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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Liquid water variability above the mountainous terrain was expected and indeed was observed as
sequential traverses over the same route often produced different spatial profiles of liquid water.
However, there was consistency in the location of liquid water maxima over midbarrier on the windward
slope. There was also consistency in the observation of smaller liquid depths above the top of the Plateau
compared to regions above the windward slope. This appeared to be primarily due to removal of SL W by
precipitation but a localized terrain effect at midbarrier might also have contributed to the result. One
experiment also suggested that the lower liquid values over the DOT site might have been caused by its
location on the downwind side of a local ridge. Analysis of additional cases will be needed to confirm
this.

The mobile radiometer provided important information on the location of the upwind edge of liquid cloud.
In one neutrally stable situation cloud edge remained fairly constant over a few hours, while during an
unstable case the upwind liquid cloud edge changed markedly. Obviously, propagating mesoscale
phenomena can affect the location of liquid over a barrier. Satellite images and the project radar data will
be used to confirm the presence of such features. The location of cloud liquid and its quantitative spatial
characteristics should prove useful to the efforts in modeling the cloud over this particular mountain
barrier.

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A two-radiometer measurement approach was briefly discussed. One case appeared to verify the
depletion of cloud water by precipitation processes. With appropriate wind measurements, the two-
radiometer systems could potentially provide quantitative estimates of changes in liquid water fluxes
across various portions of a mountain barrier. The mobility of one or more of the radiometers allows for
study of different portions of a barrier without concern for power being available at all the sites of interest.
In a future field effort, the Utah/NOAA program intends to focus more on measuring the changes in
supercooled liquid water across the Wasatch Plateau both during natural storm conditions and when
upwind cloud seeding operations are targeting the cloud over the Plateau. One future experiment is also
aimed at combining a ground-based mobile ice particle/ice nuclei measurement system with mobile
radiometric measurements to attempt to document microphysical and liquid water changes across the
boundary of a seeding plume. The mobile radiometer platform allows for much more flexibility in the
design of experiments to either assess the potential for water augmentation by cloud seeding, or to
. document changes, or lack of changes, in cloud liquid during seeding operations.

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8.3. Griffith, D. A., G. W. Wilkerson, W. J. Hauze, and D. A. Risch, 1992: Observations of ground
released sulfur hexafluoride tracer gas plumes in two Utah Winter storms. J. Weather
Modification, 24, 49-65.

ABSTRACT

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Research was conducted for a two month period over the Wasatch Plateau of Central Utah during the
winter of 1990-91. A portion of this research was devoted to tracking sulfur hexafluoride (SF 6) released
from various ground based locations during winter storms. A NOAA King Air research aircraft was the
primary "mode of tracking the SF 6' Seven flights were conducted under IFR conditions when SF 6 was
released from a foothill or valley location. SF6 was detected on six of these flights. Information on SF6
plume widths, concentrations, vertical extent, and estimated numbers of ice crystals thatwoulrl be possible
if a silver iodi~e generator had been operated from the SF 6 release site are provided for two of these
flights.

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