WMOD00554

• Title: Seeding Summertime Convective Clouds to Increase Black Hills Rainfall
• Weather Modification - Doc Type: Report

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SEEDING SUMMERTIME CONVE:CTIVE CLOUDS
TO INCREASE BLACK HILLS RAINFALU

A. S. Dennis:
U.S. Bureau of Reclamation
Denver CO 802:25

Abstract. Data from two Reclamation-sponsored randomized
cloud seeding experiments near the Black Hills of South
Dakota are combined with data on size distributions of
showers to estimate the potential for modifying summertime
precipitation. The analysis suggl~sts that increases
amounting to roughly 12 percent of the natural precipitation
are possible through a combination of microphysical and
dynamic effects. Convective clouds around 6 km deep are
judged to be the most promising targets for seeding to
increase summer rainfall.

1. INTRODUCTION
From 1963 through 1972 the South
Dakota School of Mines and Technology
(School of Mines) conducted a series of
cloud seeding experiments in the western
Dakotas to test effects of seeding upon
rainfall from summertime convective
clouds. Almost all of the experiments
were conducted as a. part of the Bureau
of Reclamation's Project Skywater.

The most important of the School of
Mines projects' near the Black Hills were
the Rapid Project and project Cloud
Catcher. Both projects dealt with
summertime convective clouds near the
eastern edge of the Black Hills. The
. purpose of this paper is to combine
findings from the Rapid Project of 1966
to 1968 (Dennis and Koscielski, 1969)
and Project Cloud Catcher I of 1969 and
1970 (Dennis et al., 1975a) with
previously published information on size
distributions of convective showers in
order to assess the potential impact of
seeding convective clouds upon total
summer rainfall over and near the Black
Hills.

2. CLOUD CATCHER: AN EXPERIMENT ON
INDIVIDUAL CLOUDS AND CLOUD CLUSTERS

2.1 Experimental Desion
Cloud Catcher was directed from a
radar site about 10 km east of Rapid
City. It was one of the first cloud-
seeding experiments to use an on-line
computer for recording and preliminary
processing of radar data (Boardman and
smith, 1974). It used a single, moving
target area to test the effects of seed-
ing on isolated convective clouds or
cloud clusters. Quoting from Dennis et
aI. (1975a), "Distinct clusters of

cumulus clouds, well away from other
showers, with updrafts of at. least
2 m. S'l below cloud base and cloud top
temperatures less than -10'C were
selected as test cases. The existence
of a radar echo did not prevent the
selection of a cluster of growing clouds
as a test case."

By definition, each test case
lasted 1 hour, which is about twice the
lifetime of a typical convective cell in
a multicell thunderstorm. The principal
response variable was the radar-
es.timated rainfall (RER), which was
determined from X-band radar data
recorded by the on-line computer. No
corrections for attenuation 'were
at'tempted. For data logging purposes,
thl~ area around the radar site was laid
off in squares 10 nautical miles on a
side. A tes't case normally occupied
tWQ squares a't one time, and all echoes
in the designated squares were
considered to be part of the test case.

In 1969 and 1970, a three-way
randomization was used, with the choices
being no-seed, salt seed, and silver
iodide seed. All seeding was done from
an aircraft operating in updrafts below
cloud base. Attention was c()ncentrated
on updrafts under new cloud 1:owers
ad:i acent to existing showers"

Salt seeding was accomplished by
releasing up to 50 kg of powdered sodium
chloride during the first 30 minutes of
each test caSE!. Seeding of a silver
iodide case was accomplished by burning
flares, each containing 120 9 of silver
iodide, one at a time in place in wing-
mounted racks. A flare burnE!d for about
5 minutes, and up to 6 flares were used

lBased on an oral presentation at the Conference on the Science and Technology of
Cloud Seeding in the Black HillS, Rapid City, south Dakota, October 16-17, 1989.

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