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

288

"I

JOURNAL OF APPLIED METEOROLOGY

VOLUME 29

Physical Response of Winter Orographic Clouds over the Sierra Nevada to Airborne
Seeding Using Dry Ice or Silver Iodide

TERRY DESHLER* AND DAVID W. REYNOLDS

Bureau of Reclamation, Auburn, California

ARLEN W. HUGGINSt

Electronic Techniques Inc., Auburn, California
(Manuscript received 10 June 1988, in final form 17 August 1989)

.1

ABSTRACT

Cloud seeding experiments devoted to physical measurements of the effects of seeding shallow stable winter
orographic clouds have been conducted in the central Sierra Nevada of California from 1984 to 1986. Seeding
was done by aircraft using either dry ice or silver iodide at temperatures between -60 and -140C. Aircraft,
radar, and surface instruments were used to measure the effects. A trajectory model was used to target seeded
precipitation to the ground where the surface instruments were deployed. Results from these c",eriments are
presented in two case studies and a summary analysis of all 36 experiments. Observations from the various
measurement platforms conformed with results expected from seeding in 35 percent of the seedlines sampled
with a research aircraft, 4 percent of those observed with radar, and 17 percent of those which passed over the
surface instrumentation; however, the complete seeding chain was believed to be documented in only 2 of 36
experiments. The failures result from difficult technical and logistical problems, and from the variability of even
simple cloud systems, particularly in the spatial and temporal distributions of liquid water and in the natural
fluctuations in ice crystal concentrations. Based on the difficulty of these experiments and the magnitude of
seeding effects observed, a statistical experiment would be.a formidable undertaking.
During the two experiments when seeding effects were detected by all measurement platforms the following
effects were observed. A high concentration, 50-100 L -I, of small compact ice crystals formed quickly along
the seedline. Although aggregation was seldom observed, riming often began 5-10 min after seeding, The seeded
ice crystals dispersed at I m S-I and cloud liquid-water evaporated in regions corresponding to the seedlines.
Seeding in a non-echoing region occasionally produced echoes of 3-10 dBZ in portions of the seedlines. At the
surface seeding effects arrived 35 to 60 min after seeding, 20-30 km downwind. Snow crystal concentrations
increased, snow crystal habits changed to small rimed particles, and precipitation rates increased by 0.1-1.0
mm h-'. The duration of these effects was short, <10 min per seedline. Changes in ice particle development
induced by seeding were similar when seeding with either dry ice or silver iodide. This was found to be the case
even at temperatures as warm as -60C using AgI NH.I NH.C10. burned in an acetone solution.

1. Introduction

With the discovery that artificial nucleants could in-
crease ice crystal concentrations in supercooled clouds
(Schaefer 1946; V onnegut 1947) cloud seeding for
winter snowpack augmentation began. Southern Cal-
ifornia Edison Company has conducted a cloud seeding
program in the San Joaquin River Basin continuously
since 1950. Other projects, mainly by utility companies,
have also been conducted for 20 to 30 years. More
detailed randomized seeding programs to better define

* Present affiliation: University of Wyoming, Laramie, Wyoming.
t Present affiliation: Desert Research Institute, Reno, Nevada.

Corresponding author address.' Dr. Terry Deshler, University of
Wyoming, Department of Physics and Astronomy, P.O. Box 3905,
University Station, Laramie, WY 82071.

@ 1990 American Meteorological Society

cloud seeding potential have been conducted since the
early 1960s (Grant and Mielke 1967; Chappell et al.
1971; Henderson 1966; Mooney and Lunn 1969).
These programs were designed to infer the effects of
cloud seeding using statistical studies. Elliott (1986)
reviewed many of these programs and concluded that
they have not been able to unequivocally determine
the amount of additional precipitation that can be
achieved through seeding. For that, statistically signif-
icant results, with supporting physical observations to
provide a plausible explanation for the statistical results,
are required.
Ludlam (1955) suggested a conceptual model for
tapping the supercooled liquid water (SLW) produced
by orographic lift, attempted to quantify the amount
of additional snow that might be obtained, and gave
some physical observations to characterize the effects
of seeding on clouds and on snowfall at the ground.
More recently there have been additional physical