WMOD00402

• Project Name: Sierra Cooperative Pilot Project
• Title: Sierra Cooperative Pilot Project - Final Report for Period November 1984 - September 1986
• Date: 6/1/1987
• Weather Modification - Doc Type: Report

2.2

3. the extent to which this technique might be developed
for covariate analysis of randomized seeding experiments.

(b)

Background:

1. Variability of Seeding Chemicals in Precipitation.

In earlier investigations in the Pyramid pilot Project (1977), it was
shown that the silver content of the snow falling in the target area (east
of the Sierra crestline), varied considerably with time throughout the con-
tinuous seeding period at'any one sampling site. It was also shown by
Warburton (1973) that the highest silver contents (from the silver iodide
seeding) occurred when the precipitation crystal types were predominantly
needles and columns. Since these crystal types are produced predominantly
in the warmer temperature ranges (-30C to -80C), it seemed reasonable to
suggest that the principal water capture and growth of these crystals had
occurred in the lower elevations. This is consistent with the later obser-
vations from aircraft that frequently there is very little liquid water in
the winter clouds over the Central Sierra Nevada above 12,000 ft elevation.

Later studies of the chemistry of snow were conducted by Warburton,
et al., (1979) and by Stone and Warburton (1981). The results of this work
showed that seeding chemicals (silver) and often special tracer chemicals
(indium and cesium) appear in the snowfall in temporally and spatially
confined manners. This evidence was obtained by two separate sampling
techniques, time sequential and vertical profiling. The time sequential
sampling method requires personnel to be located at the sampling sites
during snowfall events. The profiling method is normally used between
storms in fair weather. Both give similar information but real-time
sampling has the advantage of being able to collect ice crystal structure
information simultaneously.

The chemical analysis data from both sampling techniques have led to
the same basic conclusion, i.e., that the seeding agent from both ground-
based or airborne seeding systems appears in the snow only 10% to 30% of
the time during which seeding is conducted.

These results imply that seeding 'plumes' are spatially confined even
at distances of 70 km downwind of the release sites. Although the plumes
may be located over a given sampling site for periods of time ranging from
zero to twelve hours throughout a continuous seeding event, on the average,
the target area is not receiving seeding aerosol material in its precipita-
tion for 70% to 90% of the seeding period.

Because of these results, we plan to conduct a program of research
aimed at developing a new method for assessing the effects of seeding based
upon trace Chemistry.

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