WMOD00509

• Title: Physics of Winter Orographic Precipitation and it's Modification - Summary of Presentations
• Date: 10/1/1985
• Weather Modification - Doc Type: Report

counter showed the leading edge, defined by the first definite rise in
response, to have a minimum time of 25 sec and a maximum of 37 sec.
Applications of 9alibrated .values o,f mean and mode times of response,
though possessing more variance, gave reasonable agreement to. the
other two means of plume positioning. It should be stressed that the
NCAR counter is not a "turn key" device. Its use should assume the
presence of an experienced operator.

In all cases which had westerly wind, the plume was found over the
Bangtail Ridge in IFR conditions. The highest level the plume was
detected over the target was 11 kft (3 kft AGL). Twice the plume was
found up to 12 kft over the Main Ridge (3 kft AGL). VFR plume tracing
was done in the winters of 1970-71 and 1971-72, mostly over the Main
Ridge to verify that the ground-released plumes did indeed get
transported over the ridge. For this earlier tracing, the plume from
the southern of the two generator sites (the one reestablished for
January) was always found. Plumes were found up to 10.5 kft in this
ear lier work.

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I

The afternoon of January 15th was presented as a case study for the
workshop. On this day, the plume transport was such that there were
two control areas on the traverses; north and south of the plume.
There was a relative abundance of SLW (supercooled liquid water) as
well. For this mission the seeding generator was shut off at the
approximate time that a series of 9 kft passes was started over the
Bangtail Ridge. This introduced a temporal control as well as the
usual spatial control, i.e., north and south of the plume. Ambient
temperatures were unseasonably warm on this day as for most of the
month, and -10.6 to -8.70C were observed at 9 kft. The
hygrothermograph on top of the Main Bridger Ridge recorded -90C and
that of the seeding site recorded -5 to -40C. The seeding site had an
elevation of 7150 ft.

Figure 1 shows ice particle concentrations averaged over 2 km
intervals measured from the 2D-C probe. The approximate plume core
positions are indicated by an "X". The concentrations are clearly
higher in the vicinity of the plume. There was a tendency for
elevated concentrations over the southern control area. It is
plausible for this to be caused by the AgI plume because of horizontal
wind shear with ice particles settling downward from higher levels
where the wind was more northerly. It took approximately 1 hr after
the generator was shut off to detect a decrease of ice nuclei in the
"seeded" volume. The ice nuclei plume disappeared 20 minutes later.
It would have taken approximately 1 hr. for the 9 kft winds to advect
the AgI from the seeding site to the flight path. The additional 20
min lag was probably due to slack winds in the vicinity of the seeding
site and residual AgI within the tree cover. Prior to the decrease
ifi counts, an average of 88 counts were registered by the NCAR counter
for each pass (not adjusted for chamber losses).

Software developed by E. Holroyd, BUREC Montrose, CO., had the
capability of estimating the precipitation rate using the 2D-C data
(described in a workshop presentation). Figure 2, second panel,
shows the average precipitation rates for the 9 kft passes prior to
plume reduction over the same 2 km intervals as for Fig. 1. The
precipitation rates were clearly elevated in the vicinity of the
plume. Figure 2 also graphs ice particle concentrations showing a
strong correspondence between this and precipitation, with the high
values occurring in the vicinity of the plume. Figure 3 is analogous
to Fig. 2 but for the single pass having only one count indicated by
the NCAR counter. Both the ice crystal concentrations and calculated
snowfall for 2 km averaged intervals are far lower than for the

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