WMOD00293

• Title: Summary of the NOAA/Utah Atmospheric Modification Program: 1990-1996
• Date: 9/1/1998
• State: UT
• Weather Modification - Doc Type: Report

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statistical techniques could be very useful, but their development may be difficult in view of uncertainties
of just how seeding responses vary among the population of treated units. Fourth, and most important, a
." much improved physical understanding is needed prior to development of any future statistical design. A
decade ago Braham (1981) made a strong case for improving our physical understanding before
conducting any further "black box" experiments that have a major emphasis on demonstrating
precipitation changes at the ground. His advice is as valid today.

8.2~ Huggins, Arlen w., 1992: Mapping Supercooled Liquid Water with a Mobile Radiometer.
Symposium on Planned and Inadvertent Weather Modification, AtlaJita, GA, American Meteorological
Society, 102-107.

INTRODUCTION

~ Throughout the 1980's, and continuing into the early 1990's, ground-based microwave radiometers have
been used on numerous wintertime weather modification projects to verify the presence of supercooled
liquid water (SL W) in orographic clouds that have been the target of cloud seeding experiments or
operations. Although the instrument is transportable, in the past radiometers have been stationary during
data collection periods.

...

During the Sierra Cooperative Pilot Project (Reynolds and Dennis, 1986) a dual-frequency radiometer was
positioned at several different locations on the windward side of the Sierra Nevada, but for the final four
winter field seasons the instrument was operated in a zenith-pointing mode near the main Sierra Nevada
crest. The reasoning was that, from this vantage point, the radiometer would detect cloud liquid that was
passing over the crest and not entering into the precipitation process. The SL W measured above the crest
location was, therefore, considered an estimate of the amount that might be tapped by an appropriate cloud
seeding technology. Heggli and Rauber (1988) summarize SLW occurrence at the crest site for different
types of storms.

Winter storm research studies in the mountains of Utah have taken advantage of the scanning capability of
ground-based radiometers. Rauber and Grant (1987) show the temporal evolution of SL W over the
upwind edge of the Tushar Mountains using data from a radiometer located at the base of the mountains,
which was scanned at an elevation angle of 20 degrees. They were able to detect azimuthal variations in
liquid amounts which were related to local topographic features. Sassen et al. (1990) used a midbarrier of
the same mountain range to clearly delineate the orographic stages of winter storms and further describe
topographic influences. In both cases, because of the relative shallowness of the liquid layers and the
steepness of the antenna elevation angle (>20 degrees), the radiometer measured microwave emission
from clouds only a few kilometers from the instrument site, and thus detected only very localized spatial
differences.

. .

In a fInal example of stationary radiometer applications, Huggins et al. (1<990) describe the evolution of
SL W from two locations using two radiometers. They documented differences in cloud liquid that
occurred over a mountain crest compared to a downwind valley for various winter stoims stages, and
during the passage of mesoscale cloud bands. This study revealed the potential for using multiple
radiometers to study the water budget of winter storms by measuring the development or depletion of
cloud liquid as a function of mountain barrier position.

During the 1991 Atmospheric Modification Research Program conducted cooperatively by the state of
Utah and the National Oceanic and Atmospheric Administration (NOAA), a new technique of mobile

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