WMOD00528

• Title: The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
• Date: 12/1/1991
• Weather Modification - Doc Type: Report

6.3 Design Specifics for Physical Experiments

Based on the 1991 sampling there is every reason to believe in-cloud observation of seeding effects can
be carried out over the Wasatch Plateau with both aircraft and truck-mounted sensors. Truck sampling
of the seeding agent or tracer gas, and of ice particles characteristics, should be concentrated on the
upwind highway. However, with southwesterly flow, truck sampling is possible further downwind along
the highway that runs from the DOT snowplow shed to the lee edge sampling site (see fig. 6.1) where
radiometer observations were sometimes collected in early 1991. An aircraft can sample along
predetennined flight tracks over both the upwind and downwind edges of the plateau as was done during
the 1991 field program. This would provide infonnation on ice particles at two stages in their growth and
fallout and would document the seeding plume position across the entire plateau.

Three high-altitude seeding sites are recommended for physical seeding experiments to maximize the
chances of the AgI crossing the upwind highway with winds between south-southwest to northwest. Only
one site would be used at a time, depending upon the prevailing wind direction. This would provide a
plume of 1-2-mi width above the plateau, sufficient to compare with neighboring nonseeded cloud.
However, seeding sites should be tested approximately 500 ft lower than the high altitude seeding site used
in early 1991. AdditionaI time and distance would be provided for ice particle nucleation and growth if,
as expected, across-the-plateau transport is usually successful from lower release sites.

Two types of seeding agent would be used, AgI and propane. The latter produces ice nucleation by
cooling and high concentrations of crystals can result at temperatures as wann as 0 oC. Propane
dispensers described by Reynolds (1989, 1991) would be used when it is unlikely that seeding material
would mix as high as the -8 oC level. A large fraction of winter stonns have most of their SL W at
temperatures between 0 and -8 oC where AgI effectiveness is believed to range from none to limited.
A combined seeding program using both AgI and propane may be much more effective than AgI seeding
alone in the Sevier Basin. When propane seeding is done a tracer gas should be co-released so the seeded
zone can be monitored.

Several past physical experiments have demonstrated increases in IPC in the clouds caused by seeding.
The challenge of future experimentation will be to document corresponding changes in surrace snowfall.
Only a few past physical experiments have demonstrated snowfall enhancement at the surrace following
seeding. But such evidence is essential to validating the technology of winter orographic cloud seeding.

The best indication of seeding effects on snowfall will be from comparison between precipitation gauges
within the seeded zone and outside it High resolution gauges will be maintained along two parallel lines,
near the upwind and downwind edges of the plateau, respectively. It will be attempted to locate gauges
at about I-mile intervals along each line but this must be tempered by the topography and tree cover. It
is very important that gauges be located in similar small sheltered clearings in the conifer forest and have
wind shields in order to minimize catch errors due to wind.

Each gauge line should be long enough to provide measurements of nonseeded snow north and south of
the seeding plume for common wind directions. Truck- and aircraft-mounted instruments will monitor
the AgI plume position during seeding experiments and this infonnation will 'be used to estimate which
precipitation gauges were within the seeded zone and which were crosswind of it. As some plume
meandering can be expected, gauges near the boundary of the AgI plume may be under it at times and
crosswind of it at other times. Thus, a "buffer zone" will need to be established in which the duration of
seeding is questionable. Another reason for a buffer zone will be to handle errors in estimating the actual
seeded region that are related to vertical wind shear. Measurements of the vertical wind structure will be

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