WMOD00484

• Title: Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide Over the Rocky Mountains - Part I
• Date: 10/10/1988
• Weather Modification - Doc Type: Report

Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 27, No. 10, October 1988
American Meteorological Society

Microphysical Effeds of Wintertime Cloud Seeding with Silver Iodide over the Rocky
Mountains. Part I: Experimental Design and Instrumentation

ARLIN B. SUPER, BRUCE A. BOE AND EDMOND W. HOLROYD III
us, Bureau of Reclamation, Denver, Colorado

JAMES A. HEIMBACH, JR.*

Montana State University, Bozeman, Montana
(Manuscript received 6 April 1987, in final form 14 March 1988)

ABSTRACT

A series of winter orographic cloud seeding experiments is described in which the seeding agent and associated
changes in cloud microphysics are monitored to within.300 m of the tar~et a~eas (Montana and. Colorad.o),
and at the surface (Colorado only). This, the first paper In a three.part senes, dlscl!sses the underlYIng phYSical
hypothesis and experimental approach, and describes in detail the inst~mentat~on used. The results of the
physical evaluations, presented in P.arts 1.1 an~ III,. sh?w that marked microphysical changes were caused by
both ground-based and aircraft seedIng With silver Iodide.

1. Introduction

It is widely recognized that improved physical mea-
surements of the effects of cloud seeding are needed.
One of the specific recommendations in the American
Meteorological Soci~ty Policy Statement on Weather
Modification (AMS 1985) is the pursuit of field re-
search on the modification of the development and
growth of precipitation particles caused by cloud seed-
ing agents. .
Past weather modification experiments usually relIed
primarily on statistical evidence with only li~ited (and
often indirect) physical evidence (e.g., MIelke et al.
1981; Super and Heimbach 1983). A notewort!'ty ex-
ception was the work over the Cascade Mountams re-
ported by Hobbs and Radke (1975), who developed
innovative techniques for the physical evaluation of
seeding using airborne, ground and radar observations.
More recently, the HIPLEX-1 experiment (Smith
et al. 1984) and the Sierra Cooperative Pilot Project
(Reynolds and Dennis 1986) have demonstrated the
types of direct physical observations that can now be
made to document the effects of cloud seeding. Of
greatest importance to the present investigation is ~he
availability of two-dimensional imaging probes, whIch

* Present affiliation: University of North Carolina at Asheville,
Asheville, North Carolina.

Corresponding author address: Dr. Arlin B. Super, Bureau of Rec-
lamation, Code D-3720, Denver Federal Center, Denver, CO 80225.

@ 1988 American Meteorological Society

, \

allow estimation of ice particle concentrations, size
spectra, and habit. These observations can also be used
to estimate the precipitation rate at the sampled level
(Holroyd 1987). .
The experiments by Super and HeImbach (1988)
discussed in Part II and by Super and Boe (1988) dis-
cussed in Part III provide direct evidence of the physical
chain of events following the introduction of an arti-
ficial ice nucleant into winter orographic clouds over
the Rocky Mountains. Recent observations indicate
that such clouds often contain supercooled water but
are frequently inefficient precipitation producers
(Rauber and Grant 1986; Rauber et al. 1986; Super
1986; Super et al. 1986; Thompson and Super 1987).
The clouds' persistence over many hours makes them
excellent candidates for seeding experiments, especially
when little or no natural precipitation occurs to obscure
the seeding signal. They may also be good candidates
for enhancing the seasonal snowpack. While their short-
term, cross-barrier liquid water flux is often small
(Thompson and Super 1987), the high frequency of
their occurrence and their long duration can result in
significant seasonal flux (Boe and Super 1986).

2. Experimental design and physical hypothesis

Two separate series of physical cloud seeding ex-
periments were conducted during consecutive winters
at different locations in the Rocky Mountains. These
series were very similar in both their observational ap-
proach and in their analysis methods.
The first experiments were conducted during Jan-
uary 1985 over the Bridger Mountains of southwestern