WMOD00555

• Project Name: Sierra Cooperative Pilot Project
• Title: Results of the Shakedown Season for the First Precipitation Augmentation Experiment of the Sierra Cooperative Pilot Project
• Date: 6/1/1982
• Weather Modification - Doc Type: Report

Of the ten experimental units observed, only five were
randomized. The first two cases were deliberately seeded to
shakedown the procedures while seeding was known to be
occurring. The January 21, March 18, and March 19. cases
were conducted under suspension so no seeding was pos-
sible.

MEASUREMENTS

Seeding effects were measured with three basic systems:
1) a microphysics aircraft, 2) radar, and 3) precipitation
instrumentation at the ground. Sufficient observations were
not obtained during 1981-82 to permit conclusive state-
ments about seeding effects. Two to four years are required
to obtain enough cases to detect the seeding signal above
the natural variability. The purpose of the 1981-82 season
was to shakedown the field and analysis procedures.
Although the quality of observations from prior calibra-
tion years is less than that in 1981-82, inclusion of that
data allows some comparison of seeded and non-seeded
cases.

Microphysics Aircraft

The microphysics aircraft observed many variables as it
penetrated seeded and non-seeded clouds, (1. D. Marwitz
and R. E. Stewart, 1981). Figure la and Ib show the
average change in time of one of the primary response
variables measured. by the aircraft. Figure la shows the
average ice crystal concentration measured with a Particle
Measuring System 2D-C optical imaging probe in four
seeded and 16 non-seeded A-type cells.
It is evident that the seeded cells had considerably more
ice crystals than the non-seeded cells. In fact, after the first
few minutes the seeded cells exceeded the non-seeded cells
by over an order of magnitude. Figure 1 b shows the same
variable as Figure la but for C-type clouds. In this case the
non-seeded cells show more ice crystals than the seeded
cells.

Radar Analysis

The radar analysis for the 1981-82 season has not been
completed as of this paper. However, one of the main
analysis procedures, shown in Figure 2, is the percent
coverage of radar reflectivity within boxes downwind of
the seeding line (A. W. Huggins, 1981). Figure 2 shows
boxes centered at 30 and 60 minutes after seeding for the
14 February 1979 case. The boxes are moved downwind at
the speed of the cell motion and the sides of the boxes
diverge to allow for dispersion of seeding material laterally .
The coverage greater than 20 decibels of radar reflectivity
(DBZ) at 30 and 60 minutes after seeding is 23.3 percent
and 57.3 percent, respectively. Similar boxes are evaluated
each 30 minutes after seeding and averaged together over
the experimental unit. Percentage coverage greater than a

Vardiman

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fixed DBZ level is expected to be greater for seeded cases
than for non-seeded cases.

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Time after initial pass (min.)

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Time after initial pass (min.)

Figure 1. Average Ice Crystal Concentration Versus Time.
(a) A-type cells. (b) C-type cells.

Precipitation Instrumentation

Gages - A network of approximately 40 Belfort weigh-
ing bucket rain gages was distributed over the American and
Truckee- Tahoe River Basins. These gages were organized
in 20 kIn wide regions parallel to the crest. By overlaying
the expected seeding plume for each case over the gage
regions, the contribution of seeding by elevation zone may
be estimated. The average intensity and probability of
exceeding fixed precipitation rates were calculated for each
region within the seeding plume (Solak, et. al., 1981).
Table 2 shows the average intensity and precipitation
probability for the region 20-40 km upwind of the crest for
the 10 cases observed in 1981-82. Although the average
intensity is greater for the seeded cases than for the non-
seeded cases, the average probability of exceeding 0 mm/hr
is less. The random noise for such a small sample is too great

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