WMOD00492

• Title: Natural Ice Crystals Concentration in Northern High Plains Summer Clouds
• Weather Modification - Doc Type: Report

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NA'!'t!!\A!. ICE CRYS'!AL CONCENTRATIONS IN NORTHERN HIGH PLAINS SUMMER CLOUDS

Jack ~cl'artland. Adin Sup~r and Edmond Holroyd, III

High Plains Cooperative Program
Bureau of Reclamation
~~les City, Mon tana

1.

I~ROD':CTION

Ice particle concentra~fon measurements
vere o~ta1ned 1n ~upcrcooled convective CJ.OUlJS as
part of the 1976 H:!.P!..EX field investigations near
=-tiles City., ~!ontiln.a. TheSe meU5uremcnt9 w~rc
aeede~ ..1:0 .document the natural concentrations
found in various types of su:!!e:e!' c"louds as 1t
function of cloud c~v~lopment an~ temperature.
This infornation b ,,,,sic to ~!\e design of th~
future randomizec seed~ng experi~~nt. Most cloud
seeding hy~othcs~s for rain augT.~ntation assume
thac natural ice particle conccn~raclons are som~-
times ceficient t c!h::reby limIting precipitation
effici~ncy .

2.

DATA SOURCES

All data utilized were collected by an
Aero-CoI!!!!'.ancer 680 FT.. cloud physics aircraft
operated by Conve rgence Syste=, Inc. (CSI).
Sampling missions w~re: flolJtt ')e~\leen 10 Hay and
l'~ July 1976. ~!ca~ure!!1ents usee in this investi-
gation consisted or ic~ particl~ concentration,
temperatur~, Vl!rt I.C~:!. velocity, liquid wuter
content, cloud druplet concenc!'i!t:!.on, VOR, D~E,
heading information, and fon.rarc-looking timc-
lap:.ie Super-8 mm p:~utQs. Source:i for thi:i inform-
ation are ~rief:y c~scrl~ec je~oY.

_1 Estimates ot ice par~lcle concentrations
(1 -) were o~tainec using a CS!-modified version
of the '.!n!.versity of 'das~ling':o!1 ice particle
counter (IPe). ':'es'.:s have SllO'.ro that the detection
size threshold of. t~~ instruTe~t is approximately
50 ~m. ~~lile the u~solute accuracy is unknown.
laboratory testing lncicate::i aJout a 20% counting
effic!..egcy for l5f)-~~M crysta.!.::i grown between -16
and -20 C. Howev~r, fl~l~ tesCs in an ice cloud
showed close corrt.:sp('ln~ence ':lc,;,."een the IPC.
:?article ~!easurin3 Sys'.:e:ms (?YS) 1-0 probe data,
and oi:-coated sl~ct.: ~ata co.!..!.ected simultaneously
using a Cniverslcy 0: ~yoming type decel~rutor
(~. P. Lawson, p~rsona~ co~~nication, 1977).
Favorable intercom~a~isons 'Jere also obtained
between che CS! an~ eniversi'.:y of Washington (UW)
IPC's when both were flown '.:~rough the same cloud
during HIPLEX aircraf.t :nte!'co~~arlson teses.. The
CSI unit tended to ~ive h!g~er concentration values
than the t".l unit by a')out il :ac':.or of four. Further
discussion conc~rn~n~ com?ar!son of ice particle
counters is giv"n '>y Super~. (1977).

Temperature~ dur~ng cloud passes were
measured with n clJ.'.i1.lrate:<! r~ver~e flow sen~o't'.

Vertical velocities were measured with an
interactive system incorporating several inputs to
cancel stick-induced changes in aircraft rate of
cll mb .

.. .Liquid, wa.ter _content was measured using a
Johnson-Williams system, whil~ cloudctTopl~t C3nue'r-
trations (2 to 30 ~m size range) w~re obtained from
a PHS axially scatt~ring probe.

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Heading and VOR-DME were recorded fTom
standard aircraft instruments..

Time-lapse photos were taken at 6-second
intervals through the aircTaft windshield. A cock-
pit time display from the data acquisition system
wa~ 1n view of the CL1m~ru.

Observers '. notes and occasional photos of
inc,ividual clouds were also utilized.

3.

DATA REDUCTION AND PROCEDURES

It was desired to obtain as large a data
sample as possible. As a first step, data from all
re~leurch flighcB .....t:!;re compucel:" edited to produce an
engineer-ing unit printout for all c;ime periods when
the ai['craft was in sup~'t'cooled cloud. 1;'0"[' thi~
purpose a supercooled cloud was defined as having3a
minimum PMS cloud droplet conc~ntratlon of 25 cm -3
or liquid water content (J-W) of at least 0.1 gm m
oL(lboth. and a temperature equal to 0[' colder than
-2 C. Ten-second data averages were used. and the
pr:Lntout produced all necessary data for each 10-
s~"ond period during the flights which met the super-
cooled cloud definition. Du~ to the formatting
tedmiques used in the computer processing. lO-second
meiln ice particle concentration data were rounded to
tlll~ nearest whol~l integer. Therefore. values in the
range 0 to 0.5 9. were indicated as zero. Values
in this range are hereafter referred to as the "data
tln:'eshold. It

Passes through clouds were numbered con-
se,outively for each day. A new pass was triggered
by a l2-second or greateT break in data fitting the
cl',ud definition.

The second step in the data reduction
pNcess was to identify individual cloud types and
to estimate their state of. development. The time-
ia~se films for each period of prIntout data w"r~
carefully inspected and cloud types were visually