WMOD00567

• Title: A Report on Winter Snowpack-Augmentation
• Date: 11/11/1988
• Weather Modification - Doc Type: Report

[Reprinted from BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, VoL 69, No. II, November 1988]
Printed in U. S. A.

David W. Reynolds

A Report on Winter Snowpack-Augmentation

Bureau of Reclamation,
Auburn, California

Abstract

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Cloud seeding to increase winter snowpacks over mountainous regions
ofthe western United States have been in existence for almost 40 years.
However, our understanding of the physical processes taking place in
the clouds in response to this seeding and the expected precipitation
increases are still subjects of great scientific interest and investigation.
Recent field observations that have emphasized direct physical obser-
vations of winter clouds, their structure and liquid water content, as
well as their response to the injection of glaciogenic seeding agents
have added to our knowledge. These physical observations are helping
to provide some insight into the mechanisms of precipitation increases,
inferred from statistical analyses, that have been reported in certain
winter orographic cloud seeding programs. This paper attempts to com-
pare physical and statistical results, to show consistency, and to help
provide limits to what one might expect when winter snowpack aug-
mentation is applied within suitable cloud systems.

1. Introduction

Shortly after the discovery of the utility of dry ice and silver
iodide as cloud seeding agents, Bergeron (1949) presented a
conceptual model of their use for increasing winter snowpack
on mountain ranges. Subsequent reviews of the physical basis
of orographic snowpack augmentation have revealed increas-
ingly sophisticated views of the physical processes involved
(e.g. Ludlam 1955; Grant and Kahan 1974; and Elliott 1986).
This paper wil1 compare results from two separate types of
winter snow pack augmentation field programs. The first type,
referred to as "statistical," are projects designed to determine
the increases in snowfal1 within a predefined target area using
randomized seeding methods. Given sufficient sample sizes,
researchers hope to obtain from these projects statistical1y sig-
nificant results on the precipitation increases produced. The
second type, referred to as "physical," are those projects whose
main objective is to determine the natural precipitation pro-
cesses taking place in wintertime clouds over the region and
how these processes might be augmented through seeding. A
strong emphasis is placed on determining the temporal and
spatial distribution of supercooled liquid water (SLW) in these
clouds. Secondly, by using direct, physical measurements, the
effects of seeding are tracked, from release of the seeding
material in cloud, through the growth of the seeded particles
to precipitation size, and until subsequent fal10ut on the surface.
With this tracking approach, cal1ed the "physical-links-in-the-
chain" approach, a multitude of parameters are measured along
this chain. The statistical programs, in contrast, measure pre-
cipitation amounts only.
Fortunately, the physical experiments were conducted in close

@ 1988 American Meteorological Society

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proXlITllty to the statistical projects. The results from these
physical studies (listed in table 1) are used to provide scientific
plausibility for the statistical results reported. (For a further
description of these two different types of experiments see
El1iott [1986]).
CLIMAX (Mielke et at 1981), a randomized, ground-based
seeding program that operated during the 1960s in the central
Colorado Rockies, has reported statistical1y significant in-
creases in snow pack through cloud seeding. CRADP (Colorado
River Augmentation Demonstration Program) and COSE (Col-
orado Orographic Seeding Experiment), conducted in the early
to mid-1980s, provides supporting physical observations to the
CLIMAX statistical results.
The Bridger Range experiment was an exploratory, random-
ized cloud-seeding experiment conducted in southwestern Mon-
tana dllring the winters of 1970-71 and 1971-72. This project
also reported statistical1y significant increases in snowpack
through ground-based seeding (Super and Heimbach 1983; Super
1986). Much later, detailed physical observations w.ere made
at this same location in an attempt to provide direct physical
observations of the seeding affects (Super and Heimbach 1988).
The Pacific Gas and Electric Company (PG&E) of Califor-
nia, since the early 1950s, has been conducting an ongoing,
long-tl~rm randomized, winter cloud-seeding program near Lake
Almanor (Mooney and Lunn 1969). From 1976 to 1987, the
Bureau of Reclamation's Sierra Cooperative Pilot Project (SCPP)
(Reynolds and Dennis 1986) was conducted some 75 miles to
the south of the PG&E program site, making extensive physical
observations within natural and seeded wintertime clouds. Fig-
ure 1 shows the location of these programs.
Major observations from five physical1y based programs listed
in table 1 wil1 be reviewed. In particular, both the distribution
of SLW and its relationship to storm structure and organization,
as well as the observed temporal and spatial variability of SLW,
will be discussed. A physical basis for seeding and the advan-
tages and disadvantages of various seeding (treatment) methods
will be reviewed in the context of both the directly observed
and statistical1y inferred seeding increases reported.
Final1y, the importance of numerical modeling studies in
providing insight and guidance into the conduct of snowpack
augmentation programs will be reviewed and suggestions of
the direction for future field studies will be provided.

2. Supercooled liquid water studies

According to the World Meteorological Organization (WMO)
(1982), a cloud is considered to be seedable for increasing
precipitation if (a) the collision-coalescence process between
cloud drops is inefficient, (b) the rate of formation of super-
cooled condensate exceeds 'or is comparable to the rate of de-
pletion of supercooled water, and (c) there is sufficient time

Vol. 69, No. 11, November 1988