WMOD00315

• Title: Investigations of Winter Storms Over the Wasatch Plateau During the 1991 Utah/NOAA Field Program
• Prepared For: Utah Department of Natural Resources Division of Water Resources
• Prepared By: Arlen W. Huggins, Melanie A. Wetzel, Patricia A. Walsh
• Date: 10/1/1992
• State: UT
• Weather Modification - Doc Type: Report

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

l. INTRODUCI10N
Utah is one of five states currently conducting research in cooperation with the National
Oceanic and Atmospheric Administration (NOAA), as part of the Federal-State Atmospheric
Modification Program (AMP). The AMP has been successful in bringing a research aspect to
several summertime and wintertime operational cloud seeding projects. The overall intent of the
AMP is to evaluate and improve these well-designed and controlled cloud seeding projects.
In line with the general goals of the AMP, the goal of the Utah/NOAA research program
is to evaluate the effectiveness of the Utah State operational wintertime cloud seeding program,
and make recommendations for improving the operational program. The Utah/NOAA program
has conducted numerous field research programs aimed at understanding the evolution of winter
storms over mountainous terrain. Prior to 1989, one of the main objectives was to understand
the development of supercooled liquid water (SLW) in winter storms and to determine whether
it was available in sufficient quantities and over sufficient time periods to make cloud seeding
a viable option in increasing the snowpack over the mountains of Utah. SL W flux estimates
over the Tushar Mountains by Long (1986) and Huggins (199Oa,b) indicated that SLW was
indeed plentiful over the duration of a winter season, but was not uniformly distributed across
storms. The bulk of the seasonal SL W was typically found in a just a few of a season's storms.
For storms sampled over the Tushars in 1985, 1987, and 1989 the occurrence of SLW was most
frequent in prefrontal conditions under southwesterly flow. There was a distinct tendency for
the larger SL W amounts to be associated with warmer cloud temperatures. An evaluation of
cloud seeding criteria by Huggins (1990b), based on the occurrence of SLW, the temperature
in the liquid zone, and favorable winds for material transport, found that the criteria were
frequently not met because of unfavorable temperatures (too warm for nucleation by silver
iodide) even though SLW was present.
Since 1989 more effort has gone toward determining whether cloud seeding material
(silver iodide) is routinely delivered to clouds over mountainous terrain from ground-based
generators, which are generally located in the valleys between mountain ranges in the Utah
operational program. From seeding experiments conducted in 1990, Super and Huggins (1992a)
found generally low concentrations of silver in snow samples from the Wasatch Range and
Wasatch Plateau, and concluded that either targeting was generally poor or generator output was
too low to produce concentrations of silver higher than the natural background. Super and
Huggins (1992b) also found, from aircraft plume monitoring experiments in near-storm
conditions (simulating stable prefrontal conditions), that seeding material was also not routinely
transported over mountainous target areas from valley-based generators. Stable conditions, often

1-1