WMOD00290

• Title: 1994 Field Operation Plan
• Date: 1/1/1994
• State: UT
• Weather Modification - Doc Type: Report

than 4 h on-station time per mission. In contrast, AgI and SF 6 gas can be continuously observed day and
night at the mountain observatories, and with the van for many hours during each storm, subject to crew
fatigue and road conditions.

The T&D 'of valley-released AgI will continue to be investigated, since most operational seeding generators
are in valleys (a few are in canyon mouths). Canyon-mouth releases of tracer gas, and high-altitude releases
of both AgI and tracer gas also will be studied. A good sample of valley seeding under weakly convective
conditions was obtained in early 1991, but more stable storms should be sampled. Accordingly, investigation
of the T&D of valley-released AgI will be concentrated in the fIrst month of the 1994 fIeld project when
more stable storms are expected. These observations will enable further evaluation of the effectiveness of
the Utah operational seeding program in delivery of appropriate AgI concentrations to desired SLW cloud
regions.

Canyon releases of SF6 will be made in the mouth of Birch Creek, a typical major canyon located between
the HAS and AHS seeding sites. Birch Creek Canyon has two major branches which head near the RRS so
surface detection is practical on the Plateau top. Two high-altitude seeding sites will be tested as this
seeding method may offer a future alternative or supplement to present valley and canyon-mouth seeding.
High-altitude seeding is the preferred approach for the direct detection experiments because targeting of the
Plateau-top instrument sites is more likely.

Valley releases of AgI will be made using 3 sites chosen by NA WC as likely to target the RRS and upwind
highway.

Both AgI and propane seeding will be tested from the high-altitude sites. Propane may offer a means of
seeding the frequent slightly supercooled periods (0 to -6 oc) when AgI is likely ineffective.

As in the 1991 program, both AgI and SF 6 tracer gas will be used in T &D investigations, the latter to
simulate AgI plumes. The characteristics of available detectors are such that SF6 plumes can be observed
more precisely in space and more quantitatively in concentration than AgI plumes. The main advantage of
tracking AgI is that it is far more economical to release detectable amounts of AgI than SF 6. Consequently,
it is affordable to release AgI from multiple sites over many hours.

Ongoing analysis of the 1991 IN observations suggests that any sampling site along the upwind highway
generally indicated whether or not the valley-released AgI was reaching the Plateau top in concentrations
well above background. That is, AgI concentrations usually were similar all along the ilpwind highway when
averaged for a number of vehicle passes (a few exceptions had AgI concentrated near the head of Fairview
Canyon). In contrast, aircraft observations of valley-released AgI tended to be concentrated above the
mouth of Fairview Canyon (Fig. 1) when sampled along the upwind flight path. This was very likely because
of favorable transport up this atypically deep canyon, and the lower aircraft sampling altitudes possible over
this portion of the Plateau (3200 m compared to 3800 about 10 km further south).

Analyses of 1991 data are suggesting that it is often impractical to sample valley-released AgI (or SF6) with
an aircraft in-cloud over the west edge of the Plateau except over the lower elevation portion of the flight
track. The aircraft simply cannot sample at a sufficiently low altitude while maintaining a safe vertical
separation above the terrain. This tentative fInding makes it challenging to investigate the T &D of valley-
released AgI with an aircraft.

It is planned to again simultaneously release AgI from multiple valley sites in order to increase the chances
of targeting the upwind highway. Finding AgI along the upwind highway while not detecting it overhead by
aircraft documents the plume top to within about 600 m. This is equivalent to a 4 oC temperature difference
under average lapse rate conditions. However, 1991 sampling over the Plateau and earlier similar sampling
over the Grand Mesa of Colorado suggests the plume top would usually be nearer the aircraft than the

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