WMOD00503

• Title: Seeding of Winter Orographic Clouds: A Viable Technology for Precipitation Enhancement?
• Weather Modification - Doc Type: Report

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SEEDING OF WINTER OROGRAPHIC CLOUDS: A VIABLE TECHNOLOGY
FOR PRECIPITATION ENHANCEMENT?

-I

,

David W. R(~ynolds

U.S. Department of the Interior
Bureau of Reclamation
Sacramento, California

1. INTRODUCTION

It has been 45 years since Vincent Schaefer
conducted his famous cold box experiment (July 13, 1946)
that initiated modem weather modification. Given the
tremendous advances since then in man's ability to observe
and model the complex processes of the atmosphere, one
would have thought it unnecessary to raise the question
addressed by this paper. A well proven and widely used
seeding technology would certainly exist. However, a
dichotomy continues as to whether a viable technology
exists for augmenting winter precipitation over mountain
barriers by cloud seeding. The scientific community
demands unequivocal physical and statistical evidence of
resultant precipitation increases while the water resources
manager is willing to take risks in seeking to augment an
ever diminishing water supply. This controversy affects
both federal funding and public support of weather
modification. A series of papers published in 1978
(Simpson, 1978, Silverman, 1978, and Henderson, 1978)
which asked the question "What does weather modification
need?" proposed a number of action items that would settle
this controversy. Unfortunately 13 years later, the
controversy continues as the proposed action items were
not implemented.

This paper will briefly assess the current state of
winter orographic cloud seeding as a method of
precipitation enhancement. Both the scientist's and
operational water manager's perspective will be presented.
It is important to understand these differing perspectives in
order that progress can be made in either dismissing as
impractical, establishing, or improving on a cloud seeding
technology for winter precipitation enhancement.

A perspective possibly as significant as that of the
scientist or water manager is that of the public and the
environmental community. This perception is influenced
by. the controversy discussed above. Their perception that
seeding not only contributes to major winter storms and
floods, but that it can initiate such events, threatens future
progress in development of winter precipitation
enhancement technologies.

The context of this review will be made using the
author's experiences in California. Although California's
situation may be unique in many respects, the experiences
described here should have relevance to other parts of the
intermountain west.

4. THE SCmNTIFIC ASSESSMBNT

Reynolds (1988) provided an extensive review of
winter ~no\VP~c~~_u.g1Uentation based on examination of

,

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results from several statistical and physical experiments
conducted over the last 25 years. Since this review was
completed, no significant results from either statistical or
physical experiments have been published that would
necessitate revising its conclusions. The major conclusions
from this paper are briefly reviewed.

Extensive physical measurements have been made of
winter orographic clouds over various locations in the
western U.S. using both remote sensing and in-situ
instrumentation. These observations have greatly improved
our understanding of cloud and precipitation processes.
This improved understanding has a direct bearing on how
one might modify these natural processes to increase a
cloud's precipitation efficiency. The single most important
factor, if seeding is to affect precipitation, is the presence
of supercooled liquid water (SLW). It must be present in
sufficient concentrations, distributed over a significant
spatial domain, and last long enough to allow seeding to
convert it to predpitation that can affect a significant
portion of the target area. Observations from microwave
radiometers, airborne liquid water sensors, and
mountaintop rime ice detectors have provided the following
general conclusions. SLW over mountains 1) is highly
variable in space and time, 2) exists in rather low
I:oncentrations of 0.05 to 0.2 g m.3, 3) occurs at relatively
warm temperatures of from 0 to -10 oC, 4) is concentrated
within the lowest kilometer above the highest terrain, and
:5) is most common in shallow orographic clouds induced
by large scale synoptic weather features. Figure 1 shows
vertically integrated liquid water amounts as observed by
radiometers from three western U.S. projects. There is a
remarkable similarity in the SLW values measured in
winter orographic clouds over widely separated locations.
SLW can occur both pre-frontally and post-
frontally. The largest fluxes of SLW generally occur
during a small portion of winter storms. As much as 50 to
75 % of the seasonal flux of SL W over a watershed can
occur in just a few storms. The other 25 to 50% of the
SL W flux occurs in the large number of storm hours in
which SLW concentrations are low but continuous.

I'

The impact of natural ice enhancement processes
(Hobbs and Rangno, 1985) on cloud seedability is
important but not well understood. Various mechanisms
have been described in the literature that produce higher
l:oncentrations of ice particles in a cloud than would be
produced by primary nucleation mechanisms. These ice
(~nhancement processes may be the largest contributing
factor in limiting the amount of SL W available in
orographic clouds in the Sierra Nevada. One might l1!!.ve
E~Xpected large quantities of liquid water there based on a
significant maritime influence. However, it has been
shown that SLW's spatial and temporal distributions are