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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

and indium concentrations in seeded snowfall are very low, seldom over 100 ppt (parts per
trillion), and are not considered harmful. *Two manned sites will be operated for microphysical
measurements on top each barrier, one in the center of the expected target area, an.d the other
crosswind of the target. The latter site will provide a time history of natural cloud and snowfall
characteristics during each direct detection experiment for comparison with the seeded site.
The crosswind control observations will indicate significant natural changes that might mask
real seeding effects, or be misinterpreted as real effects. Data collection at these stations will
include two-dimensional imaging probe measurements, high resolution precipitation gauge
records, microphotographs of ice crystals and snowflakes, and frequent snow samples for later
chemical analysis of silver and indium concentrations.

Radar might appear to offer a means of estimating precipitation over the entire target area and
beyond. However, seeding effects observed by radar often are masked by natural snowfall
(Deshler et al., 1990). The backscattered radar radiation from low concentrations of large
natural snowflakes can overwhelm the return from much higher concentrations of smaller
crystals produced by seeding because radar reflectivity factor is proportional to the Eith power of
particle size. Seeding might even increase snowfall rates while decreasing radar reflectivity
under some circumstances.

Radar will be used to search for evidence of seeding effects when natural snowfall is very light
or nonexistent, but radar is not expected to yield convincing evidence of seeding effl~cts most of
the time because of returns from natural snow particles that grow large because of long growth
times and fall trajectories. Radar will provide a means to examine spatial and temporal
homogeneity during direct detection experiments to help in their conduct and interpretation.
For example, commencing a seeding experiment just prior to passage of a deep cloud band with
heavy natural snowfall would be undesirable (unless one was attempting to document that
seeding was ineffective in this situation). Direct detection experiments should bE! conducted
when natural variations in time and space are minimal.

The specific instrumentation that has been discussed in this subsection is summarized in
table 4.1.

31