WMOD00379

• Title: Potential Opportunities for Precipitation Augmentation in the Eastern Tennessee Valley
• Date: 2/1/1988
• Weather Modification - Doc Type: Report

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4. Specify the design of Phase II. This section outlines the objectives
and cost of a one-time intensive 3-month field program to obtain
detailed physical measurements on variDus aspects of the clouds in the
eastern TVA region. The purpose of this effort would .be to confirm with
in situ observations the seeding potential in candidate clouds specified
in Phase I, and to develop seeding hypotheses for a Phase III demonstra-
tion program.

2.0 Report Summary and Recommendations

A review has been made of the available meteorological data relevant in deter-
mining the potential for precipitation augmentation over the eastern TVA region
during the 3 spring months, March, April and May. Six years of data were used
in the study, 1982-1987. The data show that cloud systems inducing precipita-
tion transition from large synoptically included stratiform cloud systems to
individual convective cloud or convective cloud systems during this 3-month
period.

It has been found that the relatively shallow cold orographic cloud present over
the eastern TVA region for about 70 to 80 hours each March and April offers a
very real potential for precipitation increases through application glaciogenic
seeding material released from ground generators or by aircraft. Given that a
little over 70 percent of the precipitation which falls during March and April
subsequently runs-off, there is adequate justification for further exploration
of these cloud types to improve seedability estimates and seeding strategies.

Convective clouds that develop on approximately half the days in May, generally
induced ~y diurnal heating over the southern Appalachians, may also offer poten-
tial for precipitation increases. The available data show these convective
clouds are relatively warm-based (CST +100C). Past cloud seeding'programs and
application of 1-D cloud model suggest these clouds could be induced to increase
their vertical height by application of dynamic seeding principles. However,
model results would imply only the relatively small, moderate growth convective
clouds would respond positi'vely. These would imply that clouds have to be
selectively treated and the effects would be limited to a small portion of the
total target area. This conclusion and the fact that only 50 percent of the May
rainfall runs off show that further evaluation is necessary before final conclu-
sions can be reached on the advisability of seeding these clouds for precipita-
tion increases.

The cost effectiveness of cloud seeding was estimated using preliminary projec-
tions of basin runoff based on anticipated precipitation increases and projected
costs of an op~rational program. The calculations indicate benefit-cost ratios
in the range of 3.6:1 to 6.5:1 on an average annual basis. It must be empha-
sized that theSe benefit-cost calculations are preliminary until further
assessment of cloud seedability and treatment capability is determined.

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