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

319

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an upper limit to the amount of precipitation that could
have resulted.

(iii) Surface measurements at the fixed target

Measurements made at KGV are shown in Fig. 22.
The PPE of S I-S I 0, using the OTM advection of 265 0
at 7 m S-I, and the observed arrival ofNl and inferred
arrival of N2, N3 are also shown. Snow samples for
chemical analysis were collected at 1O-min intervals
from 2337 to 0007. Indium was found only in the sam-
ple collected from 2347 to 2357, within 5 min of the
PPE of S1O. Precipitation from Nl, which existed as
SI was dropped, arrived at KGV at 2220 and appeared
as a broad relative maximum in surface snow crystal
concentrations with a width of nearly 20 minutes. A
similar broad maximum in snow crystal concentration
was observed after seeding, when N3 was believed to
have passed. During the seeded period snow crystal
concentrations varied from 5 to 15 L -I, with eight rel-
ative maxima with average widths < 10 min. The rel-
ative maxima generally appeared with the calculated
arrival of seeded precipitation. Note that during the
PPE for the seeded interval the concentration of par-
ticles in the size range 0.5-1.0 mm was reduced while
the concentration of particles 0.3-0.5 mm increased
compared to snow crystal concentrations during the
passage ofNI and N3.
From the photomicrographic data it was observed
that needle aggregates ceased and small graupel com-
menced just before the PPE of S I. The graupel pellets
were 0.4 to 0.8 mm in diameter and conformed to the
particles expected from seeding at -80C by the cal-
culations ofHeymsfield ( 1986) and Rodi et al. ( 1985).
These particles persisted, however, throughout the pe-
riod after seeding effects were predicted to have passed.
The habit changes noted were observed with the pho-
tomicrographs and were also apparent in the habit
classification scheme of Holroyd ( 1987), which was
used to process the surface 2D-C measurements. In the
classified images (Fig. 22), needles disappeared and
the concentration of hexagonals increased coincident
with the arrival of Sl. The needle category did not
appear significantly again until precipitation from N3
arrived after the last seedline.
None of these observations are sufficient to prove
that the effects of seeding on surface precipitation were
measured at KGV. However, considering the naviga-
tional aids, the arrival of N 1, and the detection of in-
dium, if seeding with CO2 produced any changes in
surface precipitation on this day they were captured
by the measurements at KGV. Although all of the
changes highlighted in the previous discussion could
result from natural processes, they are also consistent
with changes expected from seeding.

3 ) SUMMARY OF RESULTS

For the SCPP this case represents one of the best
documented sets of observations for effects resulting

from seeding with CO2. We recognize that all of these
observations can be questioned and natural scenarios
presented that would produce the same observations.
At present our measurement capabilities are not suf-
ficient to unequivocally demonstrate seeding effects in
large cloud systems over complex terrain; however,
given the limitations of the measurements, we have
chosen to look at the data for evidence consistent with
conceptual models of the effects of seeding, and for
limits on the effects of seeding. With this in mind the
notable features of this case are: 1) The cloud physics
aircraft measured increases in ICC at places coinciding
with seedlines. 2) Navigation ofthe seeding effects pro-
duced a consistent picture in the aircraft, radar, and
ground data. 3) The seeding was done primarily in a
non-echoing cloud with natural echoes bracketing the
seeded interval. 4) Precipitation, corresponding to the
arrival of seeded snow crystals from S5-S 1 0, was only
observed downwind of the expected fallout of seeded
precipitation. Two gauges upwind of expected seeding
effects registered no precipitation during the seeded in-
terval, although they did record precipitation from the
natural echoes. 5) At KGV a change in crystal habits
was observed just before the arrival of seeded precip-
itation and conformed with the particles expected from
seeding by the microphysics calculations of Heymsfield
(1986); however, these crystal habits continued after
the seeding effects were expected to have passed. 6)
The precipitation resulting from this cloud system was
very light with maximum increases attributable to
seeding on the order of 0.1 mm h-1. 7) After 45 minutes
of travel the increased ice crystal concentrations mea-
sured at a fixed site on the ground were of short du-
ration, < 10 min. It is interesting that a vertical curtain
of dry ice produced effects similar to those observed
from seeding with a line source of AgI burned in ace-
tone on 18 December 1986.

4. Summary of seeding effects in all fixed target ex-
periments

The techniques used to delineate seeding effects in
the case studies presented were also applied to all fixed
target experiments, as well as to several similar exper-
iments conducted prior to the winter of 1984/85. Since
1980 a total of 36 experiments' were conducted on 29
days. Table 4 lists various parameters measured by air-
craft and radar at the locations where seeding occurred.
Averages from the aircraft data are constructed from
measurements 5 km to either side of the initial seedline
position. Flight procedures typically included passes
along the seedline before seeding and additional ex-
cursions back to this region while flying orthogonal to
advecting seedlines. The research aircraft averaged nine
passes per experiment through the initial seedline po-
sition. The average maxima in Table 4 represents an
average of the maximum observed during each pass.
Averages from the 2D-C and 2D-P represent the total