WMOD00591

• Title: Investigations of Winter Storms over the Wasatch Plateau during the 1994 NOAA-Utah Field Research Program - Final Report
• Date: 7/1/1996
• State: UT
• Weather Modification - Doc Type: Report

I
I
I
I
J
I
I
I
I
I
I
I
I
I
I
I
I
I
I

plumes in the vicinity of RRS, but there was an indication of a weak ice nuclei plume as it began
its first northbound pass at 1256 (Fig. 42). This is consistent with RRS NC\R counter data. which
showed enhanced ice nuclei through 1310, then nothing again until about 1515.
The gradual veering of the wind over the plateau (Fig. 7), shifted the plume interception zone
on High\'\:ay 31 further to the south with time, so that RRS was in the AHS plume initially, then
between the plumes, and finally was affected by the HAS plume late in the seeding period. Since
seeding from AHS ended at 1440, it is unlikely that IN detected by the Starship or RRS after 1530
were from AHS. This was confirmed by the Starship during its passes after 1430, when the AgI and
SF" plumes were detected in the vicinity ofRRS. No SF6 was detected after 1515 (SF6 releases ended
at 1440" but AgI was seen on the final two radiometer-Starship passes benveen 1520 and 1557. The
pass from 1540 to 1557 is shO\\oTI in Fig. 44.
The consistent detection of seeding plumes by the Starship, and the general lack of detection
by the aircraft, indicate that the plumes on this day had limited vertical extent generally less than
250 m above the plateau. At its minimum altitude on the west track, the aircraft was at a
temperature of about -10.70, compared to the Starship surface temperatures of -6.50 to -80. With
cloud base observed very close to plateau top, the seeding plumes were therefore encountering
supercooled liquid water in the range -80 to -1 O. 70~ suitable for ice nucleation by the AgI in use.
Hov,:ever. from eight Starship and mobile radiometer plume interceptions, the average liquid depth
inside seeding plumes was found to be not significantly different from liquid averages to the north
and south of the plumes. A comparison ofliquid depth distributions is shO\\'TI in Fig. 45, where the
in-plume average was 0.027 mm compared to depth means to the north (2 min from plume edge)
and south, \\"hich \l.'ere 0.031 mm and 0.019 mm, respectively.
The most puzzling aspect of the 26 January experiment was the lack of a detectable ice
nuclei plume at TAR until almost 1600 (Fig. 36),20 min after all seeding had ended. The transport
time from HAS was 45-60 min, assuming a 3-5 m S-I wind, so aerosol released during the last hour
at H.A.S could have been over TAR after 1600. The surprising observation is that nothing was
observed before 1600, given the apparent traje~tories shown in Fig. 43, and the southwesterly to
westerly flow observed at HAS, RRS and TAR. Also of interest in Fig. 36 is the apparent detection5of some sih'er concentration above background in the snow sample collected from 1510 to 1600,

58