WMOD00553

• Project Name: High Plains Cooperative Program
• Title: Seeding Experiments Using AgI in Convective Clouds
• Weather Modification - Doc Type: Report

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SEED!NG EXPERIMENTS USING AgI IN CO~EC!IVE CLO~DS

Edmond Holroyd, III, and Arlin Super

High Plains Coop~rativ~ Program
Bureau of Reclanu\tioo
Niles City, Man tana

1.

INTRODUCTION

For many years silver iodide (AgI) has
been the mO::it conunouly used sc~ding agent for nl~d-
Ifytng supercooled COllVCct.~.ve clouds. It is
.usull.!.ly dispersed from the ~urfuce or from llircraft
~y '.3unling acetone solutlon~ or by 'iglridl\1; 'fl'!l'res
w11tC!1 conta.!.n silver iodice or silver ioduce. The
e.Efectivene~s (the numl)cr of lc~ cry~tal::l r~9ulcing
per gram of AgI utilizet~) of this seeding agt:ot has
b~en measured in the lu'Jora':ory (Garvey, 1975;
Carvey et al., 1976) an<.! 1n clouds (Allee et al.,
1972; Dye (.t ul., 1976). -

This paper p~esent9 additional mea~ure-
ments in cumulus clouds of the ~ffectiveneS9 of two
types of ASI pyrotechn:t.cs and on~ type of acet.one
burner using 2% AgI complexed with ammonium iodide
(~H/ I). Several experiments usee the fHter tech-
niq(le to .confitm AgI presence. The rat~ of dis-
p~rsion of .the resultin~ ice crystal::J was dett;Hlnined
for some of the experiments.

2.

SILVER IODI9E SEEDING EXPERI~~NTS

Several eX1?er~.ments ""ere conducted near
~f!:!.es City. ~ron.tanaJ dur.lng the tiuntIner of 1976, to
~)ruv!.de b~tt:er estimateS or: th~ effec~!.veness of AgI
].~t cumuliform clouds. T~le AgI was released in one
O.F !:!t ree '-lays:
1) Xedr cloud top, e!.t!u::.r. from i1l.,ove or inside,
u!:t!.ng the ~ucldi EngJ.lll~I.:r..l!lgJ Tnc., (~I':-!.) v~ro:.lon of
~:\C TB-l. vert.ical drop flare Wlt!c~\ r.c.!..~U!:fes 50 g of
AgI in about 60 s whLLe fn.lling up CO I.A: ~(.m.
2) Near cloud top, eithc::r from above or inside,
using Navy '!..'!!-l verticill drof' Elare~ w~\lch release
.t00 g of Ag:!: in 35 to ',f)'s while falling lip to
1.R km.
3) At cloud base using hero Systems, Inc.,
a.!.rbonle generators bllr~ing a 2Z AgI-~lIl I acetone
solution at the rate of 2 g of ^gI per ,hnut~ for
cdch or two generators on ~he: s~~ding aircraft.

Each cloudowas penetratec n~ur cloud top
(or at abouc the -10 C ]~evcl) tihortly ~efore tieed-
in~ t.o measure t.he natural ice crystal conct':nt!r-
tlons which were, in mO$!: cw~e~, less than l!l.. .
'C!te: cloud was t.hen seeded by a separate aircraft.
:n nearly all CaSeS a c;.oud was st:lBctedthat was
r~asonably isolated from others sO that it could be
~dclltifled for repene:r:.ratJ.on. When select:.ed, che
c.Louds had no. visible ~laciat:!on aad had firm
~)oundat'1e~.. All of the experiment.s conduct.ed. are
t!iscu::i::ied herein.

2.1

Ice Crystal Counters

The clouds investigated were sampled by
the Convergence Systems, Inc., (CSl' Aero CQ~ndei:
and the University of Washington ('-~I) 5-23. Uoth
alrcrafc were equ.1.ppt!d with ice purtiele counters
.,' '.'(It'C) '"si'ng 'pc1adzed li.ght. T)1~ CS!-!PC was
patterned aft.e-r the 'JW-!?C, hut. diffcred. 'in ~some.'
ret)p~CI:9.. An intercompar!.son bel:\Jc~n ches~ t\JO
instruments and comments OIL chelr counting effi-
ci~ncy are in Super~. (~977).

I. ''',~>>~

In two experiments, when the resulting ice
crystals may have been too small to be dec~cted
efficiently by the CSI-I?C, ~he larger m~asurement9
from a Particle Measuring Systems 20--300 !~m :.-!)
optical arr~y probe (P~!S2) were SUOSI:!.tuted for
che near-zero IPC values ~ut not nor:~llZtc.

2.2 Other SUDport1n~ ~easureeen~s

Updrafts, downdrafts, and turbulence
redistribute ooth the Ag! and the lce crystals with-
tn the cloud.. The u?crufcs were calcul.Jl:cd ust,.ng
m~asurements fyorn acc~leTo~etex~. angle of attack
and rate of climb indicators, and o~:ler tiOut"ces.
The absolute accuracy of t;~lvt:rtica': wines Yr.ay be
no '.Jetter, than + 1 or 2 ~ s -. '::le: variance or. the
vertIcal winds ~a~ calcu2.ated fro!!:. t!atn average:d
over 5 9.

For mosC of t~le exper!.ff!~nt:l ice nuc!.ei
were measured using ?-~:Ltl.!.?ore m~!JII)!:'ane fl.!..c~rs
exposed for the duration of i:acil eX'1e::riment or as
often as each cloud pnss. In add~~!on, a rotating
ffifltn'Jrane filter pass:!.ng uncer it :;i.'!.:.t:, :lim.t.!..ar co
that cescriGed by Dy~ et al. (1,976). ~/iJ~ exoosed
on a few flights in an~mpt ~o ~~ve a tl~e res-
olution to the ice nuclei. The fi~tcr.9 of both types
were also exposed 1n clow:!s ilnd on ~i!Y5 not involv-
ing Ag! seeding so tha~ a natura: ;ac~ground cou~d
be determined. All filters TJet'e a~a~_yzec..! at -16 ~
and at _200C at NCAR under ttl\~ .supeevl.sion of
!Jr. C. Langer. Experiments und f':'ig~t: ':cve19 with-
out. nuclei documentation are .~.dt:nt.:.:iec in the
later cables. Back.!H'ounc va~~,!~:: \ for .!wm~~,e9 at clgud
uase averaged 0.2 t .- and 0.6 t - a~ -16 C and -20 C
development .temperatures, rctipectively.

Cloud dimensions were ~etermined from pen-
etration distances and from photogca9hy. Only EL few
clouds lacked photographic docum~nt.a':.ion.

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