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

326

JOURNAL OF APPLIED METEOROLOGY

VOLUME 29

1) the 200-300 ~m columns or plates at 9 min on 5
February 1983; 2) the 100-200 ~m plates at 3.5 min
on 2 February 1985; and 3) the 100-400 ~m plates at
9.5 min on 28 January 1985. For these temperatures
(-7.50, -13.50, and -12.50C) the measurements of
Ryan et al. give growth rates of 0.5, 0.9, and 0.7 ~m
s -1. The growth of crystals in seeded plumes are affected
by changes in SL W (which probably decreases with
time), the onset of riming, and changes in temperature
as crystals ascend or descend. All these effects increase
the difference between growth rates measured in cloud
and those measured in the laboratory. Also in seeded
plumes larger particles fall below the aircraft altitude,
so after 10-15 min only small crystals that have ex-
perienced a poor growth environment will remain.
Thus, aircraft measurements after 10 min are expected
to give a lower growth rate than the laboratory. Com-
bining all growth rate estimates between the temper-
atures of -60 and -120C, produced an average of 0.5
~m s-\ where Ryan et al. measured 0.3-1.2 ~m S-I
in the same temperature range.

5) SEED PLUME DISPERSION

Estimates of the rate of horizontal plume dispersion
varied from 0.2 to 4.4 m s -I for individual observations.
Where multiple estimates from the same seeded curtain
were available (5 February 1983, 5 February 1986, 18
December 1986, and 22 December 1986) the averages
ranged from 0.7 to 1.2 m S-I. The overall mean was 1
m s-\ or a spread of 0.5 m S-I in each direction. None
of these estimates are as high as values reported by
Stewart and Marwitz ( 1982) and Super and Boe ( 1988)
for cases with high wind shear or speed divergence.
The lower values noted in Table 6 may be characteristic
of the type of cloud in which seeding effects are more
easily detected.

(i) Seeding effects observed with radar

Of the 21 seeded cases in which radar data were
available, only three produced possible seeding effects.
The success rate based on the number of seedlines ob-
served was 4 percent; however, on most of these days
natural echoes would have masked any result expected
from seeding. Two of the three days when seeding was
done in a nonechoing cloud were described in detail
in section 3. The other case, 24 February 1984 pre-
sented by Rauber et al. ( 1988), was similar to the con-
vective case of 19 January 1983 described by Huggins
and Rodi (1985). Downwind of several seedlines an
echo formed 6-10 min after seeding with CO2 pellets.
The seeded cells produced weak echo to the ground in
30 min and had a duration at the surface of 15 min.
The maximum reflectivity was 20-25 dBZ. Rainfall
rates implied from the Marshall-Palmer Z-R rela-
tionship reached a maximum of 1 mm h-1. Seeding
effects were not noted in clouds with echoes at the time
of seeding. Echo formation and subsequent precipi-
tation occurred well upwind of KGV.

Measurements on 18 December 1986 and on 5 Feb-
ruary 1986 matched the original concept of a radar
response to seeding (Bureau of Reclamation, 1985).
Seeding was expected to increase the areal coverage of
10 dBZ echo by filling in nonechoing gaps in the nat-
ural cloud. This clearly occurred in the second seedline
on 18 December 1986 and to some extent in three of
the seed curtains on 5 February 1986. In both cases
echoes formed 30 min after seeding, had a maximum
reflectivity of 8-12 dBZ, advected with the seedlines,
and reached the ground 45 min after seeding. Echo
locations were consistent with OTM predictions. Pre-
cipitation rates were 0.2-0.3 mm h-I using the Mar-
shall-Palmer Z-R relationship. The experiment on 18
December 1986 reflected the importance of initial
cloud conditions since one of the other two seedlines
was also in a relatively echo-free region, yet an echo
did not develop in it.

(ii) Seeding effects observed at the surface

There were 15 cases when the OTM predicted that
seeded precipitation would fall at KGV. There is some
suggestion of seeding effects on five of these days. Based
on the number of seedlines expected to have affected
KGV, the success rate is 17 percent, and this is with
an optimistic view towards the data. At present there
is no way to tell for certain whether effects measured
at the surface result from seeding or natural processes.
The stance taken in this analysis is to claim detection
of seeding effects if changes in surface measurements
during the PPE are consistent with effects expected
from seeding.
On 26 February 1982 two prominent increases in
ICC were measured with the surface 2D-C within 3
min of the PPE from the OTM for the two CO2 seed-
lines (Humphries 1984). There was evidence in the
aircraft data, but not in the radar data, to aid in tracking
the seeded curtains to the ground on this day.
On 26 January 1985 there were increases in snow
crystal concentration at KGV coinciding with the OTM
prediction that 1 mm unrimed plates would fall at
KGV 75 min after seeding with CO2, The winds were
extremely light on this day. Plates and aggregates of
plates were observed at KGV during the PPE.
On 3 February 1986 there was an increase in snow
crystal concentration coinciding with the PPE for the
first of eight seedlines initiated with CO2. The snow
crystals were small rimed particles. An indium tracer
released with the first seedline was found during the
PPE confirming the advection calculations. The aircraft
also identified a seeding signature in the first seedline.
Increases in snow crystal concentration were not ob-
served for the other seven seedlines.
The two primary cases which documented the chain
of events from cloud to ground were 5 February 1986
and 18 December 1986, and these have been described
in detail in section 3. The experiment on 5 February

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