WMOD00299 ‎(2)‎

• Title: Scientific Weather Modification Experimentation in the United States
• Date: 8/25/1984
• Weather Modification - Doc Type: Report

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.. .. .. n. Presented at the 9th I nternat i Dna 1 Cl Dud Phys i cs
Conference, Tallinn, USSR - August 21 - 28, 1984
SCIENTI?H; WEATHER MODIFICATION EXPERIMENil>:TlOIl IN THE UNITEO STATES
.

I~~rnard A: Silverman
F.:urealJ of Reclaii,3tion
Denver. Colorado U.S.A.

1. Introduction

after treatment for the seeding to be effec-
tive. It was expected that the seeding
effects would be most easily detected in
clouds with tops in the -6 to -12 oC range
(type A-I clouds). but the experimental
design permitted selection of rain clouds
with tops in the -12 to ~20 oC range (type B
clouds) as test cases when no suitable type A
clouds were present. Type A-2 clouds were
ircluded to account for the occasional
o:currence of situations where ice multip~i-
cation by the rime-splintering process were
found.

This paper discusses racent scie~tific
weather modifi cat i on resea rch i!l tile United
States. Four large precipi:atio!1 enhancement
research programs are briefly reviewed: the
rectntly completed HIPLEX-I (Mo!1tana High
Pl';-;'i5 Experiment) and FACE-2 {confirmatory
Florida Area Cumulus Experiment) experiments.
tne ongoing SCPP (Si!rra Cooperative Pilot
Project) program and the planned CREST
(~oltra1o River Enhanced Sno~pack Test)
(iemonstration program. !n addition. high-.
lichts of selected activities in three areas
or"re1ated researcn are presented: I) dua1-
channel ~;crowave radiometer applications. 2)
1 aboratory studies of the ice nucleating pro-
perties of si1ver iodide complexes. and 3)
eVill uati cn of experi:r.ents by monitori ng the
expected chain of physical events following
seeding. Finally. the probabie nature of
future scientific weather modificatio~
experimentation in the United States is
discussed.

2. HIPLEX-I

The qualifying variables were measured d~ring
a pretreatment pass by a cloud physics
aircraft flying through a visually pro~ising
cloud at the -8 oc level and immedi~telJ eva-
luated by an cnboard real-time computer to
determine whet~er or not the selection cri-
teria for any of the s?ecified cloJd types
were met. T~e seeding was conducted by
dropping a 1ine of dry ice pellets from a jet
aircraft at a rate of 0.1 kg/km near the -10
oC lev~l within 2 min after a suitable cloud
was selected. Following the treatment. ~ry
ice or placebo, both the seeding and clo~d
physics a~rcraft made repeated passes at spe-
cified times and specified levels in and
below the cloud to document the subsequent
chain of physical events as represented by
the response variables (see Table 2).

HIPLEX was aimed at establishlng the physical
basis f'c.r enh3ncing oeneficia1 growing sea.son
precipitation from convective ,clouds on t~,e
High Plains of the United States. HIPLEX-1
(Bureau of Reclamation. 1979) was a ran-
domized. double-blind exploratory experiment
. to test the static mode seeding concept for
summer conve~tive clouds over t~e Montana
High Plains. The experimental unit of
HIPLEX-I was semi-isolated cum~lus congestus
. clouds which develop precipitation naturally
through t~e Bergeron mechan~sm. that is ice
crystal growth by va:>or diffusion followed! by
riming into g~aupel. Many of these clouds do
not rain to th2 ground at all. The objective
of the static seeding was to provide an opti-
mum concentration of ice oarticles for the~
available liQu:d wate~. about 1Q per liter'.
and initiate the precipitation process
earlier and lower in altitude in tile deve-.
l~ping clouds than would occur 1aturally. It
. was hypothesized that the efficiency of the
natural precipitation process would be
improved by the ~e~ding and thereby lead to
both addition~l Jrecipit5tior. and to an
increase' in the proportion of clouds that
precipitate.

During the course of the 2-year ex~eriment.
55 clouds were tested for acceptance as
experimental units but only 2U met all the
selection criteria. This saw~le size was
considerablv less than the 30-45 cloudS per
year that were expected from the pre i imi n2ry
field investigations. Of the 20 test cases 7
were type A-I clouds. 4 seeded and 3 not
seeded. and 13 \~ere type B c1o'Jds. B seeded
and 5 not seeded. The evaluation showeu that
the postuiated increases in cloud ice con-
centrations associated with the seeding and
the subsequent onset of riming were unequivo-
cally established despite the limite~ sample
, size. The statistical results (MielKe et
al.. 1984) are shown in Figure 1 and the pny-
sical resul~s (Cooper and Lawson. 1~84) are
shown in Figure 2 and 3. For all response
variables beyond 5 min after treatme1t,
except the average liquid water content at 8
min. changes in the sam~le average values of
the response \ariabl~s were consiste1t wit~
those suggestec by the ~hysical hypothesis,
but it was clear that in many cl~uas they
were not behaving as expected.

Oetailed criteria and procedures for
selecting and classifying test cases. for
perfor~ing the seeding and for the collection
of observations to be used in calculating
response variables were prescrijed. Based on
prelimInary ex;;loratory studies. the cloue'
selection criteria (see iable 1) were
expected to result in a sample of ClOUdS that
would be amenable to seeding according to the
'- static hypothesis and last at least 30 mirl

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The physical evaluation (Cooper and Laws~n.
1934) revealed that in 4 of the 12 clou~s
that were se~ded precipitation developed in
the hypothesi;:ed manner but physically signi-
ricant departures occurred in the remainder.
These studies indicated the following abo'Jt

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