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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for the AgI In estimates. Estimates of seeding-caused IPC are also approximations, but should be
significantly more accurate than the AgI IN estimates. .

. .

None of the estimated average AgI plume widths of Table 1 are as wide as the 40 km north-south extent of
the AgI generator network. The median plume width Table 1 is 25 km, suggesting that either some of the
individual generator plumes were not transported over the Plateau or that the aircraft were simply
overflying some plumes. The latter case is suspected to be most common because the generators were all
located in similar valley floor locations. Moreover, observations along the upwind highway indicated that.
when AgI was transported over the Plateau, it usually was found all along the 6.7 km north-south extent of
the highway. These highway observations suggest that AgI from multiple generators was usually
transported over the Plateau when any Plateau-top transport was occurring.

Observations from the network of 5 precipitation gauges could not be expected to be conclusive regarding
the effectiveness of seeding since only a limited number of non-randomized experiments were conducted.
However, the gauge observations may be suggestive and they do not set upper limits on possible snowfall
enhancement. No seeding effects would be expected at gauges FVC and HAS on the Plateau's western
slope.

The gauge observations given in Table 2 do not provide evidence of significant seeding-related snowfall
increases on top of the Plateau during two of the six experiments because hardly any snow fell. Little AgI
IN reached the high-based SLW cloud of28 February so no significant response to seeding would be
anticipated. Limited snowfall rates may have been caused by seeding during some of the other
experiments, each of which had cloud base located below the Plateau top. Seeding may have increased
snowfall particularly during the first experiment of 2 March and the single experiment of 6 March. But
any enhancement of snowfall rates was limited, especially on 6 March as shown by the average hourly
rates in Table 2. .

Table 2.-Summary of precipitation network measurements during six early 1991 experiments from
west to east over the Plateau. Values are average hourly precipitation rates in mmh-1 for the indicated
time periods. The times correspond to the aircraft sampling periods lagged by 15 min.
MMDD/Exp. MST FVe HAS DOT PTe PTE
0228/1 1345-1635 0.0 0.0 0.0 0.0 0.0
0301/1 1140-1425 0.18 0.18 0.37 0.46 0.69
0301/2 1700~ 1845 0.29 0.87 1.60 0.73 0.73
0302/2 1 030-1125 0.28 0.55 1.79 1.10 0.69
0306/1 1315-1445 0.0 0.0 0.17 0.0 0.17
0306/1 1635-1930 0.0 0.0 0.26 0.13 0.09

The minor snowfall amounts of the second experiment of2 March raise special cause forconcem.
Sup~rcooled liquid water amounts were relatively abundant as measured by microwave radiometer and an
aircraft probe. Further, the mission was terminated early because of aircraft icing. Relatively high AgI IN
concentrations were transported up to aircraft altitudes where clouds were certainly qold enough for
nucleation of much of the AgI. A noticeable increase in IPC resulted in the seeded zone. However, in
spite of the apparently quite seedable conditions and transport of AgI to aircraft altitudes, the valley
seeding did not result in significant snowfall on the Plateau top.

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