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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4.6.3 T&D of AgI and propane-created ice crystals

The T&D of seeding agents and resulting embryonic ice crystals is one of the key uncertainties
in weather modification. Tracking the seeding agent is of interest in determining whether it
reaches the desired cloud region in appropriate concentrations, and in ascertaining what cloud
volume is treated.

The overall T&D problem can be thought of as involving two phases: the T&D of the seeding
agent and the T&D of resulting ice particles. The latter is discussed in section 4.6.5 below. At
any given time some of the AgI population may have created ice crystals, which are growing
and settling within the cloud, while other AgI particles are still dispersing through the cloud
with the potential to create ice. Propane seeding produces ice crystals very near the dispenser,
so only the T&D ofthe resulting ice crystals is of concern.

In situ monitoring of AgI's T&D can be done either by measuring ice crystals formed by the
seeding agent in a acoustical ice nucleus cloud chamber (Langer, 1973), or by observing a
co-released tracer gas used to simulate the AgI plume. Both approaches will be used in the
CREST. The tracer gas will be SF 6 (sulfur hexafluoride), monitored with a fast-response
detector (Benner and Lamb, 1985). Tracer gas plumes can be used to approximate AgI plumes
because both have negligible settling velocities so trajectories and dispersion should be similar
in turbulent clouds.

Monitoring AgI or SF 6 plumes has been accomplished within cloud as close as 300 m to the
highest terrain in both proposed target areas. Reliefis limited on both the Grand Mesa and the
Wasatch Plateau, so such sampling took place about 600 m above average barrier-top
elevations. Previous studies (Holroyd et al., 1988; Super and Boe, 1988b; analysis in progress
from the Wasatch Plateau) have shown that plumes released on the windward slopes can
routinely be found over the target area in cloudy conditions, with most of the AgI within 700 m
of the mountain surface. Thus, in-cloud sampling usually monitors only the upper portions of
ground~released seeding plumes.

Silver iodide and SF 6 plumes will be monitored on top the barrier with mobile detectors on a
four-wheel drive van or instrumented oversnow vehicle, and with detectors at a fixed
installation in the target.

4.6.4 Seeded zone temperature

The vertical extent of the seeded zones will be monitored during aircraft sampling by making a
series of stepped passes normal to the wind, with 150 m vertical distance between passes over
the same flight line. The temperature of the upper portion of seeded zones will be sampled by
aircraft. Surface sensors at the seeding sites and on top the barriers will provide temperature
observations along the plume/surface interlace.

During nighttime experiments the vertical extent of the seeded zone will be unknown.
However, it will be estimated from the experience of daytime aircraft sampling and model
results.