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

to confinn microphysical changes in seeded clouds over the Bridger Range that presumably increased
snowfall (no surface obselVations were made in the limited 4-week study). Super and Boe (1988b)
showed evidence of precipitation changes at aircraft sampling levels and on the surface during a 2-month
study period over the Grand Mesa, Colorado. Deshler etal. (1990) demonstrated seeding induced
microphysical changes at aircraft levels in about 35 percent of 36 experiments conducted over a
three-winter period. Following seeding effects to the ground proved difficult in the Sierra Nevada,
partially because of. the impracticality of low-level aircraft sampling over the rugged mountains.
Moreover, ice multiplication or enhancement (processes whereby crystal concentrations far exceed ice
nucleus concentration such as described by Hallett and Mossop, 1974) is common in maritime clouds over
the Sierra Nevada, resulting in large ice crystal concentrations which can mask seeding effects. However,
the final Sierra Nevada experiments demonstrated microphysical effects at the surface after the targeting
scheme was improved with additional obselVations. Deshler and Reynolds (1990) presented a case study
in which the effects of aerial seeding were followed for over 90 min and 100 kIn.

While encouraging, the above physical experiments have been too few to demonstrate how often stonn
conditions pennit the seeding hypothesis to operate, or how much additional snowfall might result from
routine seeding. However, the need is clear to conduct a series of comprehensive physical seeding
experiments, capable of monitoring all key processes from release of seeding material to precipitation on
the ground. Both understanding and instrumentation are now adequate for this task. The remainder of
this section will be concerned with the design of comprehensive physi<;al seeding experiments for the
Sevier River Basin, Only after such experiments are conducted and fully analyzed will it make sense to
design a statistical experiment intended to document multi winter seeding effectiveness over a large area.

4.2 Factors Important In Planning Comprehensive Physical Cloud Seeding experiments

A design plan for comprehensive seeding experiments was recently completed for the Mogollon Rim of
Arizona (Super et al., 1991). It reviewed several physical cloud seeding experiments conducted over the
past two decades, and evaluated the factors affecting the success of these experiments, A number of
lessons became apparent in reviewing the various physical experiments, some of which succeeded and
some of which did not. The following discussion reviews those lessons extracted from the noted report.

First, and not surprising, it is much simpler to document seeding effects leading to snowfall on the surface
from nonprecipitating clouds than from clouds where nature is already somewhat efficient. The seeding
signal can be unambiguous for clouds with low natural IPC as illustrated by Super and Boe (1988b).

Nonprecipitating or lightly precipitating periods with at least moderate SL W available are common in Utah
winter stonns. Radar, SLW, and precipitation rate obselVations over the Wasatch Plateau in early 1991
demonstrated a number of stonn periods, often lasting hours, with little natural snowfall and abundant
SLW. Huggins and Sassen (1990) showed very light snowfall rates and significant SLW amounts during
a stonn in which physical seeding experiments were attempted.

It is, of course, important to demonstrate whether seeding can produce physical evidence of enhanced
snowfall when some natural snowfall is occurring but excess SLW still exists. Hobbs (1975b) showed
evidence of snowfall increases during naturally light snowfall. Similar documentation at aircraft levels
was given by Super and Heimbach (1988).

It will likely be increasing difficult to demonstrate that seeding enhances snowfall as natural precipitation
rates increase. When nature becomes very efficient, seeding cannot increase the snowfall because all
available SLW is already converted to ice, Fortunately, many Utah stonns are probably similar to the

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