WMOD00571

• Title: Static Mode Seeding of Convective Clouds - A Review
• Date: 5/23/1984
• Weather Modification - Doc Type: Report

During the course of the 2-year experiment, 55 clouds were tested for
acceptance as experimental units but only 20 met all the selection cri-
teria. This sample size was considerably less than the 30-45 clouds per
year that were expected from the preliminary field investigations. The
years in which the seeding experiments were carried out were considerably
drier than those used to develop the cloud selection criteria (Smith et
al., 1984). Of the 20 test cases 7 were type Al clouds, 4 seeded and 3
not seeded, and 13 were type B clouds, 8 seeded and 5 not seeded. The sta-
tistical results (Mielke et al., 1984) showed that the postulated increases
in cloud ice concentrations associated with the seeding and the subsequent
onset of riming were unequivocally established despite the limited sample
size. For all response variables beyond 5 min after treatment, except the
average liquid water content at 8 min, changes in the sample average values
of the response variables were consistent with those suggested by the phy-
sical hypothesis, but it was clear that many of the clouds were not
behaving as expected. Because of the experimental approach, the complemen-
tary physical evaluation was abll~ to detennine where, how and why the
seeded clouds behaved di fferentl:v than expected and, thereby, provi de a
better understanding of precipitation processes in natural and seeded cumu-
lus clouds in spite of the limitl~d data set that was available.

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 indicated the following about the behavior of natural and
seeded cumulus congestus clouds in Montana, and the problems in ap~ying
the static mode seeding hypothesis to such clouds:

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