WMOD00509

• Title: Physics of Winter Orographic Precipitation and it's Modification - Summary of Presentations
• Date: 10/1/1985
• Weather Modification - Doc Type: Report

WORKSHOP SUMMARY

Re-examination of Some Results from the CRBPP
William A. COQlper

1.. Summary of the experiment

Although the Colorado River Basin pilot Project (CRBPP)
has many weaknesses in comparison to more modern field
experiments (e.g., because of the lack of radar or radar
data, and the limited and early-generation airborne
measurements), it is still the last fully randomized
operational program incorporating physical observations in
the Colorado River Basin. A strength of the experiment is
that there were physical observations (surface and airborne)
collected in context of the randomized experiment, where
seeding was conducted and effects on snowfall could be
assessed. Many aspects of the results appear clearer in the
context of the last 10 yrs and subsequent field experiments
in nearby areas. The observations of high ice concentrations
and low liquid water contents have generally been supported
by subsequent measurements.
The randomized experiment was conducted for 5 yrs, during
the winters of 1970-1 to 1974-5. The experiment was
generally based on the successful results from CLIMAX,
although the CLIMAX design was not followed in detail. The
earlier CLIMAX results led to the prediction that a 10:1
cost:benefit ratio could be expected for added precipitation
in the Colorado River Basin. The experiment was a target-
only design, with a 24 hr experimental unit. The
experimental day was declared according to criteria which
varied slightly during the experiment, but included
requirements that there be forecast precipitation in the
target area, that the 500 mb temperature be >-230C, that the
mean wind be toward the barrier, and that previous snowpack
fell below the suspension criteria. There were 33 ground-
based generators, dispersing a NaIjAgI mixture. Averge
precipitation in the target area varies from 20-100 cm, and
typically falls on about 90 days. Approximately half have
500 mb temperatures suitable, and ac:count for about half of
the total precipitation. The hope was that a 50% increase on
these suitable days would thus lead to a 25% overall increase
in precipitation.

2. The statistical evaluations

The primary evaluation of the experiment was performed
by Elliott et ale (1978). The snowfall on seeded days tended
to be less than on unseeded days, although the results were

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