WMOD00412

• Title: Final Report on Utah Cloud Seeding Experimentation Using Propane During the 2003/04 Winter
• Date: 3/1/2005
• State: UT
• Weather Modification - Doc Type: Report

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4a. Examination of Early 1991 Observations

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The earliest sampling of plumes on the plateau top took place during the major early 1991 field
program. This program emphasized va1\ey and canyon mouth releases (e.g., Griffith et al. 1992) but some
high altitude releases were also done (e.g., Super and Holroyd 1994). While many types of observations
were made in early 1991, those of greatest relevance to this study were AgI plume observations
monitored with a van-mounted acoustical ice nucleus counter sometimes referred to as an "NCAR
counter." The van was driven back and forth along Highway 31 for the 7 kIn north-south distance along
which the road follows the plateau top's west edge (see Fig. 1). That portion of a1\-weather road is
hereafter ca1\ed the "upwind highway." The van data were used in a supporting role and were never
analyzed in detail for AgI releases from a high altitude site 640 m northwest of the HAS used during a1\
subsequent winters. The early 1991 van observations are no longer available. However, one of the
authors operated the van ice nucleus counter during early 1991. His recollection is that AgI releases from
the site near the "newer" HAS were routinely found crossing the upwind highway, often in the vicinity of
the TAR. Winds at the AgI release site were generally in the southwest quadrant during those
experiments. Experience with this seeding site, and the nearby but more exposed HAS location, showed
AgI plumes were routinely in the TAR vicinity until the seeding site wind direction shifted approximately
north of west. Moreover, significant SL W above the plateau usually disappeared after plateau top winds
became northwesterly (Super 1994), eliminating cloud seeding potential.

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4b. Examination of Early 1994 Observations

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The early 1994 major field program used AgI and SF6 (sulfur hexafluoride) tracer gas releases from a
new high altitude seeding location known as the HAS which was more exposed to forced uplift associated
with southwest winds. Both van and aircraft observations were made to document plume dimensions and
structure during storm conditions. Based on the 1991 results, plume experiments were attempted when
HAS winds were in the southwest quadrant. Thus, the results to be discussed provide little information
for northwest or south-southeast winds which were not expected to target the upwind highway. A limited
number of experiments had such winds because of wind shifts after they were initiated.

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The TAR was not yet established in early 1994, but numerous passes were made by that location using
an instrumented van driven along the upwind highway. The TAR was located immediately west of
Highway 31. The van data system provided second by second observations which included accurate
location using a global positioning system (GPS) receiver. A rapid response detector provided SF 6 gas
concentrations while an acoustical ice nucleus counter monitored Agl. The acoustical counter had a
slower response time resulting in considerable blurring of the AgI plume except for the entry edge.
Positions of entry edges and plume peaks were in very good agreement between the acoustical counter
and the SF6 detector. However, since the SF6 detector provided more detail and accuracy on the tracer
gas plume structure and position, only SF6 measurements were used in the fo1\owing analysis.

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A total of six days were identified which had SF6 releases from the HAS during storm conditions.
Simultaneous SF6 releases were made from another location 4 kIn due south of the HAS (6.7 kIn south
distance from the TAR) on three of these days. The SF6 plumes overlapped on one day making it
impossible to precisely locate the south edge of the HAS plume. However, plumes from the southern
release location were not observed on the other two days along the upwind highway which extends 2.5
kIn south of the TAR before turning toward the east-northeast and crossing the plateau (Fig. 1). Those
plumes were presumably further south where van sampling was impossible.

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Aircraft plume tracking took place with lowest passes about 600 m above the highway. A special
waiver from the Federal Aviation Administration permitted flight to within 1000 ft (305 m) of the highest
terrain along the flight path. The standard requirement for instrument flight rules, which apply to in-cloud
flight, is 2000 ft (610 m). Even with the special waiver, the presence oflocal hills resulted in lowest
sampling about 2000 ft above the general plateau top terrain. But even with limited 600 m vertical

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