted, following seeding effects to the ground proved

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