WMOD00509

• Title: Physics of Winter Orographic Precipitation and it's Modification - Summary of Presentations
• Date: 10/1/1985
• Weather Modification - Doc Type: Report

RECENT FINDINGS FROM THE BRIDGE RANGE - MONTANA

James A. Heimbach, Jr.
Montana State University

and

Arlin B. Super
Bureau of Reclamation

BACKGROUND

In January 1985 a winter orographic weather modification experiment
was conducted over the Bridger Range near Bozeman, MT. The intent was
to substantiate the statistical findings of a randomized weather
modification experiment conducted during the winters of 1969 through
1972 discussed by Super and Heimbach (JCAM - Dec. 1983). The target
area was the Bangtail Ridge, approximately 10 km east of the Main
Bridger Ridge. For the final two years, two ground-based generators
were run 2-5 km west of the Main Ridge crest. Statistical analyses
indicated a strong positive seeding signal for precipitation measured
by continuously recording gages. This positive signal was also found
using snow course data collected independently by the Snow Survey. At
the time of the original experiment, the microphysical influences of
seeding could not be documented because the technology did not exist.

The January 1985 experiment used one of thE~ original seeding sites and
the identical generating system; a modifiE~d Skyfire generator burning
an AgI-NH41 complex, to duplicate as nearly as possible the original
operations. Flights primarily over the Bangtail Ridge were made in
seeded clouds by the NCAR (National Center for Atmospheric Research)
King Air aircr aft.

The hypotheses addressed were: does supercooled liquid water exist in
the volume influenced by the AgI, does ground-re1eased AgI reach the
target area during westerly flow in storms, and does the AgI-NH41
complex increase the efficiency of transformation of liquid water to
precipitable ice crystals.

A waver from the FAA permitted flights down to 1 kft AGL over the
Bangtail Ridge. This was 9 kft MSL, corresponding to approximate Main
Ridge Level.

Although January was unseasonably warm and dry, eleven missions were
flown, nine of which were in IFR conditiont;.

PLUME TRACING

An NCAR acoustical ice nucleus counter on loan from the University of
Wyoming was used to track the AgI plume. Although there is some
uncertainty introduced to the plume's position by the mixing of the
sample in the cloud chamber and the time to grow detectable ice
crystals from ice nuclei, a good approximation of the plume's position
was made using two methods. Flight tracks were organized by pairs of
passes allowing a bracketing of the plume by the two leading edges.
The other was to plot the occurrence of the counts on the pairs of
flight tracks and find the centroid for the~ pair. Ground tests of the

29