WMOD00507

• Title: The Persistence of Seeding Effects in a Winter Orographic Cloud Seeded with Silver Iodidde Burned in Acetone
• Date: 6/6/1990
• Weather Modification - Doc Type: Report

JUNE 1990

TERRY DESHLER AND DAVID W. REYNOLDS

487

ing would have been below the altitude of the research
aircraft before natural particle concentrations had de-
creased to a level where the seeding effects could have
been observed. Also, CO2 may have been ineffective
on this day considering the low liquid-water content
observed at the seedline.
No effects from seeding were observed with radar;
however, considering the intensity of natural echo, rel-
ative to echoes anticipated from seeding (Deshler et
al. 1990), no radar seeding effects were anticipated.
Similarly no seeding effects were observed with the
surface instrumentation; however, the resolution of the
precipitation gauges was above what may be expected
from the passage of one seedline, and the seedline did
not pass over the heavily instrumented surface station.
The final penetration of the seedline, 10 km from
the downwind edge of cloud, Fig. 4b, still showed a
significant concentration of ice nuclei, suggesting that
seeding effects would be limited by the dimensions of
the cloud and not by the amount of seeding material
released. The unscavenged nuclei at the downwind edge
of cloud will pass over the barrier. If they enter another
cloud system they could be expected to contribute to
the ice formation process in that cloud. Thus, these
measurements lend credence to predictions of extra
area effects from seeding with AgI (Brown et al. 1978).
Although the goal of documenting ice crystal nucle-
ation at -60C with AgI NH41 NH4Cl04 was not met,
this case presents an outstanding example ofthe ability
to track seeding material for a long period of time. It
also suggests that AgI NH41 NH4Cl04 is an effective
cloud seeding agent with properties in the field that
meet or exceed those measured in the laboratory. Using
the width of the ice crystal plume on the last penetration
(7 km), the length of the seedline (40 km), and as-
suming the ice crystal curtain reached the surface 2.5
km below, the volume of cloud affected by seeding can
be roughly calculated. The result is 7 X 10 14 L. Sixteen
grams of AgI were released, which according to DeMott
et al. ( 1983) would give approximately 1016 nuclei ac-
tive at -lOoC, assuming all ice nuclei have activated.
This amounts to an ice particle concentration of 14
L -1 from seeding in agreement with measurements at
92 min that show 10-20 L -1 in the seeded curtain.
DeMott et al.'s measurements indicate that all ice nuclei
in the cloud chamber activate within 20-30 min, while
these field measurements indicate approximately 10%
of the ice nuclei still available after 90 min. Given the
1-10% counting efficiency of the ice nucleus counter,
the 30 L -1 shown in Fig. 4 may represent a relatively
high concentration of ice nuclei. This difference in ice
nucleus scavenging rate could be attributed to the lower
droplet concentration in the natural cloud compared
to the cloud chamber where droplet concentrations are
higher by a factor of 100. The collection rate in the
cloud chamber would be significantly faster because of
the high droplet concentration. Considering the ice
crystal concentrations observed and the ice nuclei still

available these observations indicate that the activity
of this AgI mixture meets or exceeds laboratory mea-
surements. In particular they suggest that contact nu-
cleation is not the primary mode of nucleation. If the
above calculation is repeated, including a scavenging
rate for the ice nuclei (Slinn 1971), the prediction is
an ice crystal concentration of 5 L -1 after 90 min.
Based on these observations, AgI is active at tem-
peratures> -lOoC, which is necessary for seeding in
the Sierra Nevada, and has several advantages over
CO2. For CO2, liquid water is necessary at the time of
seeding and all nucleation occurs instantaneously; for
AgI, this is not the case. Seeding could even be done
upwind of cloud to allow more time for the material
to disperse, before the AgI nuclei encounter liquid water
and become active (Hill 1980). Also, with AgI, nucle-
ation will be spread out over a longer time, thus perhaps
making better use of the liquid water available.

Acknowledgments. This research was sponsored by
the Bureau of Reclamation, U.S. Department of the
Interior. The support of all Sierra Cooperative Pilot
Project personnel is appreciated. Particular thanks to
the crew of the research aircraft, Dr. J. D. Marwitz,
Mr. G. V. Bershinsky, and Mr. G. L. Gordon for the
scientific and technical expertise to capitalize on this
experiment.

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