WMOD00485

• Title: Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide Over the Rocky Mountains - Part II
• Date: 10/10/1988
• Weather Modification - Doc Type: Report

OCTOBER 1988

ARLIN B. SUPER AND JAMES A. HEIMBACH, JR.

1153

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experiment. They also recommended that a limited
program of airborne measurements be carried out over
the BR T A as the most appropriate next step.
During January 1985, physical experiments were
conducted over the Bridger Range with a well-instru-
mented aircraft used primarily to search for evidence
of ground-released AgI and associated in-cloud micro-
physical changes over the BRTA. The absence of
nearby higher terrain to north and south, and the pres-
ence of a radio navigation station a short distance east,
made it possible to fly north-south (N-S) in-cloud
passes safely within 300 m of the highest BR T A terrain.
Six successful instrument flight rules (IFR) missions
were flown on four different dates. This paper describes
those missions and the results of the airborne obser-
vations as well as some supporting measurements from
surface sites. More detailed discussion of the physical
hypothesis, instrumentation, observational procedures
and evaluation techniques is included in Part I (Super
et al. 1988).

2. Experimental area and operations

The southern AgI seeding site used in the BRE was
reestablished for January 1985. Prior aircraft tracking
under visual flight rules (VFR) conditions showed that
the ground-released AgI plume from that site was rou-
tinely transported over the Main Ridge and BR T A and
that its top was about 600 m higher than the latter
(Super 1974; Super et al. 1974). The seeding site was
on a ridge almost 5 km west of the Main Ridge (Fig.
I) at an elevation of 2.2 km (all elevations are MSL)
and in a small depression well sheltered by trees from
W through N through E. Since access was by a difficult
climb from the 1.5 km level, the small personnel shelter
and supplies had to be brought into the site by heli-
copter. Propane was used for heat and AgI generation,
and for a thermoelectric generator, which powered
communication radios. Winds were measured on an
exposed knoll 100 m west ofthe AgI generator. Single
theodolite pilot balloon (pibal) measurements were
made at the seeding site prior to and during aircraft
missions.
East of the seeding site, the Main Ridge crest averages
approximately 2.6 km and is oriented almost north-
south, forming an abrupt isolated barrier to the pre-
dominately westerly flow during winter storms. Ap-
proximately 6 km to the NE of the seeding site, on top
of the Main Ridge at 2.6 km, wind sensors were main-
tained on a 10 m tower at the Crest Observatory (Fig.
I). On the same tower a Rosemount icing detector was
used to monitor SLW. Temperature was recorded in
a nearby weather shelter.
The experimental design assumed that the surface-
released AgI would pass over the Main Ridge during
periods of westerly flow. The AgI plume tracing during
January 1985 investigated in-cloud plume character-
istics further downwind over the BRTA, where the

t
N

Elevation

E1'" 2743 m
2438
2134

Shields
Valley

4t.omen \ L1VlngS'l"

I

~~ ~~~

Fla. 1. Map of Bridger Mountain Range showing locations of
seeding site, Main Ridge, Bangtail Ridge Target Area and primary
aircraft sampling track.

highest elevation is 2.4 km. The intent was to sample
the seeded cloud volume and, nearly simultaneously,
the natural cloud on either side during N-S passes
through the plume. The crosswind natural cloud re-
gions served as the control, or basis for comparison of
seeded and nonseeded microphysical characteristics.
Because of the N-S uniformity of the Main Ridge and
limited instantaneous width of the plume, differences
between treated and control cloud regions were ex-
pected to be readily detectable if they existed. Each N-
S flight line extended from about 30 km north of the
seeding site latitude to about 20 km south of it (high
terrain prevented sampling further south). This insured
that seeded zones would be detected for winds from
SW to NW.
The statistical indications of SH, and the expected
nucleation activity for the AgI complex used, suggested
that seeding effects should be observable at tempera-
tures lower than about -90C. Main Ridge crest tem-
peratures are frequently below -90C during January.
The seeded clouds with SL W present were expected to
have higher concentrations of ice particles, particularly
those types characteristic of the temperature and mois-
ture regime encountered by the AgI plume. Further,