WMOD00473

• Title: Evaluation of Bridger Range Winter Cloud Seeding Experiment Using Control Gages
• Date: 12/12/1983
• Weather Modification - Doc Type: Report

Reprinted from JOURNAL OF CLIMATE AND ApPLIED METEOROLOGY, Vol. 22, N@. 12, December 1983
American Meteorological Society
Printed in U.S.A.

Evaluation of the Bridger Range Winter Cloud Seeding Experiment Using Control Gages

ARLIN B. SUPER

Bureau of Reclamation. Montrose. CO 81401

JAMES A. HEIMBACH, JR.
Montana State University. Bozeman. MT 59717
(Manuscript received 6 December 1982, in final form 23 June 1983)

ABSTRACT

A randomized exploratory single-area cloud seeding experiment was carried out in the Bridger Ran~e of
southwestern Montana during the winters of 1969-72. Seeding was accomplished using ground-based stiv~r
iodide (AgI) generators located more than midway up the west (win~ward) slope of the north-south M~n
Ridge, thereby avoiding trapping by lower stable layers. A seconda,! ndge from 5 to .20 km east of ~he Mam
Ridge was the expected target. An extensive airborne plume tracmg p~ogram provI~ed stro.ng eVlden~e of
successful targeting of the AgI seeding material, with further evidence furnished by tracking of plbals and stiver-
in-snow analysis. . . . . ...
The experimental unit was 24 h beginning at local noon, a natural dIUrnal minimum m preclpltat~on
intensity. The response variable was daily precipitation amount as me~sur~d by a d~nse network of re~?rd~ng
gages. Locally-launched rawinsondes and a thermograph atop the Mam Ridge prOVided data for partltiOnmg
the experimental days.
A post hoc statistical analysis was conducted utilizing upwind and. crosswind control ~ge data. Results from
both the Wilcoxon rank-sum test and the recently devEloped multIresponse permutation procedure (MRPP)
strongly suggest that increased target area snowfall resulted from seeding when AgI plume temperatu~e~ w~re
colder than approximately -90C. Double ratios yielded estimates of -15% more sc:ason~ target area preCIpitatiOn
than predicted by control gages on nonseeded days, while a target-control analysIs ofmdependent snow-course
data strongly suggested seeding enhanced the seasonal snowpack ~y ~ore than 15%. . . .
. Consideration of plume tracing findings and AgI generator calibration results suggest that the artlfi.claI Ice
nuclei concentration in the seeded volume would be quite limited at temperatures warmer than approximately
-90C. This prpvides a plausible physical explanation for the results suggested by the statistical investigations.
~

1. Introduction

Researchers at Montana State University (MSU)
conducted an experimental winter orographic cloud
seeding program in the late 1960s and early I 970s.
This was one of a number of winter cloud seeding
investigations conducted in several states under the
auspices of the Bureau of Reclamation's Project Sky-
water. These programs were intended to evaluate the
potential for precipitation augmentation in moun-
tainous regions with differing topographies and cli-
mates. They were partially motivated by the apparent
success of the Climax I experiment reported by Grant
and Mielke (1967) and Mielke et al. (1970) and (1971).
The Bridger Range Experiment (BRE) was con-
ducted in southwestern Montana, near Bozeman. This
site was chosen because of the relatively simple to-
pography and good access. The Main Ridge of the
Bridger Range rises abruptly to the east of an -60
km wide valley, and is oriented approximately per-
pendicular to the prevailing westerly wind (see Fig. I).
It was hypothesized that seeding clouds above the Main
Ridge with silver iodide (AgI) could sometimes affect

@ 1983 American Meteorological Society

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snowfall farther downwind, particularly on the broad
Bangtail Ridge - 5 to 20 km east of the Main Ridge
crestline.
The BRE was an exploratory experiment in which
a single target area was either seeded or not seeded,
based on a randomized decision ( - 50% seeded). A 24
h experimental unit was used. Daily precipitation, as
measured by a network of weighing gages, was the only
response variable. Radiosondes were launched -10
km west of the Main Ridge crest line, and a ther-
mograph was maintained on the crest line. The upper
air and thermograph measurements were to provide
for partitioning of the precipitation measurements in
post hoc statistical analysis.
In addition to the randomized experiment, several
supporting investigations were carried out. These in-
cluded airborne tracing of AgI plumes, airflow inves-
tigations, analysis of snow silver content and attempts
to directly detect the effects of short-period seeding
episodes through a variety of physical measurements.
The BRE and several of the other experiments pre-
viously noted were terminated earlier than planned
due to a major reduction of the sponsoring agency's