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

1157

OcTOBER 1988

ARLIN B. SUPER AND JAMES A. HEIMBACH, JR.

.18

passes at 2.7 km. Estimates of the north and south
edges of the plume derived from acoustical counter
data are shown by vertical lines at the tops of the IPC
plots. The means of passes 7 and 8 at 2.7 km, as well
as the final pass, are shown separately to illustrate the
diminishing ofIPC with time after generator shutdown
at 1355.
Figure 2 shows that SL W persisted at the sampled
levels over the BR T A throughout the mission, with
highest amounts found south of the SSL (the origin in
Fig. 2) at 2.85 and 3.0 km altitudes. The SLW was
very limited north of the SSL at 2.7 km, the lowest
level sampled. It is not obvious that seeding markedly
reduced the SLW content. The mean vertical velocity
within the seeded zones for the 9 passes at 2.7 km al-
titude was +0.3 m S-I. Calculations by Rauber and
Grant (1986) show that even lesser updrafts can pro-
duce sufficient condensate for growth of high concen-
trations of small crystals.
Evidence of seeding was apparent in both the IPC
and AgI measurements as high as the 3.0 km level.
Total AgI counts per pass were 14 and 18 at 3.0 km,
and 71 and 113 at 2.85 km, and ranged between 73
and 120 for the first four passes at the 2.7 km altitude.
Total counts subsequently decreased to 64 on pass 5,
66 on pass 6,28 on pass 7 (at 1454), 17 on pass 8,
and only 1 on pass 9. Peak IPC in excess of 50 L -1
continued at the lowest sampling level until about 1450.
The rapid decrease of both AgI and IPC after that time
is further evidence that the enhanced IPC was due to
seeding. The observed IPC was almost down to back-
ground levels by the ninth and final pass (at about
1515), indicating that about 1.5 h was required for the
AgI plume to "flush out" after generator shutdown.
A convenient method of characterizing the concen-
tration oflarge cloud droplets is the threshold diameter,
Dr (Hobbs and Rangno 1985). This is defined as the
size where droplets ~ Dr have a concentration of 3 cm-3
as measured by the FSSP instrument described in Part
I. The mean Dr calculated for the wettest nonseeded 1
km interval of each 2.7 km altitude pass averaged 14
J.Lm, and the mean concentration of all droplets was
140 cm -3. The Hallett-Mossop (1974) ice multipli-
cation process would not be expected to be active in
these clouds, due to the scarcity of droplets >24 J.Lm
diameter, and indeed, Fig. 2 shows that the IPC was
quite low outside the seeded regions.
Figure 3 summarizes ice particle concentrations,
sizes, and habits, as well as estimated precipitation rates
for the seeded (target) zone and two crosswind (con-
trol) zones, all averaged for the first six 2.7 km altitude
passes. As noted in section 3, the seeded zone was
further subdivided into equal zones designated
N-S (North-Seeded), C-S (Central-Seeded), and S-S
(South-Seeded). The crosswind control zones, desig-
nated N-C (North-Control) and S-C (South-Control),
were each 2.5 km wide and were separated from the
seeded zones by 1.0 km buffers. The approach of Hol-

1:
E
E .15
w
..
..
a: .12
z
o
!;i
.. .09
0::
(3
w
g: .06
Q
w
~ .03
;:
<Il
W

850115
1353 -1442 mst
6 PASSES AT 2.7 KM

.00 N-C

~

z
o
!;i 15
a:
..
z
w
o
z
g 10
w
-'
o
;:
a:
~
5

"0!
' SPHERIC:'
E .16 .......... GRAUPE I
E .25 LINEAR
w.40
N HEXAGON..
- .63
en DENDRI'II
1.0 AGGRE(\:!
1,6
'RREGUL :.,

w
!:!
z
..
w
:I

FIG. 3. Ice particle concentrations and estimated precipitation rates
for the seeded zone subdivided into thirds (N-S, C-S, S-S), and north
and south control zones (N-C and S-C), shown by ice particle size
and habit. The particle size/habit shadings apply to both top and
bottom panels. Values are means for the first six passes at 2.7 km
during the afternoon of 15 Jan 1985.

royd (1987) was applied to the 2D-C probe images to
estimate the vertical flux of ice particles through the
aircraft sampling level, hereafter referred to as precip-
itation or snowfall, although these terms are usually
reserved for surface measurements. The size scale of
Fig. 3 is logarithmic (log 0.10 = -1.0, log 0.16 = -0.8,
log 0.25 = -0.6, etc.).
The average IPC in the seeded zones exceeded by
several times that in either control zone. The maximum
IPC was near 16 L -1 in Zones S-S and C-S, while the
maximum in the controls was 2 L -1.
Figure 3 shows that most of the enhanced IPC was
due to particles smaller than 0.6 mm, generally clas-
sified as hexagonal or spherical. Visual examination of
the latter suggested that they were usually tiny plates
too small to classify precisely within the limitations of
the 2D-C probe's resolution and the processing soft-
ware. Temperatures recorded over the BRTA ranged
from -tOO to -120C at 2.7 and 3.0 km altitudes, re-
spectively, so that growth of seeding-generated crystals
in the observed plate habits is reasonable (Magono and
Lee 1966). The limited sizes of these crystals suggests
that growth in a water-saturated environment had
lasted for only a few hundred seconds (e.g., Ryan et