WMOD00388

• Title: Feasibility of Snowpack Enhancement from Colorado Winter Mountain Clouds: Emphasis on Supercooled Liquid Water and Seeding with Silver Iodide and Propane
• Date: 9/30/2005
• State: CO
• Weather Modification - Doc Type: Report

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production of SL W was associated with Grand Mesa top temperatures in the -4 to -1 OOC range.
The observations were collected at 10,800 ft in west-central Colorado. Their results are in good
agreement with the findings of Hindman (1986) for the San Juan Mountains of southern Colorado
(11,750 ft observations) and Monarch Pass (11,800 ft) in central Colorado. Hindman (ibid.)
indicated somewhat colder temperatures, usually in the range -5 to -130C, for frequent and
abundant SL W at 10,400 ft in the Park Range of northern Colorado.

Microwave radiometer observations of SL W above a 8850 ft site atop the Wasatch Plateau of
central Utah were reported for a 1.5 month period in early 1991 (Super 1994). The 194 h with
vertically-integrated amounts of 0.05 mm or greater, considered to have significant snowfall
potential, were examined for this report. Only 10% of those hours had plateau top temperatures
below -60C, but much of the plateau is at higher elevations where temperatures would be colder.
Had measurements been made at an altitude of 10,800 ft, as on the Grand Mesa almost due east of
the Plateau, about 10% of the SL W hours would be expected to be colder than -1 OOC for typical
in-cloud lapse rates. This comparison suggests the Utah mountain top observations are similar to
those of Colorado for approximately the same elevations.

As previously discussed, a number of plume tracing studies have shown that ground-released
AgI plumes are mostly confined to the lowest 2000 ft above the terrain. Consequently, plume top
temperatures would be expected to be about 40C colder than mountain top observations, using a
moist adiabatic lapse rate appropriate for the mountain top temperatures and pressures involved.
When plume top temperatures are colder than about -8oe, AgI would provide adequate
concentrations of seeded crystals. The mountain top temperature ranges cited in the two above
paragraphs, adjusted 40C colder, might be considered to suggest that AgI seeding could be
effective much of the time when SL W was present.

However, it should be recalled that mountain airflow usually rapidly descends downwind of
the crestlines, resulting in rapid evaporation of tiny cloud droplets and slower sublimation of
larger ice crystals. Therefore, creating seeded crystals over crestlines often will not result in
effective seeding. In order to have sufficient time for seeded crystals to grow large enough to
settle to the mountain surface, or be swept out by larger natural snowflakes, seeded crystals need
to be in a favorable growth environment for at least 10 min. A 20-30 min growth period would
be much preferred. That requirement means that seeded crystals need to be created between 10-
30 min travel time upwind of the intended target area. The actual target area will vary with the
type of terrain. If the crestline is abrupt with a steep lee slope, most crystal growth needs to occur
upwind of the crestline. For relatively flat-topped terrain like the Grand Mesa and Wasatch
Plateau, growth may continue, but likely at a reduced rate, in the airflow over such barriers where
local higher terrain may produce additional SL W. Probably the best seeding situation is when
parallel barriers exist, without too wide a valley between, so that many of the seeded crystals
formed over the windward slope and crest of the windward barrier will still exist upon reaching
the SL W -rich zone over the windward slope of the downwind barrier. Parallel barriers are
common but valleys between them may be wide providing too much time in subsiding and
warming air for seeded crystals to survive to the second barrier. Most mountains are rugged and
complex, and airflow patterns change with wind direction and other variables. As a result,
potential target area situations should be individually evaluated concerning seeding generator or
dispenser placement.

One general rule is clear. Seeding crystals should be timned as far upwind of an individual
mountain crest as practical to maximize growth and fallout times. Therefore, the SL W cloud
temperatures of greatest interest are not over mountain tops but over windward slopes, and as far
upwind as effective seeding agent can be introduced into SL W cloud. That requirement means

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