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

Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 29, No.6. June 1990
American Meteorological Society

The Persistence of Seeding Effects in a Winter Orographic Cloud
Seeded with Silver Iodide Burned in Acetone

TERRY DESHLER* AND DAVID W. REYNOLDS
Bureau of Reclamation, Auburn, California
(Manuscript received 14 July 1989, in final form 29 November 1989)

ABSTRACT

A single case-study of a winter orographic cloud over the centrn,I Sierra Neva~a is presented in which the
effects of aerial seeding with silver iodide, an AgI NH4I NH4Cl04 mixture burned In acetone.. were observed to
persist for over 90 min after seeding and I 00 k~ downwind of the see~lin~. A research aircraft :-vas able to
locate and track the line source of AgI using an Ice nucleus counter. High Ice crystal co?centratlOns due. to
seeding were not apparent until more than one hour after seeding. This may hav~ been partIall~ due to the high
natural concentrations of ice, but post-mission analysis revealed that most sampling passes dunng the firs~ hour
following seeding were made below the AgI seeded volume. Ice nucleus measurements ~onfirmed sam~lIng of
the seedline from 1-1.5 h after seeding, with associated increases in ice crystal concentratIOns. The effectiveness
of the seeding material in the field was higher than laboratory measurements would suggest.

1. Introduction

Since the discoveries of Schaefer ( 1946) and V on-
negut ( 1947) a number of experiments have foc~sed
on determining the physical effects of cloud seedmg.
One approach has been to measure directly the effects
with instrumented aircraft, radar, and detailed surface
measurements (Leonov and Perelet 1967; Leskov 1974;
Weickman 1974; Hobbs 1975; Super and Heimbach
1988; Super and Boe 1988). The research conducte?
during the Sierra Cooperative Pilot Project (SCPP) IS
a continuation of this effort (Reynolds and Dennis
1986). This paper deals specifically with identifying
and tracking seeding material in a wintertime, fairly
shallow, stratiform cloud over the central Sierra Nevada
with the use of a research aircraft during the final year
of the SCPP. It complements earlier studies over the
central Sierra Nevada (Stewart and Marwitz 1982;
Deshler et al. 1990) in which cloud seeding was done
from aircraft using primarily either dry ice or droppable
pyrotechnic flares containing AgI. These two droppable
seeding agents were used during the SCPP because,
according to clear air tests over a lake, they would pro-
duce a vertical curtain of ice crystals that extended 700
m below the seeder aircraft. Such an effect was expected
to be more easily found and tracked with a research
aircraft than a line source of AgI burned in acetone;

* Present affiliation: Dept. of Physics and Astronomy, University
of Wyoming, Laramie, Wyoming.

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

however, the success rate for detectin~ seeding effects
with these materials was low, even WIth the research
aircraft (Deshler et al. 1990). This led, in the final year
of the project, to the experimental use of AgI released

as a line source.
The problems encountered from seeding with 20 g
AgI flares or dry ice, and the low success rates, as well
as the advantages of seeding by burning AgI in acetone
have been described by Deshler et al. (1990). Mea-
surements in a cloud chamber indicate that an AgI
NH4I NH4CI04 mixture will produce 10 13 nuclei g-1
of AgI at -60C and 6 X 10 14 nuclei g-1 at -1OoC
(DeMott et al. 1983). There is little increase in activity
at temperatures below -woe. With activities this high
this mixture may produce detectable levels of ice crys-
tals in clouds as warm as -60e. Also the AgI is useful
as a tracer since it can be measured with an ice nucleus
counter. This paper presents one case study using AgI
NH4I NH4Cl04 that demonstrates the activity of this
seeding material and the capability of a research aircraft
to find and track a line source of AgI.

2. Instrumentation

The research aircraft, a Beechcraft Super King Air
200T operated by the University of Wyoming, carri~d
a full complement of instrumentation for cloud phYSICS
research (Cooper et al. 1984). Instruments on the air-
craft measured altitude, indicated turbulence, position,
temperature, dewpoint, and winds. The aircraf~ a1~o
carried Johnson-Williams (JW) and CSIRO hqUld
water meters, a Rosemount icing rate meter, an ine~ial
navigation system, an ice nucleus counter, a PartIcle
Measuring System (PMS) Forward Scattering Spec-