WMOD00508

• Title: Physical Response of Winter Orographic Clouds over the Sierra Nevada to Airborne Seeding Using Dry Ice or Silver Iodide
• Date: 4/4/1990
• Weather Modification - Doc Type: Report

APRIL 1990

DESHLER, REYNOLDS AND HUGGINS

315

TABLE 3. Seedline penetrations on 5 February 1986 by the cloud physics aircraft.

Penetration Age of Seedline Width of Mean Mean
Advection time seedline width Dispersion elevated 2D-C 2D-P LWC
Seedline (degfm S-I) (min:sec) (min) (m) (m S-I) ICC (m) (L-') (L-') (g m-3)
10 304/11 2301:57 8 640 1.1 2250 19 2 0.2
10 270/10 2312:00 17 1500 0.7 3240 36 3 0.1
9 304/11 2302:30 17 720 0.6 1890 19 5 0.1
9 270/10 2311:14 25 1350 1.0 2070 38 11 0.0
8 270/10 2310:30 31 900 0.4 1080 21 4 0.05
7 270/10 2309:40 37 8 3 0.1
Nat 2301:45 1350 65 2 0.2
Nat 2312:20 1530 77 5 0.0
Nat 2314:00 4000 41 8 0.05

measured an average liquid water content of 0.05 g
m -3 along the seedline. Seeding was done with dry ice
dispensed from a grinder on the aircraft at a rate of
approximately 0.7 kg km-1 from 2142 to 2257. Ten
28 km long curtains, S I-S I 0, of dry ice were released.
The tenth seed curtain was tagged by dropping indium
sesquioxide (ln203) flares.

2 ) RESULTS OF SEEDING
(i) Aircraft measurements

Due to the shallow clouds over the barrier the re-
search aircraft was unable to fly beneath the seeder
aircraft and track seeded parcels downwind. Instead
the aircraft was limited to a single ascent and descent
across the barrier at the end of seeding. The flight track
is shown in Fig. 18, along with aircraft measured winds
and altitude. Note the directional shear of the winds
in the layer 2.9 to 3.5 km as a function of location.
Winds measured by the research aircraft on ascent be-
tween 2.9 and 3.3 km were from 3040 at 13 m S-I,
while the winds on descent 10 km to the north and 7
min later were 2700 at 10 m S-I.
Measurements from the particle sensors on the air-
craft are shown in Fig. 19 for the ascent (2301-2304)
through the southern end of the seedlines and descent
(2309-2313) over KGV on a 2700 heading. Advection
of the seed curtains using 3040 at 13 m S-1 and com-
parison with the aircraft data indicated that on ascent
S9 and S 1 0 were intercepted within cloud, but that no
increases in ICC were noted at the intercept times;
however, when an advection of 3040 at 11 m S-1 was
used the interception of S9 and SIO coincided with
increases in ICC, Fig. 19a, although not all of the ice
observed could be attributed to seeding. To indicate

the portion of the ice crystal region which may have
been caused by seeding, estimated seedline widths as-
suming a dispersion of I m s -1 are shown. Clearly S 10
has a large natural region of ice associated with it while
S9 has a smaller natural component. Eleven m S-1 was
within the standard deviation of the winds measured
on ascent. For S9 and S 10 a 2 m s -I uncertainty in
the advection speed corresponds to a 1-2 km uncer-
tainty in position. Liquid water up to 0.2 g m -3 was
observed on the edges of these ice crystal regions. Using
an advection of 2700 at 10 m S-1 for the descent pass
indicated that S7-SIO were sampled at the times and
with the ages and widths shown in Fig. 19b. Increased
ICC are observed for the penetrations of S8-SIO, but
not S7; however, S7 was 37 min old and, based on
previous case studies (Martner 1986), the effects of
seeding had probably descended below the aircraft al-
titude of 3.1 km by this time. The ICC in curtains S8-
S 1 0 are clearly decreasing with age.
The characteristics of the four seed lines penetrated,
as obtained from the particle sensors, are listed in Table
3. The averages were calculated over the width of the
ice crystal plumes thought to be caused by seeding.
The dispersion rates were 0.4 to l.l m S-I, lower than
might have been expected given the turbulent nature
of the cloud. Comparable dispersion rates have been
observed by Martner (1986), although Stewart and
Marwitz (1982) measured twice this dispersion rate
for AgI seeding of a stratus cloud with similar wind
shear, 0.005 S-I. For comparison the characteristics of
three regions of cloud containing ice crystals of natural
origin are also included in Table 3. These regions were
sampled at nearly the same times as the seeded regions.
The 2D-C images recorded in the seeded regions of
high ICC and in the surrounding regions of low ICC
were quite similar. The ice could be classified as small

FIG. 19. Measurements, as in Fig. 5, from the research aircraft during, (a) ascent, 2300-2304, and (b) descent, 2309-2313,
through the seed curtains. Liquid-water content (solid line) and temperature (dashed line) are shown in the bottom panel, and
altitude and I D-C concentration in the next panel. The plots are annotated with the expected time of seedline penetrations'. The
width of the bar indicates seedline width assuming a dispersion of I m S-I.