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

ARLlN B. SUPER AND BRUCE A. BOE

1175

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boundaries can be defined. Those portions of cloud
not clearly either seeded or nonseeded were excluded
from further evaluation. The 5 km interval upwind of
the Snow Lab was evaluated separately for the seeded
and nonseeded classifications to determine the micro-
physical differences.
Although the first airborne seeding experiment was
no more successful than some of the others, it is dis-
cussed in some detail. In the interest of brevity, how-
ever, only the essential points of the other five similar
experiments are presented. Section 4g summarizes the
results of all airborne seeding experiments and presents
supporting information for each.

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a. Airborne seeding experiment 1

At 0500 on 18 Mar 1986 a deep trough layover the
Rocky Mountain states. At 50 kPa the trough axis ran
from EI Paso, Texas, over the Oklahoma Panhandle,
across central Nebraska, and northward to Bismarck,
North Dakota. A strong short-wave trough had passed
through southern Arizona and New Mexico in the pre-
vious 12 hours.
The GMO tower-mounted icing rate detector (Fig.
1) indicated continuous SL W in the orographic stra-
tocumulus cloud at 70 m above the mesa top from
1600 on 17 Mar until 1300 on 18 Mar. Thereafter near-
surface SLW was observed between 1900-2100 on 18
Mar but not at all during the experiments on 19 Mar.
flow was northerly after the passage of the long-wave
trough.
Three pretreatment passes were flown between 1013
and 1041 on 18 Mar, parallel with the winds (essentially
on a north-south axis) at 3.8 km altitude, where the
temperature was -140C. These passes detected max-
imum SLW content of 0.15 g m-3 and essentially no
ice except 5-10 km south of the Snow Lab, where the
mean IPC was less than 2 L -1. Highest cloud tops were
4.0 km and -15.50C, while bases were near the surface
(3.2 km). The aircraft sounding recorded on climbout
from Montrose (Fig. 2a) revealed slight instability from
just below 71 kPa up to about 68 kPa.
The first seeding arc on 18 Mar was generated near
1057 at 3.8 km altitude and about 37 km upwind of
the Snow Lab target. Satellite imagery at 1100 revealed
solid orographic cloud for some 20 km upwind of the
Snow Lab, with broken clouds extending about another
10 km farther north. This proved to be the greatest
upwind cloud coverage observed during any of the ex-
periments.
Ten postseeding passes parallel with the wind doc-
umented the plume and cloud evolution (Fig. 8). On
the first three passes, the seeded zone was marked by
the presence of AgI and twice by very high IPC in quite
narrow zones. Natural ice remained limited to the area
5-15 km downwind of the Snow Lab. Airframe icing
necessitated a descent to deice after the third pass.
A considerably broader ice crystal plume was en-

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countered coincident with AgI on the fourth pass, about
1150. Twenty-two minutes had elapsed since the seeded
volume had last been sampled and the AgI had entered
the continuous orographic cloud deck.
Aircraft winds recorded along each north-south pass
over the mesa were examined to determine the accel-
erations induced by the barrier in the northerly flow.
For the three flights on 18-19 Mar, the mean wind
speed at the 3.8 km aircraft altitude was 8-10 m S-I.
Wind decelerations were consistently recorded from
about 20 to 10 km upwind of the Snow Lab, followed
by accelerating flow from 10 km upwind to overhead
of the Snow Lab. The speed minimum of around 7 m
s -1 was consistently found about 10-15 km upwind of
the Snow Lab, while the maximum speed of 13-15 m
S-I was invariably found over or within a few kilo-
meters upwind of the Snow Lab. The accelerating flow
presumably stretched the seeding plume as it crossed
the Mesa. It is likely that the enlargement of the seeded
volume was further aided by turbulent mixing and ver-
tical wind shear within the stratocumulus cloud.
The seeded zone continued to broaden as it passed
over the mesa (passes 5, 6, 7). Observed IPC gradually
began to decrease, and AgI counts diminished sharply,
probably due to the combined effects ofturbulent mix-
ing, diffusion, nucleation, and scavenging of the AgI
by cloud droplets and ice particles.
The final three passes (8, 9, 10) showed diminishing
IPC, most likely resulting from both precipitation and
leeside subsidence and sublimation. Little ice or SL W
was detected farther than 15 km south of the Snow
Lab because the aircraft was generally out of cloud.
The mean IPCs recorded within seeded and 0.00.-
seeded cloud in the 5 km immediately upwind of the
Snow Lab throughout the duration of the experiment
are shown in Fig. 9. The mean IPC in the seeded cloud
volume was 6.6 L -1, compared to less than 1 L -I in
the nonseeded cloud. Most of the increase was attrib-
uted to small crystals, classified as hexagonal and
spherical, whose images often appeared to be embry-
onic dendrites too small to be adequately classified by
the software.
The mean estimated precipitation rate in the seeded
zone was 0.16 mm h -1, greatly exceeding the 0.01 mm
h-1 recorded in the nonseeded zone. The increase was
largely attributable to crystals classified as aggregates
(mostly of dendrites), as well as irregular particles, large
numbers of smaller hexagonal crystals, and a trace of
graupel-like snow. The importance of aggregation in
the lower regions of winter orographic clouds has re-
cently been demonstrated by Cotton et al. (1986). Most
of the observed IPC enhancement appeared as crystals
less than 0.6 mm in diameter, while most of the esti-
mated precipitation rate in the seeded zone was attrib-
utable to crystals 1.0 mm or larger.
Surface snowfall rates were derived from Snow Lab
ice crystal photographs in the following manner:
Chilled glass collection plates were exposed to snowfall