WMOD00490

• Title: The Effects of Mountain Lee Waves on the Transport of Liquid Propane-Generated Ice Crystals
• Date: 9/9/1996
• Weather Modification - Doc Type: Report

REYNOLDS

1455

SEPTEMBER 1996
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FIG. 28. Sulfur hexaflouride measured by sequential sampler lo-
cated at site 8 on Fig. 2 (near ICC). The first rise is from a plume
from site 7, and the second rise is from a plume from site 9.

estimate of the cloud liquid water, a relatively good
estimate of crystal fallout can be achieved. This con-
clusion is predicated on the belief that the enhanced
precipitation is a direct result of seeding.
As shown, the SF6 from site 9 was on a trajectory
passing over the Jackson Creek observatory. The SF6
sequential sampler (No.8), about a kilometer west of
JCC, indicated SF6 from both site 7 (1730-1800 UTC)
and site 9 releases (1815-1845 UTC) (Fig. 28). How-
ever, no SF6 above background levels was observed
with the continuous analyzer at the JCC Observatory.
There were indications of ice crystal concentration in-
creases on the 2D probe at JCC during the period from
1700 to 1815 UTC of up to 30 L -I above a background
of 15 L -I. However, these data were compromised by
wind-blown snow into the 2DC aspirator1 thus making
it difficult to quantify the magnitude of seeding effects
(Deshler 1988).

5. Conclusions

Rawinsonde balloon ascent rates and aircraft and
ground-based tracer studies indicate that gravity waves
are a common phenomena during winter storms to the
lee of the Sierra Nevada. These data, along with simple
modeling studies, indicate these lee waves have a sig-
nificant impact on the trajectory of ice crystals pro-
duced from liquid propane seeding atop the Sierra Ne-
vada.

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The tracer SF6 was released from two separate high-
altitude propane dispenser sites during many winter
storms over several consecutive winter seasons. Sulfur
hexaftlouride sampling from ground-based measure-
ments taken 600-700 m below the release altitudes in
a downwind valley indicate that the tracer is often ob-
served within 30 min after start of the tracer release.
Aircraft sampling of the tracer indicated that the plumes
rarely rose above 2700 m, or 500 m above the release
altitude.
Rawinsonde balloon ascent rates have been used to
calculate the magnitude of the vertical velocities within
these gravity waves. A technique has been presented
that subtracts the nominal balloon ascent rate from the
observed ascent rate to infer atmospheric vertical mo-
tions across the target area. Given that the SF6 and,
thus, the ice crystals are confined to within 1000 m of
the ridge top, a simple extrapolation of the balloon-
derived vertical motions within this layer seems appro-
priate. This three-dimensional wind field is then input
into a simple two-dimensional Gaussian plume model
(GUIDE) with empirical microphysics to infer plume
dimensions and ice crystal trajectories.
On 17 February 1993, distinct seeding effects were
noted by the research aircraft on four of six plumes
intersected from one of the dispenser sites. This al-
lowed comparison of GUIDE model predictions of
crystal trajectories with these aircraft observations.
Model-predicted fallout of seeded crystals correlated
well with projected fallout from aircraft observations
and gauge-observed precipitation increases. Precipita-
tion at several gauges directly in line with the aircraft-
observed seeding plume showed a factor of 2-5 in-
crease in precipitation over surrounding gauges. Ap-
parent dynamic seeding effects were noted on several
aircraft passes on this day and one other. This was
somewhat unexpected but consistent with seeding rates
used and numerical model simulations done in strati-
form clouds using glacio genic seeding agents.
This paper points out the complexities associated
with the transport and the successful targeting of seed-
ing-induced ice crystals initiated over complex terrain.
It shows that nearly continuous horizontal and vertical
wind fields, along with a vertical profile of temperature,
are needed to accurately predict crystal growth and fall-
out. The 9l5-MHz wind profiler with RASS (radar
acoustic sounding system), as described by May et a1.
( 1990), may provide the technology necessary to ob- -
tain nearly continuous measurements of the three-di-
mensional wind field and temperature profile, if a
method can be developed to eliminate the effect of hy-
drometeor fall speeds on the vertical wind component
calculation.
The LOREP was intended to be a five-year program.
It was terminated after three years in part due to the
limited periods in which lee waves did not have a det-
rimental effect on particle growth and trajectories. This