WMOD00550

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

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RESPONSE VARIABLES

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FIGURE 1 -- STATISTICAL RESULTS OF HIPLEX-1
(AFTER MiELKE ET AL, 1984)

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the behavior of natural and seeded cumu-
lus congestus clouds in Montana, and the
problems in applying the static mode
seeding hypothesis 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 precipitation effi-
ciency. 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 precipita-
tion. In this respect, the cloud
selection criteria had failed to pro-
vide the intended sample of clouds
whose liquid water would persist at
least 30 min after treatment as
'required by the physical hypothesis.
The finding that entrainment exerts a
controlling influence on the lifetime
of HIPLEX-l clouds and that, in turn,
limits precipitation development in
natural and seeded clouds is con-
sistent with the findings from ather
experiments on similar clouds (Braham,
1960; Schemenauer and Isaac, 1984).
The dilemma for cloud seeding applica-
tions is to be able to predict which
of the clouds in a population will
have liquid water that tasts long
enough for seeding to be effective.

2. Precipitation development in
most of the seeded cloudS'did not
proceed via the graupel process as
hypothesized. Primarily because of

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FIGURE 2 CUMULATIVE DISTRIBUTIONS OF THE
RESPONSE VARIABLES CIC2 (SOLID LINES) AND
CICS (DASHED LINES) FOR HIPLEX-l CLOUDS
(AFTER COOPER AND LAWSON, 1984)

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FIGURE 3 -- CUMULATIVE DISTRIBUTIONS FO~ THE
CONCENTRATION OF RIMED ICE CRYSTALS 5 MINUTES
AfTER TREATMENT OF HIPLEX-l CLOUDS (AFTER
COOPER AND LAWSON, 1984)

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the high ice concentrations produced
by the seeding, a combination of
aggregation and low-density accretion
onto the loose aggregates was the
dominant precipitation process. Only
small raindrops were produced in this
way, which were insignificant in com-
parison to those produced by the
natural accretional growth process.
In this respect, the seeding operation
failed to produce the target con-
centrations of 10 per liter without
penalty. The formation of aggregates
following seeding convective clouds
apears to be a common occurrence (see,
e.g., Dye et al., 1974; Strapp et al.,
1979; English and Marwitz, 1981; Rodi,
1984) despite the fact that graupel
development is usually postulated and
expected. A better understanding of
the formation and growth of aggregates
is needed to determine if and when
aggregation can lead to precipitation
in the time available, in convective
clouds. If aggregational growth is
relatively slow it would mean that the
seeding process is driving the cloud
into a less efficient precipitation
process than its natural one with
counterproductive consequences and the
.formation of aggregates that result
from high ice concentrations produced
by seeding can be regarded as another
definition of "overseeding."

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