WMOD00581

• Title: Targeting of AgI in a Utah Winter Orographic Storm
• Date: 8/2/1992
• State: UT
• Country: United States
• Weather Modification - Doc Type: Report

556

IRRIGA nON AND DRAINAGE

1515-1620. High IN concentrations were still found from 1720-1740 along the valley
floor with the van.
Ice nucleus count rates per minute are equivalent to IN rl for the 10 rl intake flow
used with the acoustical counters and the X 10 correction suggested by Langer (1973)
for known ice crystal losses. Thus, the DOT IN concentration averaged about 500 rl
at the cloud chamber temperature of -200C. In order to estimate the concentration of
IN effective at the prevailing DOT air temperature of -40C, the temperature
dependence of effectiveness values (ice crystals produced per gram of AgI) for the
Utah operational generators (Griffith, et al. 1991) must be considered. Assuming a
lower canyon wind speed of 1-4 m S"I, effectiveness values at -60C, -120C and -20oC
are about 2 X 1010, 1014 and 3 X 1015, respectively. Extrapolation from-2boC to _60C
would reduce the effective IN by 5 orders of magnitude. Calculations with typical
SLW condensate production rates indicate that approximately 10-100 IN rl are needed
for effective cloud seeding (Yo\lng, 1974). Thus, the 500 rl measured at the DOT at
-20oC would provide negligible IN concentrations at -40C.

Silver Iodide Sampling Along the Upwind Highway: Silver iodide was detected all
along the highway on all 9 van passes between 1230-1548. The entire 8 km upwind
highway appeared to have AgI present throughout the sampling period. This implies
that the density of valley AgI generators may be adequate for periods with embedded
convection. (However, the density used by the operational program is less than used
in the early 1991 experiments). It is speculated that the widespread dispersion was due
to a combination of convection within the cloud elements, and mechanical turbulence
over the rugged windward slope.
The average IN concentrations for each of the 9 passes along the upwind highway
ranged from 300-650 rl, effective at -200C. The IN concentration fell to about 50 r1
during the final 1/3 of the last pass as the storm ended. Air temperatures averaged
-3.50C on the north end of the upwind highway (elevation 2680 m MSL), and about
-5.50C on the south end (2990 m). Negligible effective IN concentrations would result
from AgI at these temperatures. The AgI would need to be transported to higher,
colder cloud levels to provide adequate IN concentrations.

Table 1. Summary of van measurements of SF6 tracer gas in parts per trillion (ppt).

Mean time Mean SF6 max. SF6 N-S plume Plume center dist.
(MSn JIm1L JIm1L width (lan) S. of DOT (lan)
1319 40 220 4.0 6.6
1350 30 110 4.4 5.0
1414 20 90 4.2 2.3
1503 100 170 5.5 2.1
1512 35 60 0.8 -0.2
1538 0 0 0.0 N.A.

Tracer Gas Sampling Along the Upwind Highway: Table 1 summarizes surface SF6
observations. Mean SF6 concentrations were in the range 20-100 ppt (by volume) and
plume widths were between 4.0-5.5 km prior to 1512. The tracer gas thereafter
decreased, presumably because gas release was terminated about an hour earlier. The