WMOD00486

• Title: Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide Over the Rocky Mountains - Part III
• Date: 10/10/1988
• Weather Modification - Doc Type: Report

1172

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

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~ 30 MEAN OF PASSES 1, 3, & 4

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505
DISTANCE FROM SEEDING SITE LONGITUDE IN KM

10

FIG. 6. As in Fig. 4 for three crosswind passes during the ground-based seeding experiment of 20 Mar 1986.

4.0 to 7.0 m S-1 near the 3.8 km aircraft sampling
level. With these speeds, AgI transport times to the
sampling line about 11 km downwind ofthe generator
would have been between 0.5 and 1.0 h. Thus, AgI
released until 1225 from Site 1 might have affected the
cloud as late as 1325, but AgI from Site 2, first produced
at 1251, should have been at or near the sampling line
by that time. No decreases in IPC or plume widths
were apparent in the three crosswind passes between
1251 and 1319.
Figure 6 shows the mean IPC, SL W content, and
AgI plume edge positions observed on the three cross-
wind passes made near 11 km from the AgI generator
(referenced to the seeding site longitude). Peak SL W
values on the individual passes (not shown) were near
0.05 g m -3. The width of the enhanced IPC zone ranged
from 7 to 8 km on the individual passes with bound-
aries meandering only over 1 to 2 km. The plume width
was obviously much greater than on the previous day.
An analogous plot for the single pass parallel to the
wind (Fig. 7) revealed peak SL W contents near 0.1 g
m -1 from above the seeding site to 7 km downwind
of it. Ice particle concentrations in excess of 100 L -]
were found coincident with the leading edge of the AgI
plume. A secondary maximum was found about 15

km south of the seeding site, which rapidly decreased
further south in leeward subsidence.
The edges of the seeded zones for the 3.8 km cross-
wind passes were defined by IPC exceeding 2 L -], and
the seeded zones thus defined subdivided into thirds.
Control zones 3 km wide were established beyond
buffer zones of 0.1 km width. Mean concentrations in
the seeded zones ranged from 9 to 16 L -1, and the
estimated precipitation rate reached 0.11 mm h -1,
compared with essentially nil precipitation in the con-
trols. Crystals in the seeded zones were again predom-
inantly hexagonal and less than 0.6 mm in size, yet
larger particles contributed two-thirds of the precipi-
tation.
At 1331, a final crosswind pass was made through
the precipitation shaft of the visibly dissipating shower
at the 3.25 km altitude (150 m agl). The precipitation
shaft was 2 km wide and 15 km SSW of the seeding
site at that time, and a new shower could be seen to
the north. The mean IPC was 6 L -] and the estimated
precipitation rate near 0.04 mm h -1. The acoustical
ice nucleus counter registered 128 counts.
The second shower was also sampled in cloud and
below cloud base. The AgI plume edges were again
coincident with the beginnings of the enhanced IPC