WMOD00476

• Project Name: Colorado River Basin Pilot Project
• Title: Executive Summary of Comprehensive Evaluation
• Date: 12/1/1976
• Weather Modification - Doc Type: Report

predictions applied to treated cases were then used as covariates in the sta-
tistical analysis.

Using data from the balloon-borne rawinsondes (shown in Figure 4) in an
orographic flow model, the evaluators computed the total condensation as the
air mass cooled in its ascent over the barrier. The computation was done
separately for cold top and warm top, stable and unstable clouds, and with
and without seeding. The unstable category of clouds contained all those in
which deep convective activity was indicated from the sounding; whereas, the
stable category contained all others, including many with a shallow unstable
layer. The bulk condensation amount was then compared with the average pre-
cipitation observed over three equal segments of the San Juan range: the
western upwind slope, the crest, and the downwind slope.

Figure 5 shows, for the stable category of clouds, the average percent-
age of the total condensate falling out over each seglnent when cloud-top
temperatures were warmer than -290C (left) and colder than -290C (right).
One curve (8) is for seeded clouds, the other (N) for not-seeded.

For the "warm" clouds, sample (S) shows fractions ranging from .090
on the upwind slope to .205 at the crest, and .125 on the downwind slope.
These total 42.5% of the total available moisture that was produced over the
profile. In the control or natural sample (N), the range was from .050 on
the upwind slope to .095 on the crest, and .050 on the lee slope, for a total
of 19.5%. The natural efficiency of removal of condensate as snow over the
whole barrier was thus under 20 percent, a little over 80% evaporating
to the lee, while seeding raised the overall efficiency to about 42 percent,
more than double that of the natural case.

For the "cold" clouds, natural precipitation is quite efficient, amount-
ing to 86 percent for the barrier as a whole. The seeding sample curve shows
the ravages of overseeding. The overall barrier efficiency for the seeded
sample is 43 percent. Thus, seeding all cold-top clouds appears to decrease
their precipitation by about half. However, as will be discussed below, there
is a sub-category of cold top clouds that yield increases in precipitation
with seeding.

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.J