WMOD00430

• Title: A Diagnostic Technique for Targeting During Airborne Seeding Experiments in Wintertime Storms over the Sierra Nevada
• Date: 7/7/1988
• Weather Modification - Doc Type: Report

JULY 1988

RAUBER ET AL.

823

computed for each of the fixed target experiments of
1985-86 by integrating the errors along the entire par-
ticle trajectories. The error in the center of the fallout
area due to errors in the wind fields was almost always
within 10 km of the Kingvale site and often within 5
km. This shift generally, but not always, left Kingvale
within the expected area of effect.

I.

b. Ice particle growth rates

Ice particle growth rates used in targeting were based
on the laboratory measurements of Ryan et al. (1976)
and Fukuta and Wang (1984). These experiments were
conducted under water saturated conditions. In the
Ryan et al. experiments, water saturation was main-
tained by growing crystals in an environment with liq-
uid water contents near 3 g m-3. These diffusional
growth rates therefore represent maximums expected
in real atmospheric conditions.
During SCPP, ice particles created by seeding have
been observed in a number of experiments (e.g., Stewart
and Marwitz 1982b; Martner 1986; Deshler and Reyn-
olds 1987). Natural conditions in the vicinity of seeded
regions have varied substantially. This natural vari-
ability has had a substantial effect on the identification
of seeding effects. Martner (1986) found that only 25%
of the seedlines penetrated by aircraft showed unam-
biguous seeding effects. In the remaining 75%, seeding
effects did not appear either because highly variable
and large ice particle concentrations already existed
naturally or large droplets masked the signal on the
optical array probes (Rauber and Heggli 1988).
When seeding effects were observed in the orographic
cloud systems, somewhat consistent information
emerged. With few important exceptions, particle
growth rates were found to be consistent over a broad
temperature range. Figure 9 shows a plot of observed

!~

1.6

'.

'T",
~ 1.2

(f)
UJ
~
a:
I 0.8
I-
~
o
a:
Cl
~ 0.4
u
j::
a:
~

-4 -5 -6 -7 -8 -9 -10 -II -12 -13 -14
TEMPERATURE (Oe)

FIG. 9. (Solid lines) Variation of crystal axial growth rates with
temperature from Ryan et al. (1976) experiments. (Line segments)
estimated crystal growth rates based on optical array probe images
collected during seedline penetrations (see text). Coincident line seg-
ments are indicated by different symbols.

particle growth rates in seeded regions derived from
cloud physics aircraft data collected at various tem-
perature levels. The particle growth rates were deter-
mined by measuring sizes of particles associated with
seeding from optical array probe images at the time of
seedline penetration and dividing these values by the
time elapsed between the release of the seeding material
and the observation. Particles associated with seeding
were identified by comparing particle spectra measured
during the seedline penetration with those directly out-
side the region of seeding effects. These data are also
tabulated in Table 3. The table also describes ambient
cloud conditions and other pertinent information as-
sociated with each observation. The diffusional growth
rate curves of Ryan et al. (1976) are also shown in
Fig. 9.
Although scatter exists in the observations, the ob-
served growth rates in the seeded regions of clouds gen-
erally fell below the rates measured in the laboratory.
With one exception, no enhanced growth was observed
along the c-axis between -4.50 and -7.50e. The ex-
ception occurred during one seedline penetration on
5 Febmary 1983 where needlelike habits were observed.
In all other cases, small thick columns, hexagonal
plates, or small irregular particles were observed. These
habits are characteristic of sub-water saturated con-
ditions. The high ice particle concentrations, reductions
in supercooled water observed in plume penetrations,
and consistent particle habits and slow growth rates all
suggest that sub-water saturated conditions existed lo-
cally in plumes due to overseeding. It is interesting to
note that supercooled liquid was observed on the seed-
line during the case when larger needles were observed.
In the temperature range < -80C, growth rates and
habits more closely matched the laboratory measure-
ments. It is important to point out that the seeding
agent in several cases was Agl. Small particles were
observed on the seedline through 40 and 81 min, re-
spectively, on 18 and 22 December 1986. These small
particles may indicate slow growth rates, but, more
likely were due to continued nucleation well downwind
of the release point.
According to Heymsfield (1987), preferred mecha-
nisms of growth occur after initial diffusional growth
depending on the habit of the particle. He found that
particles with small cross-sectional area grow by accre-
tion much more efficiently than by aggregation. In
contrast, particles with large cross-sectional areas, such
as needles growing at -60C, or dendrites growing at
-I50C, grow efficiently by aggregation. Direct obser-
vations within seeded plumes suggested that the pri-
mary growth mechanism in shallow orographic cloud
systems seeded in SCPP during the later period of crys-
tal growth was accretion. Aggregation was rarely ob-
served. Heymsfield's results indicate that for liquid wa-
ter contents normally present in these cloud types
(0.05-0.20 g m-3), maximum dimensions reached be-
fore impact would be 500-800 ILm and terminal ve-