WMOD00484

• Title: Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide Over the Rocky Mountains - Part I
• Date: 10/10/1988
• Weather Modification - Doc Type: Report

1150

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

Mesa in a sheltered clearing about 300 m south of and
100 m below the Mesa caprock. Winds were rarely
more than 2 m s -1 at this location. Snow fell down a
31 cm diameter chimney onto cold glass plates for
timed intervals. These plates were photographed with
sufficient detail to identify the particle habits as well
as the concentrations. The area of the collection plate
photographed was always the same, so any possible
tendency of the camera operator to select areas con-
taining the larger or more interesting crystals was
avoided.

4. Methodology

The ground-based seeding techniques used over the
Bridger Range and the Grand Mesa were essentially
identical, while the airborne seeding method used over
the Grand Mesa employed a different approach. In each
of the experiments, a single seeding generator was used.

a. Ground-based seeding experiments

For each of the ground-based experiments, a single
AgI generator was operated well up the windward side
of the barrier. The plume thus generated was advected
up and over the barrier into the orographic cloudmass,
as will be shown in Parts II and III.
An instrumented aircraft monitored the seeded cloud
volume and neighboring natural cloud volumes by
making constant-altitude passes downwind of the gen-
erator and approximately perpendicular to the wind.
Aircraft altitude sometimes varied from pass to pass
but was usually the minimum allowable: about 300 m
above the highest terrain in the vicinity. Reciprocal
passes at a single altitude were utilized to delineate the
leading edges of the AgI plume as defined by (1), so
that the width of the plume was approximated for each
pair of passes. Any corresponding increases in IPC were
presumed to be due to the Agl. Natural clouds cross-
wind on either side of the plume were evaluated as
controls for comparison with the regions containing
AgI and enhanced IPC.

b. Airborne seeding experiments

To initiate each airborne seeding experiment, the
aircraft AgI generator was ignited and an arc flown at
a constant distance from the surface target at the lowest
permissible altitude. Each arc was centered on the line
intersecting the surface target and parallel with the
mean wind at the seeding altitude. The distance of each
arc from the target was varied from experiment to ex-
periment in an effort to assess the impacts on plume
spreading, nucleation, and precipitation development.
After each seeding pass the instrumented aircraft
made a series of sampling passes at the seeding altitude,
over the Snow Lab and parallel to the mean wind. The
evolution of the AgI plume as it spread, encountered
SLW, and produced enhanced IPCs was thereby mon-

itored. As in the ground-based experiments, regions
having enhanced IPC coincident with AgI were pre-
sumed seeded. Some additional knowledge about the
deformation and stretching of the plume was also ob-
tained.
The ultimate evaluation of each airborne seeding
experiment was based on the characteristics and inten-
sity of the precipitation observed at the Snow Lab sur-
face target, which were monitored for the duration of
each experiment by means of the previously described
ice crystal photography system.

5. Summary

Several winter orographic cloud seeding experiments
were carried out over mountain barriers in Montana
and Colorado. Silver iodide was released either on the
windward slope or by aircraft. Recent advances in
technology made it feasible to detect (i) the AgI in-
cloud to within about 300 m of each target area's high-
est terrain; (ii) corresponding changes in ice particle
concentration, habits and sizes; and (iii) corresponding
changes in SL W content. Relative changes in the pre-
cipitation rate were estimated at aircraft levels with a
recently developed computerized approach. In addi-
tion, surface observations of seeding-induced changes
in ice particle characteristics and precipitation were
made in Colorado.
In this first part of a three-part study, we have de-
scribed the experimental design and physical hypothesis
underlying the experiments and discussed in detail the
instrumentation used, its associated calibrations and
limitations, and the methodology employed in each
type of experiment.
The results of applying the instrumentation and
methodology [described here] are found in Parts II
and III. They show that marked changes in cloud mi-
crophysics were detectable with both airborne and
ground-based AgI seeding of winter orographic clouds.

REFERENCES

American Meteorological Society, 1985: Planned and inadvertent
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Boe, B. A., and A. B. Super, 1986: Wintertime characteristics of
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orado. J. Wea, Mod., 18, 102-107.
DeMott, P. J., W. G. Finnegan and L. O. Grant, 1983: An application
of chemical kinetic theory and methodology to characterize the
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