WMOD00485

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

1162

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

physical observations completely verified earlier sta-
tistical findings based on a large number of storms. Yet
there are no apparent inconsistencies between the
physical and statistical approaches. The January 1985
seeding was effective in some but not all cases. The
successful seeding was done with Main Ridge temper-
atures of -90C or colder, moderate to strong westerly
flow, warm cloud tops, and SL W present. This is in
agreement with earlier statistical suggestions and cur-
rent physical understanding.

6. Crosswind widths of the seeded zones

It is of interest to document the widths of the AgI
seeding plumes and the associated ice particle plumes
over the BR T A, because such information can suggest
the appropriate crosswind distance for placement of
AgI generators for any future seeding programs with
terrain and clouds similar to those at the Bridger Range.
The width of the ice particle plumes caused by AgI
seeding were determined as described for the enhanced
IPC zone in section 3. These were estimated by ex-
amination of buffer- by-buffer IPC measurements from
the 2D-C probe. Ice particle plumes clearly caused by
seeding were apparent on the 10 and 15 Jan missions.
The widths of these plumes are shown in Table 2 for
each pass except for the last three of 15 Jan, which had
decreased IPC due to earlier cessation of seeding. The
mean IPC is also noted. Supporting wind and stability
data can be found in Table 3.
The mean plume widths at the lowest sampling level

TABLE 2. Summary of ice particle plume widths and mean ice
particle concentrations (IPC) for N-S passes with enhanced IPC due
to seeding.

Date
(1985)

Altitude Width Mean IPC
(km) (km) (L-')
2.7 2.1 12
2.7 5.8 11
2.7 4.9 8
2.7 8.1 3
3.0 3.5 9
3.0 3.3 14
2.85 5.2 17
2.85 3.3 15
2.7 5.3 15
2.7 10.7 7
3.0 4.1 6
3.0 2.0 7
2.85 7.7 18
2.85 10.6 14
2.7 7.3 19
2.7 9.1 12
2.7 8.6 14
2.7 9.1 13
2.7 9.0 21
2.7 7.3 21

10 Jan

15 Jan
(a.m.)

15 Jan
(p.m.)

(2.7 km) were 5.2 km for 10 Jan, 8.0 km for the morn-
ing of 15 Jan, and 8.4 km for the afternoon of 15 Jan.
Comparison with Table 3 shows that the mean ice par-
ticle plume was narrower than the mean AgI plume in
the 10 Jan case, presumably due to the limited cross-
wind extent ofSLW. Table 2 also shows that the plume
was narrower at 3.0 km than at 2.7 km on both the 15
Jan missions. Mean ice particle concentrations were
remarkedly consistent, ranging only from 3 to 21 L -1,
with only two values below 7 L -1. Table 3 lists AgI
plume widths estimated with the acoustical counter
for pairs of2.7 km passes along the N-S sampling line
over the BRTA. Also listed are wind and stability in-
formation, both upwind of the Bridger Range and over
the BR T A. The mean concentration of AgI was esti-
mated by dividing the mean adjusted total counts by
the plume width. In this case the total counts per pass
were multiplied by ten to adjust for chamber losses, as
suggested by Langer (1973). The mean concentration
is effective at -200C, the acoustical counter cloud
chamber temperature. Concentrations would be about
an order of magnitude less at -1 OOC but only a factor
of two less at -120C, according to the AgI generator
calibration of Garvey (1975). Comparison of 10 Jan
IPC values in Table 2 with corresponding AgI concen-
tration values in Table 3, adjusted downward by the
factor of two for the prevailing temperature of -120C,
suggests that a small fraction of the available AgI nu-
cleated ice crystals. This may be due to the long time
dependence of contact nucleation, discussed by Demott
et al. (1983), for the low water content cloud found
on 10 Jan, Agreement between IPCs and ice nucleus
concentrations was much better on the wetter 15 Jan
missions if Table 3 values are reduced by a factor of
ten for the warmer prevailing temperatures.
Excluding the apparently meandering plumes in the
very stable atmosphere of 19 Jan, AgI plume width
estimates ranged from about 2 to 8 km. The mean
value for the 12 estimates in 5.2 km, which compares
with a mean of7.3 km for all 2. 7 km ice particle plumes
in Table 2. Tables 2 and 3 both suggest that the seeded
zone at 2.7 km over the BRTA was infrequently less
than 4.7 km wide. The range of conditions samples
was somewhat limited, since five of the six missions
had a moderately stable atmosphere over the BRTA.
Wind speeds there varied from 6 to 15 m S-I and di-
rections from 2680 to 2990. However, as shown by SH
and by Super (1986), these were typical values during
the BRE.
The BRE used two AgI generators with about a 6.4
km crosswind separation. The present work suggests
that another generator between those two would have
been advantageous, especially since time-averaged
plumes tend to have Gaussian distributions with con-
centrations tailing off toward each edge. Although this
was suspected from AgI plume tracing during the BRE,
resources did not permit operation of a third seeding
site.

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