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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reach by snowmobile. The information collected by such sites can be limited because of
uncertainties of routinely targeting them with meandering seeding plumes.

Remote sensing of seeding-caused ice particles, or radar chaff simulating their trajectories, is
possible under some conditions. Radar has long been used to detect ice particles. However, in
most circumstances, radar can detect seeding-caused crystals only if natural ice crystals are
scarce. A recently developed technique, discussed by Martner et al. (1992), permits radar
tracking of chaff fibers by monitoring their circular depolarization ratio.

Radar chaff consists of almost microscopically thin aluminum-coated glass fibers cut to half the
radar's wavelength. Unlike past chaff tracking by radar, the circular depolarization approach
permits chaff detection even in the presence of moderate precipitation. Very small
concentrations of chaff fibers can be detected by radar using this technique. The approach
should offer a first approximation of the ice particle T&D because individual chaff fiber fall
velocities (about 30 em S.l) are similar to those of many ice crystals. Thus, radar monitoring of
chaff co-released with a seeding agent will provide information with which. to estimate ice
particle trajectories.

This chaff tracking approach has limitations. Co-released plumes of AgI and chaff will separate
with time, especially if significant time is required for AgI nucleation, or if resulting ice crystals
fall with significantly different speeds than the chaff. Chaff may settle to the surface well before
seeding-caused ice crystals if ground-releases are made in the stable lower atmosphere
characteristic of many orographic storms. Improved release methods are needed to reduce chaff
clumping, which results in most ground-released chaff settling to the surface near the release
point, substantially reducing the signal strength. However, as noted, a very low chaff
concentration is detectable.

The possibility of the chaff itself affecting cloud microphysics will be given serious consideration
during the direct detection experiments. Chaff is known to produce high concentrations of ions
through corona discharge when in high electric fields. The ions may influence cloud
precipitation processes in ways still poorly understood. Electric field strengths are not known
during winter storms over the Grand Mesa and Wasatch Plateau, but will be measured during
seeding experiments involving .chaff releases.

In spite of these potential problems, the circular depolarization ratio method of chaff tracking is
expected to be an important part ofthe CREST T&D studies. This method allows monitoring of
the important layer between the lowest safe aircraft sampling level and the target surface.
Much of the seeding-caused conversion of SLW to snow takes place in this layer, which is
impractical to measure by direct means.

4.6.6 Ice particle growth environment

The temperature/moisture/wind environment will be routinely monitored at a number of
surface locations from the seeding sites to the target area. Tower observations of wind,
temperature, and humidity will be made at the seeding sites, and at selected sites further
upslope, by automatic weather stations. Microwave radiometers, discussed in section 4.6.1
above, will indicate liquid cloud, and a laser ceilometer will indicate cloud base altitude.