WMOD00299 ‎(2)‎

• Title: Scientific Weather Modification Experimentation in the United States
• Date: 8/25/1984
• Weather Modification - Doc Type: Report

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tory studies, the cloud selection criteria (see Table 1) were expected to result
in a sample of clouds that would be amenable to seeding according to the static
hypothesis and last at least 30 Olin after treatment for the seeding to be effec-
tive. It was expected that the seeding effects would be most easily detected in
clouds with tops in the -6 to -12 oC range (type A-I clouds), but the experimen-
tal design permitted selection of rain clouds with tops in the -12 to -20 oC
range (type B clouds) as test cases when no suitable type A clouds lr4ere present.
Type A-2 clouds were included to account for the occasional occurrence of
situations where ice multiplication by the rime-splintering process were found.

The qualifying variables were measured during a pretreatment pass by a cloud
physics aircraft flying through a visually promising cloud at the -8 oC level
and immediately evaluated by an onboard real-time computer to determine whether
or not the selection criteria for any of the specified cloud types \'iere met.
The seedi ng was conducted by droppi ng a 1 i ne of dry ice pe 11 ets from a jet
aircraft at a rate of 0.1 kglkm near the -10 oC level within 2 Olin after a
suitable cloud was selected. Following the treatment, dry ice or placebo, both
the seeding and cloud physics aircraft made repeated passes at specified times
and specified levels in and below the cloud to document the subsequent chain of
physical events as represented by the response variables (see Table 2).

During the course of the 2-year experiment, 55 clouds were tested for acceptance
as experimental units but only 20 met all the selection criteria. This sample
size was considerably less than the 30-45 clouds per year that were expected
from the preliminary field investigations. Of the 20 test cases 7 were type A-I
clouds, 4 seeded and 3 not seeded, and 13 were type B clouds, 8 seeded and 5 not
seeded. The statistical results (Mielke et al., 1984) showed that the postu-
lated increases in cloud ice concentrations associated with the seeding and the
subsequent onset of riming were unequivocally established despite the limited
sample size (see Figure 1). For all response variables beyond 5 Olin after
treatment, except the average liquid water content at 8 Olin, changes in the
sample average values of the response variables were consistent with those
suggested by the physical hypothesis, but it was clear that in many clouds they
were not behaving as expected.

The physical evaluation (Cooper and Lawson, 1984) revealed that in 4 of the 12
clouds that were seeded precipitation developed in the hypothesized manner but
physically significant departures occurred in the remainder. These studies indi-
cated the following about the behavior of natural and seeded cumulus congestus
clouds in Montana, and the problems in applying the static mode seeding hypothe-
sis to such clouds:

1. The liquid water content in many of the HIPLEX-1 clouds decayed
rapidly due to entrainment, accounting for their low natural precipi-
tation efficiency. The exponential decay time constant of liquid
water, defined as the time for the maximum liquid water to reduce to
lie of its initial value, was 14 min. The liquid water depleted
faster by entrainment than seeding could exploit it to develop preci-
pitation. In this respect, the cloud selection criteria had failed to
provide the intended sample of clouds whose liquid water would persist
at least 30 Olin after treatment as required by the physical hypothe-
sis. The finding that entrainment exerts a controlling influence on

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