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

OCTOBER 1988

ARLIN B. SUPER AND BRUCE A. BOE

1179

d. Airborne seeding experiment 4

The fourth and final line of AgI generated on 18
Mar was released near 1635 only 9 km upwind ofthe
Snow Lab and within the accelerated flow over the
mesa top. Digital satellite imagery indicated cloud tops
as cold as -180C over the Snow Lab and -20oC 15
km north of it.
On the first sampling pass IPCs in excess of200 L -)
were found coincident with the AgI, followed by SL W
contents farther south of 0.1-0.2 g m -3. The following
two passes revealed some broadening of the ice particle
plume and depletion of the SLW. However, the along-
the-wind width of the enhanced IPC zone was less than
5 km as it passed over the Snow Lab. In contrast, widths
exceeded 10 km during the first three experiments. The
final sampling pass, only 31 minutes after seeding,
showed that much of the seeded cloud had already been
subjected to the lee side subsidence and had dissipated.
Mean seeded IPCs were 7 L -) in the 5 km zone
upwind of the Snow Lab, compared to 1 L -1 when the
zone was nonseeded. 'Estimated precipitation rates at
the 3.8 km level were 0.16 and 0.04 mm h-I in the
seeded and nonseeded zones, respectively. The esti-
mated precipitation rate in the seeded zone was much
less than observed in experiment 3, 1.5 h earlier, prob-
ably reflecting the short time available for crystal
growth after nucleation.
Precipitation at the Snow Lab began at 1700, 12
minutes after the passage of the leading edge of the
seeded zone aloft and 4 minutes after the passage of
the trailing edge (Fig, 11). Thereafter, natural snow
continued at most gage sites for several hours. It is
believed that seeding did not cause any significant
snowfall during this experiment, because time was in-
adequate for the growth and fallout of seeding-caused
crystals to the Snow Lab.

"'.

e. Airborne seeding experiment 5

Northerly flow persisted on the morning of 19 Mar,
again producing orographic cloud. Clouds tops were
at 4.6 km and -190C. Mean IPCs were initially 3-4
L -1, and cloud base was measured at 3.4 km, about
200 m higher than the previous day. The climbout
sounding is shown in Fig. 2c. After about two hours
of ground-released seeding (see section 3), it was
thought that enough SL W was present to warrant ad-
ditional aircraft seeding experiments.
The fifth aerial seeding arc (the first of 19 Mar) was
generated 23 km upwind of the Snow Lab, again at 3.8
km altitude and -130C. Seeding took place near 1356
and was followed by six sampling passes. The somewhat
broken orographic cloud deck extended 17 km upwind
of the Snow Lab. In contrast to the previous day's sam-
pling, the ground near the lab was sometimes visible
from the aircraft.
Ice particle concentrations in the seeded zones
ranged from 20 to 100 L -), while natural IPCs often

i
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reached 15 L -I, much higher than those observed on
the previous day. Evaluation of the 2D-C images re-
corded within the 5 km immediately upwind of the
Snow Lab revealed a tripling ofIPC, from 1.8 L -I when
nonseeded to 7.2 L -) when seeded, while the seeded
precipitation rate was twice the nonseeded. However,
the enhanced IPC zone was not very pronounced and
was only a few kilometers wide (N-S) prior to and
during passage over the Snow Lab. It achieved much
greater IPC values and became 5-10 km wide when
observed downwind of the Snow Lab. In experiment
2, when AgI was released a similar distance upwind,
an enhanced IPC zone over 10 km wide was apparent
while passing over the Snow Lab. The much narrower
width and later development of high IPCs during ex-
periment 5 may have been due to more limited inter-
action with SL W in the drier, broken cloud deck of 19
Mar.
No appreciable precipitation was recorded at the
Snow Lab until 1453, some 20 minutes too late to
have been related to the passage of the seeded zone.
The gage network recorded no precipitation throughout
the duration of the experiment; consequently, there
was no evidence that seeding increased snowfall on the
mesa.

f Airborne seeding experiment 6

The sixth and final airborne seeding arc (the second
of 19 Mar) was generated near 1454, 28 km upwind
of the Snow Lab. Satellite imagery showed the broken
cloud deck still extending about 18 km north of the
Snow Lab. Cloud top temperatures at the time of seed-
ing were - 180C over the target and -220C some 8 km
upwind of it.
On the first of seven sampling passes, the AgI plume
was missed, and the (natural) IPC often reached 7 L-1
over the mesa. Increased IPCs ranging from 15 to 100
L -) were found coincident with AgI on five of the re-
maining six passes (only AgI was detected on the sec-
ond pass), while natural IPCs often reached 5-10 L-1
from 5 km north to 10 km south of the Snow Lab.
Liquid water was generally encountered only in the
few kilometers immediately upwind of the Snow Lab.
The N-S extent of the enhanced IPC zone was again
limited to a few kilometers while upwind and over the
Snow Lab, and maximum IPC was only about 15 L-1
on the two passes with the seeded zone just upwind of
or over the Lab.
Mean IPC in the 5 km zone upwind of the Snow
Lab was 2.5 L -1 when seeded but 2.6 L-1 when non-
seeded. The estimated precipitation rates were also es-
sentially unchanged. Precipitation began at the Snow
Lab at 1520, some 15 minutes before the seeded zone
began to pass overhead, so it was almost certainly of
natural origin. Snowfall continued until 1550. The
seeded plume passed over the Snow Lab between 1535
and 1542, and any precipitation that might have re-