WMOD00293

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

-20 oC, and 10 percent of the hours exceeded 2,000 L-1 . Therefore, the AgI was usually transported from
the valley up to the canyon head, a vertical distance of over 900 m. This is remarkable when IS it realized
that valley-based inversions are common during winter storms. Most hours with SL W amounts of
0.05 mm or greater had weak embedded convection present, which likely assisted vertical AgI transport.

As discussed by Super (1994, section 8.3) these 100 sampled hours may be optimistic of typical hours
with SL W present because of the frequency of embedded convection and associated large SL W amounts.
Many other hours of observations were made that have not been reported when very large AgI
concentrations were confined to a shallow layer over the Sanpete Valley. A large fraction of these hours
also had SL W present above the Plateau, although average amounts were more limited than the 100 h
sample just discussed. During a number of experimental periods, the only AgI that could be found with an
instruinented vehicle carrying an NCAR counter were within about 100 m elevation of the valley floor.
During a number of experiments when only background INC were found along the Plateau top, the vehicle
was driven down a major canyon toward the town of Fairview in the valley. The INC effective at -20 oC
would abruptly shift from background levels of 1- J L-1 within the canyon to many thousands per liter as
the vehicle passed below the canyon mouth just above Fairview. The NCAR counter would become
saturated with ice crystals so absolute INC cannot be quantified beyond stating that they were at least
thousands per liter. Clearly, as the eight valley generators were operated hour after hour, they produced
vast quantities of AgI aerosol, trapped in a shallow layer above their valley elevations. Valley winds
during these events were no more than a light northward drift.

~-

In spite of observations of trapped AgI during a number of experiments, it appears that the valley-released
AgI was often transported to Plateau top altitudes whenSL W was reasonably abundant as represented by
the 100 h data set discussed above. However, it is necessary to consider the adjusted NCAR counter INC
values in the context of typical Plateau top temperatures which are much warmer than the -20 oC cloud
chamber operating temperature. For example, the median temperature during all 100 h was a mild
-2.6 oC, while the median for the 50 wettest hours, with SLW exceeding 0.16 rom, was an even warmer
-1.5 oC. The latest calibration of the NA WC generator shows that the effective INC at -6 oC, the wannest
temperature sampled in the CSU Isothermal Cloud Chamber, is only 1/4 of 1 percent of that at -20 oC.
Hence, even an adjusted NCAR counter INC of2,000 L-1 yields an estimated effective INC of5 L-1,
considered too low for significant snowfall production (Super 1994, section 8.2). Furthermore, only 9 of
the 100 h was colder than -6 oC at the Plateau top altitude of 2700 m.

A more recent data set by Holroyd and Super (1998) showed that the Plateau top temperature was colder
than -7 oC during only 20 percent of AgI seeding experiments conducted during the 1994-95 and 1995-
96 winters. Temperatures were even warmer duringmany other experiments when liquid propane seeding
was attempted. These large data sets leave no doubt that temperatures near the Plateau top are typically
only mildly supercooled during storm episodes.

The CSU generator calibration showed that at -8 oC the effective INC was 8 percent of that at -20 oC,
while the effective INC increased to 33 percent at -10 oC. If the AgI concentrations measured on the
Plateau existed at higher altitudes corresponding to these temperatures, the median effective INe would
be 40 and 165 L-1, respectively. Such concentrations would be considered sufficient for effective seeding.
However, for typical in-cloud lapse rates, the stated temperatures would be about 800 and 1,100 m above
the Plateau for the median Plateau top temperature of -2.6 oC.

Aircraft observations were made during six experimental days when valley-released AgI was transported
to the lowest permissible aircraft sampling altitude, about 600 m about the average Plateau top terrain as
reported by Super (1995a). Four of these experiments were conducted on some of the same days which
provided the 100 h Plateau top data set just discussed. The aircraft missions showed that AgI

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