WMOD00550

• Title: Scientific Weather Modification Experimentation in the United States
• Weather Modification - Doc Type: Report

SCIENiIFIC WE.a.THER MODIFICATION EXPERmENT.~TION IN TIlE UNITED STATES
I

Bernard A: Si 1 verman
Bureau of Reclamation
Denver, Colorado U.S.A.

1. Introduction

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 experimental
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 were present. Type A-2 clouds were
included to account for the occasional
cccurrence of situations where ice multipli-
cation by the rime-spl intering process were
found.

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1

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This paper discusses recent scientific
weather modification research in the United
States. Four large precipitation enhancement
research programs are briefly reviewed: the
recently completed HIPLEX-l (Montana High
Plains Experiment) and FACE-2 (confirmatory
Florida Area Cumulu5 Experiment) exoeriments,
the ongoing SCPP (Sierra Cooperative Pilot
Project) program anC the planned CREST
(Colorado River Enhanced Snowpack Test)
demonstration prograrr. In addition, high-
lights df selected activities in three areas
of related research are presented: 1) dual-
channel microwave radiometer applications, 2)
laboratory studies of the ice nucleating pro-
perties of silver iodide complexes, and 3)
evaluation of experiments by monitoring the
expected chain of physical events following
seeding. Finally, the probable nature of
future scientific weather modification
experimentation in the United States is
discussed.

2. HIPLEX-l

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 eva-
luated by an onboard real-time computer to
determine whether or not the selection cri-
teria for any of the specified cloud types
were met. The seeding was conducted by
dropping a line of dry ice pellets from a jet
aircraft at a rate of 0.1 kg/km near the -10
oC level within 2 min after a suitable cloud
was selected. Following the treatment, qry
ice or placebo, both the seeding and cloud
physics aircraft made repeated passes at spe-
cified 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).

HIPLEX was aimed at establishing the physical
basis for enhancing beneficial growing season
precipitation from convective clouds on the
High Plains of the United States. HIPLEX-l
(Bureau of Reclamation, 1979) was a ran-
domized, double-blind exploratory experiment
. to test the static mode seeding concept for
summer convective clouds over the Montana
High Plains. The experimental unit of
HIPLEX-l was semi-isolated cumulus congestus
clouds which develop precipitation naturally
through the Bergeron mechanism, that is ice
crystal growth by vapor diffusion followed by
riming into graupel. Many of these clouds do
not ra1n to the ground at all. The objective
of the static seeding was to provide an opti-
mum concentration of ice particles for the
available liquid water, about 10 per liter,
and initiate the precipitation process
earlier and lower in altitude in the deve-
l~ping clouds than would occur naturally. It
,was hypothesized that the efficiency of the
natural precipitation process would be
improved by the seeding and thereby lead to
both additional precipitation and to an
increase' in the proportion of c 1 Duds that
precipitate.

During the course of the 2-year experiment,
55 clouds were tested for acceptance as
experimenta1 units but only 20 met all the
selection criteria. This sample size was
considerably less than the 30-45 clouds per
year that \~ere expected from the prel imi nary
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 evaluation showed that
the postulated increases in cloud ice con-
centrations associated with the seeding and
the subsequent onset of riming were unequivo-
cally established despite the limited sample
size. The statistical results (Mielke et
a1., 1984) are shown in Figure 1 and the phy-
sical results (Cooper and Lawson, 1984) are
shown in Figure 2 and 3. 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 resoonse variables were consistent with
those s~ggested by the physical hypothesis,
but it was clear that in many clouds they
were not behaving as expected. '

Detailed criteria and procedures for
selecting and classifying test cases, for
performing the seeding and for the collection
of observations to be used in calculating
response variables were prescribed. Based on
preliminary expl'Jratory 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 filin

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 signi-
I ficant departures occu rred in the rema i nder.
L These studies indicated the following about I

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