JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101,NO. D14, PAGES 18,961-18,977,AUGUST 27, 1996
Microphysical and electrical evolution of a Florida thunderstorm
1. Observations
JeffreyR. French,
• JohnH. Helsdon,AndrewG. Detxvilcr,andPaulL. Smith
Instituteof Atmospheric
Sciences,
SouthDakotaSchoolof MinesandTcchnology•
RapidCity
Abstract. This studydealswith the microphysical
and electricalevolutionof a thunderstorm
that occurredon August9, 1991,duringthe Convection
andPrecipitationf
Electrification(CAPE)
Experimentin easternFlorida. During its approximately1-hourlifetime,the stormwaspenetratedseveraltimesby the Instituteof AtmosphericSciences'
T-28 aircraftat midlevels. It was
alsopenetratedat low andmiddle-levels
by a NationalOceanographic
and AtmosphericAdministration(NOAA) P-3 and scanned
by threeradars,oneof whichhad multiparameter
capabilities,operatedby the NationalCenterfor Atmospheric
Research.Two stagesof the storm's
evolutionare analyzedhereinduringwhichthe stormgrewto produceprecipitationandlightning. The first stage,sampledduringthefirst T-28 penetrationat 5.25 km (-3øC) andthe P-3 at
6.4km(-10øC),wascharacterized
bya 2- to3-kinwideupdraft
(maximum
14m s'•) withcloud
liquidwatercontents
upto4 g m-3,lowconcentrations
ofgraupel
at-10øC,andsmalltomedium
raindrops
in concentrations
oflessthan200m-3at-3øC. A downdraft
region
alsoexisted
that
wasdevoidof cloudliquid water,but containedgraupelup to 2 mm. Radardata(Zr)•) are consistent with a coalescence-dominated
precipitationgenerationmechanismfollowedby transportof
dropsin theupdraftto heightswith temperatures
colderthan-7øC, wherefreezingformedgraupel that continuedto growby riming. Electrificationduringthis stageremainedweak. The secondstage,sampledduringthe secondandthird T-28 penetrations
andthe secondP-3 penetration, wascharacterized
at midlevelsby a narrowerupdraftanda morediffuse,broaddowndraft
separated
by a 1- to 2-km wide transitionzone. The updraftcontinuedto showsignificantcloud
liquidwater(•2 g m-3)withfewprecipitation
particles,
whilethedowndraft
hadverylittlecloud
liquidwithgraupel
in concentrations
>1 e-•. Thetransition
zoneshared
bothupdraft
anddowndraft characteristics.The increasein ice concentration
wasaccompanied
by a rapid increasein
theelectrification
ofthecloudwithpeakelectric
fieldsreaching-20
kV m-• atT-28altitude
and
thedetection
of lightningby ground-based
sensors
andpilotreport. As timeprogressed,
precipitationparticleconcentrations
reachedseveralper liter at midlevelsin bothupdraftsand downdrafts. The observations
areconsistent
with electrification
througha precipitation-based
mechanisminvolvingthe development
of the ice phase.
1. Introduction
and
large-scale
separation
togravitational
sorting
oftheparticles.
Questions
dealing
withtheelectrical
natureof thtmderclouds
Thistheory
hasbeenexplored
tbrvirtually
all typesofhydromehavebeenpuzzling
atmospheric
scientists
forcenturies,
yetmost teorcollisions
[e.g.,LathamandMason,1962;Attfdetynattr
and
majoradvances
in thefieldhavecomein onlythelastfewdec- Johnson,
1972;Brooks
andSaunders,
1994].Thenoninductive
ades.Despite
these
advances,
manyquestions
concerning
chargehypothesis
hasbeendeveloped
through
a setof laboratory
exstructure,
mechanisms
for charge
separation,
andelectrical
el- periments
datingbackto Reynolds
et al. [1957]. It hasbeen
fectsona thunderstorm's
microphysical
evolution
remainlargely shown[e.g.,Reynolds
et al., 1957;Church,1966;Takahashi,
um'esolved.
1978a;Jayaratneet al., 1983'Saunders
et al., 1991]thatcharge
It is unclear
at thistimeexactly
whichmechanisms
arepri- will betransferred
during
rebounding
collisions
between
graupel
marilyresponsible
fortheelectrification
of thunderstorms.
The andsmallerice crystals
in thepresence
of supercooled
water.
convective
hypothesis,
firstproposed
byGrenet
[1947]andlater Large-scale
chargeseparation
thenresultsfromgravitational
byVonnegut
[1953],
attributes
charge
separation
tothemotion
of separation
ofthetwodifferent-sized
hydrometeor
species.
A recharged
cloud
particles
andsmall
ions
inthecloud
updraft
andin viewofthese
theories
along
withtheirstrengths
andwealo•esses
thedowndraft
atitsedges.
Theinductive
hypothesis,
originallyinexplaining
observed
electrical
characteristics
ofthunderstorms
proposed
byElsterandGeltel[1913]forraindrop/cloud
dropletisgiven
bySaunders
[1993,1995].
Recently,severalfield investigations
involvingboth remote
collisions,attributeschargetransferto the interactionof polarand
in
situ
measurements
have
been
mounted
to characterize the
ized hydrometeorsin the presenceof an ambientelectric field
microphysical
and electricalstateof convectivecloudsin orderto
•Now at Department
of Atmospheric
Sciences,
University
of Wyoming,
Laramie.
Copyright1996 by the .tunerican
Geophysical
Union.
cal stormssuchas occuralongthe eastcoastof Florida. Studies
of subtropical
stormshaveinvolvedradaranalysisandgroundbased electric field data to infer the interior electrical structure of
Papernumber96JD01625.
0148-0227/96/96JD-01625
shedlightonthequestion
of chargeseparation.
It is noteworthy
thatrelativelyfew of theseinvestigations
havestudiedsubtropi-
$09.00
the thunderstorms
[e.g., ,lacobsonand Krider, 1976; Krehbiel,
18,961
18,962
FRENCH ET AL.' OBSERVATIONS OF EVOI.UTION OF A THUNDERSTORM
1981, 1986; Krider, 1989; Lhet•itte and Williams, 1985; 34aier
and Krider, 1986;3•lurphyet al., 1996]. The resultshavecharacterizedthe electricalstructureof thesestorms(basicallytri-polar)
without characterizingthe accompanying
microphysics
to aid in
identifyingthe electrificationprocesses.
Somestudieshave obtainedin situ electricaland microphysical datafromsubtropical
andtropicalclouds. Talcahashi
[1978b]
made balloon-borne,large-hydrometeor
chargemeasurements
in
two maritime thunderstom•s
near Penape,Micronesia. They
exhibited different degreesof electrificationcorrelatedwith
lightningproduction.One storm,sampledprior to the initiation
detennineboththe microphysical
and electricalevolutionof the
storm. In section2 we discussthe collectionandanalysisof the
datafrom the August9 storm. Section3 coversthe resultsof the
data analysis. Section4 summarizesthe observations,
and sec-
tion 5 discusses
how our restiltsshedlight on the questions
raisedearlierin comparison
with otherstudies. A companion
paperdealswith the numericalsimulationsof this storm.
2. Data Set
Two main data setswere usedin the analysisof this storm.
Data collectedby radarsconstitutethe first set. The secondset
of lightning,
hadcharges
ofbothsigns
at levelsfroin0 to-60øC comprises
in situmeasurements
madeusinga varietyof probes
with a slightexcessof positivechargeat warmertemperatures on boardinstrumented
aircraft. The majorityof the in situ data
and net negative charge at colder temperatures,reachinga was collectedby the SouthDakota Schoolof Mines and Technolmaximum aroundthe -40øC level. The secondstorm was al- ogy T-28. Data collectedby the National Oceanicand Atmosreadyproducinglightningat the time it was sampled.This storm phericAdministration's
(NOAA) P-3 werealsoanalyzed.Since
had a more distinct tri-polar structurewith the net negative ourviewof theevolutionof thisstormis highlydependent
onthe
charge
peaking
at around-20øC.Takahashi
inferred
fromthese interpretation
of thesedata,we t•cl it necessary
to discuss
the
data that the tipper positivechargewas carriedby ice crystals measurement
devicesusedandhowmeasurements
madeby each
and the main negativechargewas carriedon graupel,whichhe devicewere interpreted.
foundto be consistent
with thenoninductive
rimingmechanism.
2.1. Radar Measurements
Takahashi[1990] flexva balloon-borneparticleimagingsystem, correlatinglow lightningfrequencyxvithlow graupelconThe stormonAugust9 wascontinually
scanned
by threeNacentrationsbelow the -30øC level, and highergraupelconcen- tionalCenterfor Atmospheric
Research(NCAR) radars,CP-2,
trations,but lack of liquid water above the -30øC level. This CP-3, and CP-4. Figure 1 showsthe locationof the stormin relack of collocatedgraupeland liquid water wouldproduceweak lation to the radar installations,
Cape Canaveral,and the Melelectrificationvia a noninductive
rimingprocess.
bourneAirport. Boththe CP-3andCP-4DopplerradarsoperGoodmanet al. [1989] useda polarization-diversity
radar in atedin the C band,at wavelengths
of 5.45 cm and 5.49 cm, reAlabamato examinecloudsthat were maritimein nature. They spectively.During the August9 storm,the antennafrom CP4
foundthatlightningoccurred
onlyaftersignificant
concentrationshad elevationcontrolproblems.Becauseof this, the radardata
of ice were intbrredto have formedbasedon multiparameterra- discussed
in the followingsections
comefromeitherthe CP-3 or
dar signatures. Willis et al. [1994] obtainedobservations
l¾om the CP-2 radar.
aircraftpassesthrougha thunderstormduringCAPE. Electrified
regionsat the-7 ø to-10øC level within this storm contained
graupel,relatively little cloud water, and either downdraftor a
:..Atlantic
• • • ' I
CP-2
relative minimumwithin an updraft. The synthesisof theseob. ';:'•Ocean
.
servationssuggestthat ice formationis a precursorto significant
:--:;:..::':.:'!:::i":"."
":'
electrificationin subtropicalmaritimestorms.
In a studysimilar to the one describedbelow,Ramacha•drcm
et al. [1996] examinedtwo stormson the sameday duringCaPE
usingaircraftand multiparameterradar data. Both stormswere
in closeproximityspatiallyand temporallyand both exhibiteda
multicellularcharacter.Both stormsshoweddevelopment
of icephaseprecipitationthroughthe freezingof supercooled
raindrops
and the growthof electric
fieldsto tensof kV in'• at the6 kin
(-6øC) level subsequent
to the initiation of ice. While the aircraft-measuredelectric fields were of coinparablemagnitudein
bothstorms,onlythe moredynamicallyvigorotiscellsin the secCP-4
Melbourne
ond stormproducedlightning. In each of thesecells, the time
betweenthe first freezingof rain and first lightningwas between
10 and 12 min. They concludedthat the observations
were consistentwith a chargingmechanisminvolvingcollisionsof cloud
and precipitationparticlesin regionswith modestamountsof
cloudwater abovethe 6-kin level. Electrifichtionto the stageof
lightningwas dependentuponthe vigor of the updraftsand the
maximumdepthof the cloud.
In this anda companion
paper,xvetry to shedadditionallight
Figure 1. Locationof August9 CaPE stormin relationto radar
on the relation betweenthe microphysics
and electrificationof
sitesandMelbourneAirport(baseR)rT-28). The stipplingindisubtropicalstormsthroughthe analysisof both observational cateswater. To the far right (east) is the Atlantic Ocean. The
data and an associatedmodelingstudy. In this paper, xvedeal outer banks (Cape Canaveral)are separatedfrom mainland
specificallyxviththe analysisof in situ and remotelysensedob- Florida by the Bananaand Indian Rivers. The scanarea indiservationaldata from a thunderstonnthat occurredon August9, catesthe regionin whichthe radarswere scanningfor this par1991, duringCAPE. The datawere analyzedin detail in orderto ticularstorm.ModifiedfromFoote[ 1991].
CP-3
FRENCH ET AL.: OBSERVATIONS OF EVOLUTION OF A THUNDERSTOILM
The CP-2 was a dual-wavelengthradar with multiparameter
capabilities.It operatedsimultaneously
in boththe S bandand
the X band,corresponding
to wavelengths
of 10.68 cm and 3.20
cm, respectively.Both bandswere able to providereflectivity
andDopplerdata. In addition,the S bandchannelprovideddifferentialreflectivity,ZD}•,and the X bandprovidedlinear depo-
18,963
with respectto the Earth's surfaceand the componentthat is
perpendicularto the plane'sheadingand horizontalwith respect
to the Earth's surface. Willis et al. [1994] describethe method
used to calculate the electric field based on measurements
from
the field meterson boardthe P-3. Contraryto the sign convention used by Willis et al., in this discussiona positive vertical
electricfield (z component)rel•rs to net negativechargeabove
larizationratio (LDR).
ReflectivitydatacollectedfromCP-2 were comparedto those the aircraft,and a positivehorizontalelectricfield (y component)
collectedfrom CP-3, andthe two datasetscomparedwell in both refersto net negativechargeto the rightof the aircraft'sflight distructureand value even thoughthe CP-3 data usedare raxvre- rection. This sign conventioncorresponds
to a right-handedcoflectivity data, while the CP-2 data are griddedand smoothed. ordinatesystemin which the positivex direction(a component
Bothreflectivityfieldswerethereforeusedin tile analysisof this not resolvedin the electricfield calculations)pointsout of the
storm. In addition,we analyzedtile difl•rential rellectivityfield tail of the aircraft.
The T-28 madesevenpenetrations
at 5.25 'Ion(-3øC,ambient)
recordedby CP-2. Analysisof ZDRplayeda vital role in determiningthe microphysical
characteristics
of this storm. Differen- throughoutthe stonn'slifetime. The first penetration(1835:15)
tial reflectivityis a measureof the differencein backscattered beganat about 11 rain after detectionof first echo,and penetrapowerfroma wavebothtransmitted
andreceivedpolarizedhori- tions continueduntil the stormwas well illto its decayingstage.
zontallyas opposedto onepolarizedvertically. Mathematically, The T-28 madeits last penetration(1920:00) about55 rain after
ZDR is defined as
first echo. Hereill we discussdata collectedduringthe first three
T-28 penetrations,corresponding
to times up to about 25
after first echo (about 9 rain after first lightning). The R)urth
penetrationwas madeconsiderably
southof the main reflectivity
Zvv
core. The remainingpenetrationswere made while the storm
was in its decayingstage,after collapseof the "ZD• column,"
whereZHHrepresents
the reflectivityfactordue to horizontally
indicatingat this stagethe lack of sufficientupdraftto maintain
polarizedtransmittedand receivedwavesandZvv represents
the
supercooled
raindropsaloft.
reflectivityfactordue to verticallypolarizedtransmittedand reMeasurmnents
of cloudliquid water concentrations
(CLWC)
ceived waves.
from the T-28 were made usinga Jolmson-Williams(JW) probe.
Our interpretations
of ZDRmeasurements
were basedon the
The accuracyof this deviceis not as great as that of the King
conclusions
reachedby Herzeghand Jameson[1992]. Basedon
Probe. Tile JW respondsmainly to liquid water ill the form of
analysesof Alabama thunderstorms,
they concludedthat from
dropletsless than 30 to 50 •tm ill diameter. Recent intercomZDRmeasurements
one could clearly delineateregionsof ice
parisonsbetweenthis JW and a King Probe flown simultaneprecipitation
(ZDR'4)dB) fromregionsof significantrain precipiouslyon the T-28 showthat this JW tendsto give readings---1/2
tation(ZDP,>2dB). Further,it •vasnotedthat in convectivesysthoseof the King Probeill wann-basedcloudswith broadercloud
tems, ZDRcan providethe informationnecessaryto detectthe
dropletspectra.
presenceof supercooled
rain (liquid water dropsabovethe 0øC
The vertical wind estimates(l¾omthe T-28) were based on
isotherm).We refer the readerto Bri,gi and Hendty [1990] for
changesin aircraftpressurealtitude,pitch,andverticalacceleraflirtherdiscussions
on the topicof difl•rential reflectivityand its
tion,usinga methoddescribed
by Kopp[1985]. The noiselevel
usefulnessin radar meteorology.
for thesecalculationsis about2-3 m s'• Tile verticalwind calculationmaybe biasedby up to a few metersper second,positively
or negatively,dependingon the trim of the airplane. The base2.2. In Situ Measurements
line zerovalueshouldbe takenas tile valuecomputedin straightData collected frmn two instnnnented aircraft were used in
and-levelflightjust prior to cloudentry.
Data from two probeson the T-28 were used in analyzing
the analysisof this storm. The P-3 made two penetrationsinto
the storm,oneat 6.4 km meansealevel (MSL) (-10øC, ambient) particlesizesand types:a PMS 2D-P imagingprobeand a foil
of the phases(wateror
about 2 rain after first echo (•-1825:00 UTC) and anotherat impactor.In this analysis,detemfination
was consideredvery important. Since
4.1 km MSL (+3øC, ambient),20 rain after first echo(all alti- ice) of the hydrometeors
tudesreferredto henceforthwill be with respectto mean sea the determinationof water phaseis somewhatsubjective,we dislevel, and all times will be with respectto universaltime con- cuss how this was done.
The PMS 2D-OAP probesare imagingprobesdesignedto revention).
image(or shadow)of eachsampledparData collectedby the P-3 and used in this analysisinclude corda two-dimensional
ilparticle imagesfrom both the ParticleMeasuringSystemsInc. ticle. Each probeconsistsof a linear arrayof 32 photodiodes
(PMS) two-dimensional
imagingprecipitationprobe(2D-P) and luminatedby a laserbeam. The diodearrayis scannedrepeatcloudprobe(2D-C). Operationof theseprobesis discussed
fi•r- edly as particlespassthroughthe beam, yielding a two-dimenther, below. Also usedwere vertical wind calculationsbasedon sional shadowimage. The opticsand opticalpathsdiffer beinertial navigation data, true air speed, and angle of attack tween the 2D-P and 2D-C models, giving different effective
(describedby Willis et al. [1994]). Liquid water concentrations resolutions.For the 2D-P probeson both the T-28 and P-3, the
were derivedfrom measurements
made by the King Liquid Waeffectivediodespacingis ---200[tm, while for the 2D-C probeon
the P-3 tile spacingis 50 [tm.
ter Probe[King et al., 1985].
Last, we analyzedthe two components
of the ambientelectric
Basedon the shapeof tile particle image, one can oftell defield derivedfrom measurements
madeby four electricfield me- terminethe phaseof the hydrometeor.Water dropswith diameters on the aircratl. Tile two resolvablecomponents
of the elec- ters greaterthan-600 [tm tend to cant(elongationalongan axis
tric field measuredby the P-3 are the componentthat is vertical of---45ø) in the flow field aroundtile probe[Blackand Hallet,
ZDR
=10log
ZHH
(1)
18,964
FRENCH ET AL.: OBSERVATIONS OF EVOLUTION OF A THUNDERSTORM
1986]. Graupelparticles,on tile other hand, do not cant, and
thereforetend to result in less regularly orientedimages. Becauseof resolutionlimitationsimposedby the 2D-P probe,it is
difficult to determinephasefor hydrometeors.For this reason
we analyzeddatacollectedby the foil impactorto help discriminate the phase of hydrometeorsin regionspenetratedby the
T-28.
The foil impactorconsistsof a thin movingstripof aluminuln
(-7.5 cm wide and---0.003 cm thick) that passesover a ridged
drum behindan openwindow (with dimensionsof 3.75 by 3.75
cm). As particlesstrike the foil, they leave ridgedimpressions.
The ridgesare spacedat 0.25 iron, which definesthe resolution
of the instrument;the minimulndetectablesize is approximately
0.5 mm. The foil is exposedandtransported
onlywhenthepilot-
probesalongwith tintinginformationencodedinto tile image
data. In regionswith highparticleconcentrations,
theseprobes
are oftenlimited by memoryand data transferrate constraints
to
obtainingimagesfor only a portionof the total time in-cloud.
However,the disadvantages
of this intermittentsamplingare
mitigatedby the abilityto ignorefor countingpurposes
thoseim-
agesthat are obviousartifacts,andto compute
an appropriate
size-dependent
samplevoltimefor largerparticles
onlypartially
imaged.
DatafromtheP-3, withboth2D-C and2D-Pprobes,suggest
thatthe latterseriously
underconnts
particles
sosmalltheytrip
the probebut are not imaged,i.e. smallerthan-43.4min. For
largersizeswe believeour computed
concentrations
are representativeof actualparticleconcentrations.One additionalconactivated"in-cloud" switch is on. At other times, the tbil is not
siderationin the concentrations
computedfroin the T-28 2D-P
moving,and a metal shuttercoversthe foil at the windowposi- probein this studyis that the imagesin the first halvesof each
tion. Each time the in-cloud switch is turned off, the shutter
4 kilobytebufferwereoftencorrupted.Thisrequiredconcentracloses,anda punchmark is madein the foil. The foil movesat a tionsto be estimated
basedonjust half (the "good"half) of the
constant
speed
(-3.4 cms'l) duringa penetration,
punchmarks totalnumberof recordedimages.
denotethe boundariesbetweenpenetrations,and the times assoDetermination of the electrical evolution of this storm relied
ciatedwith thesepunchmarksare recordedon the data system.
heavilyon measurements
madeby four electricfield lnetersoil
Thusonecandeterminethe locationill the cloudthat eachpartitheT-28. Thesefourfieldmetersareableto resolvetwocompocle struckthe foil with an nncertaintyof about 100 ill (the nominents of the ambient electric field, vertical and trailsverseto the
nal accuracyof the globalpositioningsystemreceiveroil the airdirection
of motion(thesameasthoseresolved
by theP-3). The
craft).
field metersare modelE-100, rotating-shutter
type instruments
The method used to determinethe phasesof hydrometeors
described
by Wim•[ 1993].
that left impressions
oil the foil is basedoil the shapesof the iln(Ey)
prints. Water dropsare assumedto leave circularimpressions, The two field metersusedto determinethe horizontal
while ice or ice-liquid combinationparticlesare asstunedto componentof the electric field are locatedoil the end of each
leave "splatter"marks. This is basedoil a reportpreparedby wingtip. Thetwoothermeters(locatedonthebottomandtopof
the vertical(E•) component
Miller et al. [1967]. Laboratory experimentsdiscussedby the fuselage)are usedto determine
of
the
electric
field.
Details
are
presented
by Ramachandran
et
Knight et al. [1977] producedsimilar restilts,but also demonstratedthat water-dropand ice-sphereimpressions
were indistin- al. [1996]. A brief summaryis givenhere.
guishablefor particleslessthan-3 mm in diameter. As the lbil
impressionswere analyzedonly for their character,and not
detailed concentration
and size statistics,overlappingimpressionsin regionsof high precipitationparticle concentration
did
not hinderthe analysis.
We carefi•llyconsideredboth setsof particledata (2D-P and
foil impactor)beforeconcludingwhat typesof particlesexisted
in a given region in cloud. Using thesetwo setsof data, we
coulddiscriminatebetweenwater and ice regionsin cloud for
particleswith diametersgreaterthana few nm•with a reasonable
degreeof certainty.
In this work the primaryemphasisis on precipitationparticle
The four electricfield metersin the four differentpositions
responddifferentlyto a unit-magnitude
field component
normal
to their faces, due to differences in curvature and local surface
featuresat eachmounting
location.The readings
of the individual metersrespondingto the samefield magnitudeare scaled
relativeto eachotherusingself-charging
testsin negligible
ambient fields and roll maneuvers in constant ambient fields with
negligibleself-charging.
Eachof the ambientfield components
is thencomputed
by calculating
the scaleddifferences
fromeach
pair of oppositely
facingfield metersandcorrecting
for the roll
andpitch(usingsimpletrigonometry).Overallenhancement
of
theambientelectricfieldaround
theaircraftis accounted
forby
measurements.
Particle concentrations are estimated in two
multiplyingthe measurements
by an enhancement
factordeterwith the New Mexico
ways. A rough estimateof precipitationparticle concentration minedthroughin-flightintercomparisons
of MiningandTectmology's
(NMIMT) SpecialPurpose
canbe obtainedby dividingthe PMS probeshadow/orcount(the Institute
Research(SPTVAR) as docutotal number of particlescountedper second)by tile optical Test Vehicle for Atmospheric
sampling
volume
sweptoutby theprobe(-0.15 m3 s'•, forex- mentedby Giori andNanevicz[1992].The electricfield meters
in ambient
fieldsof approximately
+200kV m'•
ample,for the T-28 2D-P probe).This concentration
includesall saturate
sizesof particles,fromthosejust big enoughto activatetheprobe
Whentheaircraftis highlycharged,
asit oftenis duringthunpenetrations
dueto precipitation
particleimpacts,variwhile not leavingan image(-0.2 mm, for the 2D-P) to thoseso derstorm
largethat they couldnot be entirelycontainedwithin the sample ouslocations
ontheairframeandpropellerwill discharge
plumes
volume. It alsoincludesartilhctsdue to water streaming
off of ions. Theseplumesdistorttile electricfield sensedby the
probetips, foggedoptics,and otherphenomena.The precipitation particle concentration
can be estimatedsimilarly froln the
foil impactorby dividingthe numberof valid impressions
within
an areaof foil, by that foil areamultipliedby the distancemoved
duringthe time of expostire(roughly90 ill per secondof expo-
meterson the fuselageandwingtipsand may introduceartifacts
in estimatesof mnbientfield components
[e.g., doneset al.,
1993]. Experiencewith the T-28 has shown that when the
magnitude
of the ambientfield is greaterthanthe fieldowingto
chargeon the aircraft(computed
froin scaledsumsof readings
sure).
from the wingtip pair of meters,usingthe sameoverallenA more rigorousestimateof precipitationparticleconcentra- hancement
factor)signsof distortions
like thosedescribed
by
tion can be obtainedusingthe imagedata recordedby the PMS
doneset al. [1993] are infrequent.
FRENCH ET AL.' OBSERVATIONS OF EVOLUTION OF A TI-IUNDERSTO•
18,965
We feel that T-28 measurements are accurate within about a
this stormis somexvhat
atypicalof otherCaPE tlnmderstonns
defactorof 2, basedon clear-airformationflight intercomparisons scribedin the literature. Althoughmostof the incastiredparameetc.) were comparable
madeduringCaPEwith theNCAR SailplaneandKingAir, and ters (i.e., CLWC, particleconcentrations,
throughcontinuity
of spatialstructure
in electricfield estimates to those measured in other storms, the overall structure seemed
madeduringsuccessive
penetrations
madeby theT-28,P-3, and different. In particular,mostof the other stormsin CaPE develNCARKingAir in anotherstonnonthissameday(V.N. Bringi opeda distinctmulticellularstructurewith the individualcells
et al., Evolutionof a Floridathunderstonn
duringthe Convecton relatively easyto distinguishon radar plots. This stormdid not
andPrecipitation/Electrification
Experiment:
Tile caseof 9 August 1991, submittedto Monthly kVeather
Review,1995), and by
share that characteristic.
Figure 3 presentsa seriesof ConstantAltitude Plan Position
Indicators(CAPPIs) constructedfroin CP-3 data at four times
aircraftchargingappearto be minimalin the electricfield data duringthe observingperiod(1830, 1835, 1842, and 1849), from
discussed
below, exceptpossiblyduringtile final P-3 penetra- 5 min after first echountil tile end of the third T-28 penetration.
tion. Duringthis penetration,
aircraftchargingxvashigh;how- The different heightsapproximatetile +3, -3, -10, and -20øC
ever,the variationacrossthe cloudof the ambientelectricfield levelsin the cloud,respectively.During tile first threetime periods,the cloudwas essentiallystationaryand wc believe characcomponents
still looksreasonable.
terizedby a singleupdraft. This conjectureis supportedby the
presenceof a singleZr• columnduringthis period(as discussed
below). Tile presenceof two distinctmaximain tile reflectivity
3. Analysisof ObservationalData
at 1842 in the 6 km and 5 'kmCAPPIsis likely to be precipitation
3.1. Environmental
Conditions
cascadingoff the top of the nearlyverticaltipdraft. There is an
indicationof the developmentof a secondupdraftregion in tile
The stormon August9 waslocatedoverthe westernedgeof
northeastquadrantby 1849, whichis supported
by tile spreading
CapeCanaveral,abovethe field mill networkoperated
by the
of the stormin that directionin the 1849 columnof Figtire 3.
KennedySpaceCenter. The stormformedalonga convergence
While there is evidencefor the developmentof a secondcell
boundary
[Bringiet at., 1993]. Figure2 showsa sounding
from
a radiosondethat was launchedat 1810:00, 15 min before detec- within this cloud, it seemsto occurafter tile original cell had
evolvedelectricallyto tile pointof producinglightningat or betion of first echo. The sonde was launched froill a mobile CrossChainLORAN AtmosphericSoundingSystem(CLASS) facility fore 1842. So, we believethat xvhatis describedin tile lbllowing
approximately
3 'kmsouthof wheretile stormtBrmed.It showsa discussionis the microphysicaland electricalevolutionof a sinconditionallyunstableatmosphere
(lifted index -3.5, K index gle thunderstormcell.
3.2.1. P-3 penetration 1. At about 1827:00, tile P-3 pene36.8) thatis relativelymoistthroughout
its depth. Tile shearis
16.1m s'• through
thelowest6 l•n, whiletileconvective
avail- trated this cloud at 6.4 'km (-10øC, ambient) flying from the
the T-28 and P-3 in tile stonn described belo•v. Tile effects of
northwest
to the southeast.
Tile cloud at this time and altitude
able potentialenergy(CAPE) for this soundingis 1872 J/kg.
The CAPE value is characteristicof illany subtropicalthunder- was only 2-3 Ion in diameter,with a peak reflectivityof 15 dBz
vertical winds at
stormenvironments
both over land and over ocean[Lucaset at., near the penetrationlevel. Aircraft-measured
thislevelwereall positive
0naximum
•-14m s'•). TheKing
1994].The bulk Richardson
numberis 15.
Probeindicated
a peakCLWCvalueof4 g in'3.Table1 indicates
that the 2D-C and 2D-P probesencountered
a few small precipitation-sizeparticles,nonelargerthan 1 min. Electricfields(both
When comparingdata from this stormwith data froin other horizontaland vertical) encounteredby tile P-3 were less than
stormsobservedduringCAPE,we cameto the conclusionthat 2 kV m'• andmayhavebeeninfluenced
bya mature
stonnlo-
3.2.
Storm Evolution
cated a few kilometers
to tile southeast.
These measured electric
fields were smaller than the uncertaintyof tile measurements
[Willis et al., 1994].
3.2.2. T-28 penetration 1. The maximtimreflectivityin the
cloudreached20 dBz by 1830:00,and 45 dBz by 1835:00.The
T-28 beganmakingits first penetration8 rain after the P-3, enteringcloudat 1835:15. Flying from the southeast
to the northwest, the aircraft penetratedat an altitude of 5.25 'km (-3øC,
ambient). Figure 4 showsa CAPPI of reflectivityat 5.25 km.
ThesedatawerecollectedfroinCP-2 nearandduringthetime of
the first penetration;the CP-2 site is tile originfor the coordinate
systemused. In the southeastern
half of the cloud,the T-28 flew
throughan area of relativelyhigh reflectivity(in excessof 40
dBz). In the northwesternhalf, the refiectivitieswere much
lower, between 20 and 40 dBz.
The lower panelin Figure5a showsa verticalcrosssectionof
CP-2 reflectivityalongthe flight path of the T-28. The ordinate
is altitudeaboveground,andthe abscissa
is plottedin time units
but can be thoughtof as a spatialvariable. Sincethe T-28 was
flyingat approximately
90 m s'l (although
the trueair speed
varies
between
80
and
105
m
s'•),
each
10
s
represents
4).9 kin
Figure 2. Plot of a sounding(temperatureand dew point)
launchedby mobileCLASS at 1810:00on August9. The soilde of data. Tile two graphsabovethe reflectivitycrosssectionshow
the 2D-P shadow/orcount,whichrepresents
the numberof times
wasreleasedapproximately
3 kin southof the stonn'slocation.
18,966
FRENCH ET AL.' OBSERVATIONS OF EVOLUTION OF A TH-UNDERSTORM
E
--•
E
E
E
---
c5oi•
"•0
0
•,--,
r.,0 0
,4.., 0
"
.,..,
FRENCH ET AL.' OBSERVATIONS OF EVOLUTION OF A TIIUNDERS'FORM
18,967
Table 1. MicrophysicalCharacteristics
of tile Different CloudRegionsDiscussedin the Text
Begin
P-3
1826:57
T-28
End
Pen 1
Alt, km
T, øC
6.4
-10
5.2
-3
1827:06
Pen 1
Max CWC
VW
Total
4
up
220
>400 mm > lmm
>2mm
>3 mm
Max D, mm
0.95
35
1835:15
1835:21
dn
2400
370
16
1835:27
1835:27
dn
2700
690
100
16
Phase
ice
1.6
ice
2.4
ice
1835:28
1835:40
dn
2600
790
260
80
37
7.5
ice
1835:40
1835:44
dn
2000
430
21
9
6
7.4
mixed
2
up
2900
160
32
6
3
3.4
liquid
2
up
1200
300
64
10
10
5.6
liquid
1835:50
T-28
1836:24
Pen 2
5.2
-3
1840:48
1840:51
1840:51
1840:59
2
up
1300
200
29
12
1841:00
1841:09
1.5
tr
2900
420
74
19
1841:10
1841:20
tr
3800
680
72
10
1841:20
1841:48
dn
4800
1700
690
110
dn
850
530
190
35
5
7.2
ice
5
9.5
mixed
0.8
ice
2.4
Ice
P-3
Pen 2
1845:10
1845:40
1845:40
1846:16
T-28
Pen 3
4.1
liquid
6.6
mixed
6
6.8
mixed
41
7.2
ice
9
3
3
5.2
2.6
up
1100
460
110
15
-3
1847:51
1847:54
dn
4800
280
1847:55
1848:00
dn
4000
340
34
1848:00
1848:18
dn
3800
1000
320
120
41
6.6
ice
1848:18
1848:30
tr
5900
1800
620
250
160
6.3
mixed
up
2100
730
350
200
130
7.6
m•xed
5.6
m•xed
1848:30
1848:40
0.8
1848:43
1849:00
0.3
dn/up
3400
1000
370
120
36
"Begin"and"end"columnsdelimitthetime intervalwhentile aircraftpassed
throughtheregion,Air istheaveragegeopotential
altitude,T istheaveragetemperature,
Max CWC isthemaximumcloudwaterconcentration
encountered
duringtheperiod,VW characterizes
theregionasupdraft(up),
downdraft(dn), or a transition(tr) betweenthetwo.Total is thetotalparticleconcentration
of particleslargerthan200 l•tm(estimated
from2D-P probe
observations
exceptfor tile firstP-3 penetration.
wheretheestimateis basedona 2D-C probe)whilethesucceeding
columnsdenotethenumberlargerthan
or equalto the indicatedsize.In thefinal column.Phasenotesthepredominant
phaseof thehydrometeors
encountered
duringthe period.
per secondthe 2D-P probewas 'lripped"(top), and the vertical
electric field component(bottom) as computedl¾omT-28 field
meters. Figtire 5b showsa corresponding
crosssectionof differential reflectivity along the flight path of the T-28. The two
graphsabovethis panel show the CLWC measuredby the JW
probe(top) and the verticalwind (bottom).
Referringto Figures5a and 5b, it is apparentthat as the T-28
flew ttu'oughthe southeastern
half of the cloud (1835:15 to
1835:50)it encountered
a doxvndraft
with verticalvelocitiesbe-
drops. In the updraftportion,where the reflectivitywas rather
weak, Zi:)• valuesas greatas 3.5 dB were measuredat the level
of the T-28. Tile 'lligh Z•)• column"(1.5dB contour)was lbund
to extendup to a heightof 7 km (-15øC, ambient). This is indicative of an area in which relatively low concentrations
of
largerliquid water dropsare present. Also, CLWCs in the up-
draftwererelatively
high(maxinmm
of -1.7 g m-3asmeasured
by the JW probe),while tile doxvndrafi
was virtually devoidof
anycloudliquid water.
tween-5 and-10 m s-•. Asmentioned
earlier,thereflectivity A sampleof particle imagesdetectedby the 2D-P probedurvalueswere highestin this portionof the cloud. Also, as one ing the downdraftin tile first penetration
is shoxvn
in Figtire6a.
would expect,the 2D-P shadow/orcounts,proportionalto pre- Particle concentration
statisticsare given in Table 1. This
cipitationparticle concentration,
were relativelyhigh in this por- downdraftregionis far frombeinghomogeneous
in particlesize,
tion. In the northwesternhalf of the cloud (1835:50 to 1836:30) habit(shape),andconcentration.
In the southeastenunost
portion
the T-28 encountered
moderateupdrallswith verticalwinds be- of the downdraft(tlu'ough1835:21),tile 2D-P probedetecteda
tween 5 and 15 m s'•. The reflectivitiesand shadow/orcounts relatively
lowconcentration
(-10 m'3)of graupel
particles,
all in
here were generally lower, except for one local peak in the 1- to 2-ram sizerange. For the next 7 s (-630 m) tile T-28
shadow/orcountsnear 1836:20, on the northwestedge of the flew througha regionof predominantlysmallerparticles(less
strongestupdraft, where the highestconcentrations
of the pene- than 1 mm in diameter)with a muchgreaterconcentration.From
tration were encountered. These concentrationsare relatively 1835:28until 1835:40,the 2D-P probedetecteda considerably
low(---300
m'3)compared
tothose
thateventually
develop
in the broadersize distributionwith hydrometeors
up to 7 mm in distorm.
ameter. Both tile 2D-P and the foil impactordata indicatedthat
up to this pointwere in tile
In the downdraftportion(1835:15to 1835:50),Zr• values(at all of the particlesin the doxvndraft
the level of the T-28) were low, lessthan 1 dB. This is indica- ice phase;this is consistentxviththe concurrentlow valuesof
tive of an area made up mostly of ice particlesor small water Zi•}•.
i8,968
CP-2
FRENCH ET AL.: OBSERVATIONSOF EVOLUTION OF A THUNDERSTOI•,'M
and T28
Altitude=
5 25
183500-183700
UTC
9 August
1991
km
55
As the T-28 flew tlu-oughthe northwesternmost
kilometerof
the downdraft(1835:40 to 1835:50) ZD}:increasedfrom 0 to
about1.5 dB. In this region,the 2D-P probedetectedan average
concentration
of 9 m'• of particles
with diameters
2 nun or
-18
greater. Both the 2D-P and foil impactordata indicatedthat
someof the particleswere in the ice phase(graupel)and some
-
were in the water phase(raindrops).
The T-28 enteredthe updraft at 1835:50. The entire 4-'•n
-20
:::::::.::::::::
-22
ß
width of the updraft(at 5.25 l•n) was characterized
by high Zo::
(•eater than 1.5 dB), relativelyhighCLWC, andlow concentrationsof precipitation-size
particles.Duringthe first 2 s that the
aircraftwas in the tipdraft(---180m), the 2D-P probedetecteda
... :::..:. '.:..
localconcentration
of morethan40 particles
m-3xvithdiameters
-24
greaterthan 1 1ran(Figure6b). The 2D-P imagesand foil imprintsindicatethat all of thesehydrometeors
were water drops.
Ttu-oughout
the remainderof the updraft,neitherthe 2D-P probe
nor the foil impactordetectedanyparticlesgreaterthan 1.5 mm
in diameter. During this 2.9 km of tile updraftpenetration,tile
.....
...:.:::..
-26
-28
2D-Pprobe
andtbiliinpactor
each
xvould
havesampled
4.6m3of
-30
15
3
5
7
9
11
13
15
dBZ
X DISTANCEFROM CP-2 (km)
cloud. The reflectivityin this regionmusthavebeendominated
by large dropsin concentrations
too low to be detectedby the
T-28 probes[cf. Illi,gm,orth,1988]. Toxvardthe middle of the
updraft,a large numberof very sillall (d < 0.4 mln) particles
were detected. The phase(water or ice) of thesedrizzle-drop
sizehydrometeors
couldnot be determinedbecauseof the reso-
Figure 4. A CAPPI displayat 5.25 kin constructed
froIllgriddcd lution limit of the instn,nents; however, in an tipdraft at the
and smoothedCP-2 data between 1835:00 and 1837:00 during
of-3øC, one Wotlldexpectthem to be
thefirstT-28 penetration.
The solidline indicatesthe flightpath penetrationtemperature
of the T-28 (SE to NW). The origin Ibr this figureis the CP-2 liquid. The 2D-P probe also imaged a large number of
(long, thin imagescausedby water flowingoff the
radarsite. Reflectivitiesare plottedfrom 15 to 55 dBz in incre- 'Sstreakers"
probetips), which are indicativeof relativelyhigher CLWCs.
mentsof 10 dB as indicatedby the scaleoil the right.
B
CP-2
and
T28
183500-183700
UTC
CP-2
9 August 1991
and T28
183500-183700
UTC
9 August
1991
3 JWCLWC
(g/ms)
•saa
IaDe-Sh/or
(#/•)
55
10•0
4.5
2
20
3½
VERTICALELECTRICFlED (kV/m)
.......
::::::::::::::
•
-30
z
-10
• •2
3 GHz Zn
I
1.5
3 GHz
Zrm
8
o
25
o
<
4
4
ß
.::..':;:::'
:::::::'.::::.i::":"
:.'::::..
:::::::::::::::::::::::::::::::::::::::::::::::
.........
'....
-1.5
15
dB
dBZ
o
183500
FLIGHTTIME (UTC)
1837013
SE --) NW
83500
FLIGHTTIME (UTC)
• 83700
SE --) NW
Figure5. (a) Thelowerpallelshows
a rcflcctivity
cross
section
alongtileflighttrackof theT-28duringits
firstpenetration.
'Ille upperpallelsindicate
theverticalelectric
fieldderived
frointhethselage
millsandthe
shadow/or
countderivedfrointhe2D-P probe. (b) The lowerpanelshoxvs
a ZD• crosssectionalongtheT-28
flightpath. Theupperpanelsindicate
theverticalwindcalculated
from%28 measurements
andthecloud
liquidwaterconcentration
(CLWC)derivedt¾om
theJW probe.Eachtic on theabscissa
indicates
10 s of
flighttime,or about0.9 •n of aircrafttravel.
FRENCH ET AL.' OBSERVATIONS
OF EVOLUTION
18,969
20-
A
Buffer 316
OF A TItUNDERSTORM
2D-P
Stad Time = 183521.65
Duration
= 0.88 sees
iiillllllllUIIIIIIIIl•llltllilN
l.F-7•- 1I-I •--•--•UI••im••t•[•1••[[•
iig•i•i•i•1•i•H•lU•i•i'•i•i•Fri•U•i•i•i•'•H•Hi•ri•i•i•F•ii[i•L•[•i•L•$•
Buffer 322
2D-P
Start Time = 163530.70
Duration = 1 26 secs
Vet'
L teal
Hop
tzon
Lal
•[•k•L•i•"•illilL•L•-i•L•t•[•i•i•r•LB•L•r•!•-•IFIlIFILl•i•i•IH•i
Buffer 324
2D-P
Slart Time -- 183533.35
Duration = 0.68 secs
•U•i•i•il•t•i•iiH•H•F•i•FH•i•i•E•'•L•Ell'•P•U•i•P•L•
Buffer 326
2D-P
Slart Time = 183535.35
•L•"L••i•i•iH•'H•CL•-•[•i•
Buffer 328
2D-P
Slart Time = 183537.35
blh
Duration = 0.21 sees
Duration
•11111tlllli
IIIlLI_LIIIHlt--L.I-kt--IFI
= 0.66 secs
•i••L•r•'b•i•-f.•H•FF•LB•b.[•1•H•iL•P•n•rf•F•H•r•iL•F
Buffer 330
2D-P
Start Time = 183539 35 Duration = 0.36 secs
o
B
Buffer333 2D-P Slart Time = 183544.00 Duration= 7.77 secs
-IIIBl'•ItiEIIUtI--HIIilIfi•It-1111!-11•11111•
•i•i•i••i••i•[I•|•ig
Buffer334 2D-P StartTime= 183551.95 Duralion= 2.80 sec.s
-2e
II
I
183•
[
[
[
]
836
[
l
1837
T I H E
.ILiHI
IIIIt[ll
LliliiSillblliiib,Llil,lililJEILilt_tllt
ILl •11•1
[i1__lI-'-IFil-I-[IIIIit•ilIIIIBII I
Buffer336 2D-P Start Time = 183557.40 Duration= 1.05 secs
-Ll'-tl_llL_-L
- Ill-L_=-I11-t1'-'-'-1--I_l_ll-II[
I1__
LLLI_
I--I--I-tM- Figure7. Ambientelectricfield(kV nf]) derivedfromT-28
Buffer338 2D-P Slarl Time = 183559.85 Duration= 1.84 secs
duringthe firstpenetration.The solidline repre-- IIIII-I-1111111-M----I--I•l
___i._l--!i-kLl-!--llllilH_•-•
LIIIbl-I-- measurements
Buffer340 2D-P S!art Time = 1836O5.75 Duration: 2.77 •ecs
sentsthe vertical(z) component,
andthe dashedline represents
_ill
t-IIII•IIiIt--ilHt
iilLI--iLI__ILiI'I•IILII-llI--k_lLI-ilLliki-tt-IilLtLI thehorizontal(y) component.Seetext lbr signconvention.
Buffer342 2D-P Start Time = 183609.70 Burnlion= 3.21 secs
----I-.-tHt-l--fil-il-[tlJI
•t--ilI-II•11-}_I71tLI-IIILIIIIIII[IILIi
li--P--ilillilil•
Buffer344 2D-P Start Time = 183614.45 Duration= 2.82 secs
-II
il -- -i•lil_LllllLJllllHl-::•il-lillllililllIFFi[11l__l[[l-i_l-I--iliillli
Buffer346 2D-P Staff Time = 183618.45 Duration= 0.77 sees
this time the stormwas approximately
9 km longand 5.5
•i•i•i•i•ii•i•i•i•i•i•i[•iiil.---••i•i•i•i•iii•i•i•
wide, still arrangedalonga northwestto southeast
axis. The
BUffer348 2D-P Slart Time = 183620.55 Duration= 1.05 secs
high-reflectivity
core
(greater
than
50
dBz)
•vas
located
slightly
-iillllllllllil
__11111il•111iltlllllUlll
IUlI_•!I
-----------FIl--It-IBuffer350 2D-P Slart Time = 183623.15 Duration= 126 secs
--
northwest
of the stormecho'scenter. Throughout
muchof the
Hllll•111111•111111-!!llll!lt
-Iilllii•1111111•lffililliilliElliiilllilillillillilil_l--t--H-I_
penetrationthe T-28 was in an area with refiectivities between
Figure 6. A sampleof particlei]nagesrecordedby tile 2D-P
probefrom the (a) downdraftand (b) updraftduringthe first
penetration. Vertical bars representthe 6.4-ram vddth of the
probeaperture.Eachline corresponds
to one buffer(4 kbytesof
data). Tile buflL-rheadingsshow tile bul'l•zrnumber, tile start
30 and 50 dBz, with greaterreflectivitiesencountered
earlier in
the penetration.
Figure8 showsa 5.25-kmCAPPIwith the T-28 flighttrack
superimposed,
andFigtire9a showsa crosssection
of reflectivity
time of each bufi•2r(lbnnat is 183533.35 = 1800 hours, 35 ]nin,
33.35 s, for example)and the duration,or total elapsedtime
duringxvhichimageswere recorded,in decimalseconds.It takes
a finite time tbr the datato be dumpedto the datarecordingsystem; duringthis time, no datafire recordedby the probe. ]'herefore this recordrepresentsa noncontinuous
set oœparticleimages. Only selectedbufibrsare shoxvn.
CP-2 and T28
Altitude=
-18
5.25
184000-184200 UTC 9 August 1991
km
-
Tile existenceof thesestreakerimagesin tile PMS data supports
themeasurements
oftiptonearly
2 g m-3o1'
CLWCmade
bythe '•JWprobe.TheT-28 exitedthecloudat 1836:30.
• -22
The electric fields (both the vertical and horizontalcompo- &
nents)measuredby the %28 during the first penetrationwere •
weak(Figure
7). Bothcomponents
werenegative
throughout
the p,-24
entirepenetration,
andthemagnitudes
wereonlya fewkV m'•
•
near
what
isbelieved
tobethetillcertainty
ofthemeasurements.
• -26
Also, there was no correlation betxveenelectric field measure-
a
25
ments
andupdraft/dovmdraft
structure
orparticle
clusters.
These b
measurenmnts
and earlier ones made by the P-3 indicate that
electrification
up to thispointwasrelativelyweak,at leastin the
vicinityof the penetrations.
3.2.3. T-28 penetration2. The secondpenetration
madeby
the T-28 beganat 1840:30, 15 rnin after the initial radar echo.
The aircraftenteredthe cloudon the northwestside,flew directly
-28
-302
15
4
6
8
113
12
14
dBZ
X DISTANCEFROMCP-2 (km)
through
thehighZDRcolumn
andthehigh-reflectivity
core,and Figure8. As in Figtire4, exceptfor tilesecond
penetration,
exitedthroughthe southeasten•
edgeof the cloudat 1842:00. At
1840:00-1842:00.
18,970
CP-2
FRENCH ET AL.: OBSERVATIONS OF EVOLUTION' OF A THUNDERSTORM
and T28
184000-184200
UTC
9 August 1991
CP-2 and T28
•sea 2DP-Sh/or
(#/s)
50•
184000-184200
UTC 9 August 1991
3I JW
CLWC
4.5
••
30
v•x•ca•
•L•CTmC •'Im)
(•V/m)
.......
:i:i:i:i:i:i:
o
-10
.:.:.:.:.:.:.
.............
-30
J
•
.......
.......
3 GHz Z•
1.5
3 GHz ZDR
8
.o...........
===========================
:.:.y.:.:.:.::..'..'.::::::iiii•i•iiiiiiiiiiii!iiiii:i:•:i:
...;.:-:.:.:.:.:.:4
-1.5
dBZ
84000
1842•0
FLIGHTTIME (UTC)
NW-)
dB
184000
FLIGHT TIME (UTC)
NW-)
SE
184200
SE
Figure 9. As in Figtire5, exceptIbr the secondpenetration.
alongthe flight pathof the T-28, alongxvithtile verticalelectric
field and the 2D-P shadow/orcount. Figtire 9b shoxvsthe ZDR
crosssectionalongthe flight pathalongwith the JW CLWC and
the verticalwind. In the northxvestenunost
portionof the cloud
(1840:48 to 1841:00) the T-28 encountereda narrow,moderate
updraft
withvertical
speeds
between
10and16m s-•. Afterexit-
Imagesrecordedby the 2D-P probewhile the T-28 xvasin the
updraft(1840:48 to 1841:00) are shownin Figure 10a. Concentration and size statisticsare indicatedin Table 1. The updraft
was characterizedby a numberof streakerimages. In the very
begilmingof the updraft(northwcstcnunost
edge)a clusteringof
11 particlesrangingfrom0.5 to 1 mrn in diameterwasdetected
ingtheupdraft,theaircraftflew througha 2-kin-wide"transition in a regiononly 100m across;thiscruxesponds
to a localconcen-
zone,"wherevertical
velocities
variedbetween
-2 and+4 m s'•.
tration
ofnearly70in'3oftheselarger
particles.
Thelbili•npac-
h• this region, reflectivity values along the flight track were tor also detectedthis clustering,and the characterof the impresgreatest(in excessof 50 dBz). In the downdraftregion(1841:20 sionsindicatedthat theseparticleswere all liquid drops. About
to 1841:48),
vertical
velocities
werebetxveen
-1 and-7 m s'• 2 s (---180m) later, the 2D-P probedetecteda very large liquid
and reflectivities were ---40 dBz. Shadow/or counts were much
waterdropwith a diameterof 5.5 min. This is a significantfind,
higherin the downdraftthantheywerein the updraftandtransi- in that the T-28 was locatedin a regionof very high Zm (-3.5
tion zones.
dB) with reflectivityof 43 dBz. This indicatesthat a relatively
Similarto the firstT-28 penetration,the updrafthadrelatively low concentrationof large liquid-waterdrops can return high
highCLWC(JWreadings
between
1.5and2 g m4) andwas valuesof Zm togetherwith moderatereflectivity. The foil imcollocatedwith a highZm column. Throughoutthe tipdraft,Zm pactor,despitesamplinga similar voltime,did not detectany
rangedfrom 1.5 to 3.5 dB. The 1.5-dB contourextendedtip to dropsof this sizein the tipdraft. Sevenotherdropsof diameter1
about6.5 km (-12øC) in this region,again indicatingthe pres- to 2 mm were detectedby both the 2D-P probeand the foil imence of supercooled
raindrops. In the transitionzone, xvhich pactorthroughoutthe remainderof the tipdraft. The total volume
sharedcharacteristics
of boththe tipdraftand the douq•drafi,the sweptout by the 2D-P probeand foil impactor,each,in the tip-
CLWCvariedbetween
0.5 and1.5g ma andZD•ranged
l¾om draftwasapproximately
2.4 m3 (although
2D-Pimages
were
0.5 up to 2 dB. The presenceof substantialliquid water,the absenceof significantupdraft,the moderatelyhigh reflectivity,the
increasingprecipitationparticle concentration,
and the particle
imagesdiscussed
belowindicatethat this regionreflectsthe decayof a portionof the originaltipdraft,probablydue to precipitationloading. Note that the tipdrafthasnarrmvedconsiderably
in Figure 9b comparedto Figtire 5b, althoughthe locationremainsthe samexvithrespectto the cloudstn•cture.In the downdraft,whichagainxvasvirtuallydevoidof anycloudliquidwater,
measured values for Zm were between-0.5
and 0.5 dB. This
collectedfor onlya fractionof thistime).
Data from the 2D-P probe and foil impactorindicatedthe
presenceof both liquid water and ice particlesin the transition
zone. Figtire 10b shows2D-P imagescollectedxvhiletile T-28
was in the transitionportion of the storm. The presenceof
streakerimages(fewer than in the updraft)indicatesthe continued presenceof cloudliquid water; This agreeswith measurementsmade by the JW probe. h• the northwesternhalf of the
transitionzone(1841:00to 1841:10;---1kin) the 2D-P probedetectedat least four particleswith diametersgreaterthan 2.5 mm
volume
of---1.4
m3. Thetxvolargest
ones,
-7 nundiindicatesthat this portionof the stormwas madeup of graupel in a sample
particles,which xvill be sho•vnto be consistentwith measure- ameter,appearedto be ice. Data from the foil impactoras well
as the 2D-P imagesindicatedthat someof theseparticleswere
mentsmadeby the T-28 2D-P probe.
FRENCH ET AL.' OBSERVATIONS OF EVOLUTION OF A THUNDERSTORM
18,971
A
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Figure 10. As in Figure.6, exceptduringthe (a) updral•.,(b) transitionzone,and(c) doxvndrafi
in the second
penetration.In this sample,all bul'Ik2rs
are shoxxqi.
probablywater drops,while somewere probablygraupcl. During the last 10 s (southeasten•hall) that the T-28 was in the
transitionzone, both devicesdetecteda larger concentrationof
smallerparticles,with only a few particlesgreaterthan 3 mm in
diameter. Foil impactordata indicatedthat most, if not all, of
the particleswere ice.
Particleimagesfrom the first 0.5 km (1841:20 to 1841:25) of
the downdraft(Figure 10c) indicatedyet higherconcentrations
of
smallparticles,and a relativelyhigh numberof particlesgreater
than 3 mm in diameter.Those2D-P imagesof a•fficient size,
andthe foil impactordata,indicatedthat the particlesdetectedin
this region,as well as the rest of the downdraft,were ice. As the
T-28 flew farther throughthe doxvndrafi,both devicesindicated
that rest of the do•vndrafiwas fairly homogeneous
in particle
sizes and concentrations,
with someM•atfewer very small and
verylargeparticles.
The magnitudesof the electric field componentsmeasured
duringthe secondT-28 penetration(Figure 11) were significantly
largerthanthosemeasuredduringthe first penetration.The vertical componentof the electric field was near zero as the T-28
enteredthe cloud from the northwest(1840:30). It increasedal-
20-
_
1•-
_
_
_
_
_
Ver'
LLcol
Hør'
LzønL71_••.z_.
.,Z /-II
_½-10
-1•
-20
1 B40
1841
T
T
1842
H
F
mostlinearlyto about7 kV m'• at theilaredgeof thetransition Figure 11. As in Figure7, exccpttbr the sccondpenetration.
18,972
FRENCH ET AL.: OBSERVATIONS
OF EVOLUTION
OF A THUNDERSTORM
zone(1841:20). The T-28 then experienceda relativelystrong, of tipdraftswith relativelyhigherCLWC (King Probereadingsto
positive,
verticalcomponent
(up to 14 kV m-l) overa spanof
3 g m'3)andupdrafts
of 10to 14m s'•.
10 s (4).9 km flight path) as it enteredthe downdraft. Figure 9
showsthat the 2D-P probe particle detectionrate (shadow/or
count) increaseddramaticallyin this region, while Figtire 10c
showsthis to be a regionwith a mixtureof small and largeprecipitationparticles,predominantlyice. The verticalcomponent
The particleprobeson the P-3 detectedmainlygraupelin the
dom•draftregion,with a mixtureof graupelandrain in the up-
of the electric field relaxed to zero as the T-28 flew on toward
draft region. Particleconcentrations
were generallylower than
foundby the T-28 4 rain earlierand 1.1 km above,with generally
similarpeak sizes(cf. Table 1).
The electricfield measuredby theP-3 wasconsistent
(in sign)
with that measuredby the T-28 during its secondpenetration.
The P-3 encountered
positiveverticalelectricfields tlu'oughout
the southeastern
edge of the cloud,througha region of higher
concentrations
of smallgraupel.
(thepeakvalueof 4 kV m'• occurred
in the
The horizontalcomponent
of the electricfield (Figure 11) was the penetration
smallerin magnitude,but its variationwas similarin structureto downdraftregion), indicatingnet negativechargeabovethe airthe vertical component. The horizontalcolnponentincreased craft. The magnitudeof the vertical componentwas relatively
linearly
fromzero(at 1840:30)
to about
4.5 kV m'xat 1841:20. smallerthanthat measuredby the T-28 1.1 km above. This can
This, coupledwith the positiveverticalcomponent,
indicatedthe be interpretedas consistentwith a region of negative charge
presenceof negativechargeaboveandto the right (south)of the above the T-28 altitude with the P-3 flirther removed from it and
aircraft at this location. At the same location where the vertical
thus measuringa lower field magnitude. An inductionring incomponentincreasedsharply,the horizontalcomponentbecame stmmenton the P-3 measuredparticle chargesof both signsin
negative
(-5 kV in'X).Thisisindicative
ofa localized
region
of the downdraftregion,xvithan excessof negativeparticlesover
negativechargeslightlyaboveand to the left (north)of the T-28 positiveones[Brooks,1993]. No particlechargedeterminations
position. The horizontal componentthen recoveredback to could be made in the updraft region due to signal interference
3 kV in'• and decreasedto zero as the T-28 exited the cloud.
fromwatersheddingfromthe inductionring.
The electric field measurements
tlu-oughout
this penetration
3.2.5. T-28 penetration 3. The T-28 beganmakingits third
at 1847:30, some22 min afreflectedsomecorrelationwith microphysical
variables. In par- penetrationfrom the east-southeast
passticular, the dominantpeak in both resolvedcomponents
of the ter the initial echo. The planeflew to the west-northwest,
electric field was encountered in the transition zone and northing tlu'oughthe southernhalf of the highestreflectivitiesas
westportionof the downdraft,in a regioncoincidentxvitha rela- shownin Figure 12. It passedabout2 km southof the high Zm
portion
tive maximtimin the concentration
of large graupel,as seenin column,whichwas then locatedin the northwesternmost
of the storm. The T-28 wasoutof cloudby 1849:30.
Figtire 10b.
A reflectivitycrosssectionalong the flight path of the T-28
The T-28 pilot reportedlightningduringthis penetrationnear
1441:22,just beforethe peakin measuredverticalandhorizontal (Figtire 13a) revealsthat the high-reflectivitycorewas locatedin
field components
abruptlyended. Closeexaminationof the in- the easternhalf of the storm(beghminghalf of the penetration).
dividual T-28 field mill recordsindicatesa small field change The T-28 just penetratedthe top of the regionof highestreflec(œ1kV m'l) at thistime.Examination
ofdatal¾om
several
indi- tivities,whichwere between55 and60 dBz. The easternportion
vidual field mills in the Kem•edy Space Center ground-based of this corewas locatedin a weak to moderatedowndraftregion
networkshowsindicationsof an intmclouddischargeat this time
but a more restrictiveanalysis[M. Murphy, personalcommuniCP-2 and T28
184730-184930 UTC 9 August 1991
cation, 1995) indicatesthe first resolvableintracloudlighming
Altitude=
5.25
km
occurredat 1842:31,just afterthis penetrationended.
55
At 1845:00,while the T-28 was turningin preparationfor a
thirdpenetration,the stormshowedslowmovementto the north,
-18
whereasit had beenprimarilystationaryuntil this time (cf. Figtire 3). The highestreflectivitiesat 5 km (over 60 dBz) were
found in the centerof the storm.This time corresponded
to the
-2O
highestreflectivitydetectedat the 5-hn level duringthe entire
life of the storm.At this level, the stormhad a diameterof ap-
proximately10 kin. The high Zm column•vaslocatedin the
northwesternportion of the highestreflectivitiesat 5 km; the
•.• -22
::::::::::::::.:
1.5-dB contourextendedup to about6 kin. The 1.5-dB contour
wasforrodat a heightof about4 .l•nthroughout
the remainderof • -24
the high-reflectivitycore, indicatinggraupeldescending
below
the freezinglevel andmelting.
•
-26
3.2.4. P-3 penetration2. The P-3 beganmakingits second
and final penetrationat about1845:00. It flew from the south-28
eastto the northwestat an altitudeof •4.1 .l•n(+3øC, ambient).
The aircraft flew directlytlu'oughthe centerof the stormand
penetrated
the high-reflectivity
core(-60 dBz at thislevel).
-30
The horizontalprofile of verticalwindsmeasuredby the P-3
4
was consistentwith that of the T-28 in its secondpenetration.In
25
6
8
10
12
14
16
15
dBZ
X DISTANCEFROMCP-2 (km)
the center of the storm and to the southeast,dox•n•draflsof about
2 to4 m s'• in magnitude
•vereencountered.
Thenorthwestern
Figure 12. As in Figure 4, exceptfor the third penetration,
sideof the storm(corresponding
to theZm column)wasmadeup
1847:30-1849:30.
FRENCH ET AL.' OBSERVATIONS OF EVOLUTION OF A THUNDERSTORM
18,973
A
CP-2
and
T28
184730-184930
UTC 9 August 1991
CP-2 and T28
15002DP-Sh/or
184730-184930 UTC 9 Augus[ 1991
3•-JW
CLWC
(g/m
•)
55
2-
4.5
_
500
1
i
i
30 VERTICAL
ELECTRIC
FIELD
(kV/m)
0
t
•
:
20 VERTICAL
VELOCITY
(rn•s)
ii!ii!!!iiiii
;
I
:i:i:•:i:i:i:
3
.:.:.:.:.:.:.
......
-30 '
12[
i:i:!:i:i:i:i
ß.'.'.'.'.'.'.
i:i:i:i:i:!:i:
3 GHz
ZH
•
:
f
:•.•:
..
:::::::::::::
......
-, 0
.':' ß
3GHz
Z•R
....
---
ß
...
.'i':
ß
ß
8
8
ß.......
"
: -..:':.....-....
ß
:
25
15
184730
-1.5
dBZ
o
FLIGHT TIME (UTC)
ES E -) WNW
184930
dB
184730
FLIGHTTIME(UTC)
184930
ESE -) WNW
Figure 13. As in Figtire5, exceptfor the thirdpenctratiol].
of larger
(1847:50to 1848:18)xvitha maximumnegativeverticalvelocity (Figure14bandTable 1) revealedhigherconcentrations
of-7 m s-•andmoderate
shadow/or
counts.Thewesten]
portion graupelparticlescomparedto the dovmdraft.The largenumber
of the core was in the transition zone (1848:18 to 1848:30), of "splatter"imagesevident in the PMS recordshouldnot be
where vertical winds between-5 and 5 m s'• were encountered. confused
with streakerimages.Thesesplatterimagesare the reIn this regionthe shadow/orcountpeakedat over 1000 particles sult of mixed-phaseprecipitationparticleshitting one of the
s'•, corresponding
tototal2D-Pparticle
concentrations
ontheor- probetips and thereforethere is no correlationbetweenthese
derof6000Ill'3. Theupdraft
(nlaxiilltln]
---10111
S'i) was½ncotlll-imagesandhighCLWC.
During the first 10 s (-0.9 kin) of the updraft, 1848:32tered on the northxvestemlnost
edgeand just outsideof the reflectivitycore. The shadow/orcountwas significantlylower ill 1848:42, tile T-28 flew throughan area with a concentrationof
large particlessimilar to that in the transitionzone (Table 1).
this region.
to the highestreflectivitiesand
The ZDRprofileis shownill Figtire 13b. Throughlnuchof the This 1-km regioncorresponded
penetration,
the 1.5-dBcontourremainedbelmvthe flight level ZDRthat were measuredin the tipdrafton this pelletration. Relaof the aircraft. Yet, in the centerof the tipdraft,the 1.5-dBcoil- tively fewer small particles(0.4 mm < d <1 mm) were detected
tour did extendup to 5.25 kin. This is alsothe regionin xvhich in the updraft comparedto the transitionzone. Data from both
maximum
CLWCwasencountered
(-0.7 g in'3). Northof the the2D-P probe(Figure14c)andfoil impactorwere in agreement
T-28 track, on the northwestemlnost
sideof the reflectivitycore, thatthe vastmajorityof the particlesdetectedwere ice (gmupel),
the 1.5-dB contourdid extend above 6 kin. The strongestup- althoughsomeraindropswere present. Someof what appearto
be streakerimagesare actuallydue to a stuckbit in the probe
drafts•vereprobablylocatedin this regionat this tilne.
Particle data revealed fewer difl%rences betxveen the various
and do not representreal images(a stuckbit is generallyrepreportionsof the stormcomparedwith earlierpenetrations.hi the sentedby a perfectly"straight"image,that is, only one pixel,
that folloxved,and
southeastern
portionof the dmvndrafi(1847:51 to 1847:54)the 0.2 nm] wide). Durillg a narrow dox•qldraft
2D-P probedetectedrelativelylow concentrations
of sinallgrau- the remainderof the tipdraft(endingat 1849:00),both devices
than
pel particles,up to-0.8 mm in diameter(Figtire 14a and Table revealeda broadsizedistributionwith higherconcentrations
1). For the next 0.5 kin, the probemeasuredpredominantlyvery in the main updraftportionof the penetration.Many of the parsmallparticles,but with maximumsizessteadilyincreasing
with ticles detectedin this regionappearedto be liquid, and splatter
particlesam alsoevident.
time to 2.2 nun at 1848:00.At this point, the T-28 enteredan imagesindicativeof mixed-phase
The peak magnitudeof the vertical electric field component
areaof greaterreflectivity(50 to 55 dBz). Throughout
therestof
the downdraftregion,data•¾o111
boththe 2D-P probeand foil im- during the third penetrationwas not much different than that
pactor indicatedhigher concentrations
of particlesin all size duringthe secondpenetration,but the horizontalcomponentwas
categories,up to 6.6 mm in diameter. All of the particlesde- much larger than that measuredduringthe secondpenetration
(Figure 15). The vertical componentof the electric field was
tectedin the downdraftappearedto be graupel.
There was no distinctboundary,at leastin tile particletypes, positivethroughthe first 40 s (-3.6 Ion) of the penetration.
betweenfile downdraftregionand the transitionzone. The 2D-P Fromthis point,the field decreased
dramatically
overa 10-speimagesand statisticsfrom the transitionzone, 1848:18-1848:31 riod (---0.9 km) andwent througha nfinilnumof approximately
1-8,974
A
FRENCH ET AL.' OBSERVATIONS OF EVOLUTION OF A THUNDERSTORM
of the aircraftat this time. The horizontalcomponentrecovered
to about-7 kV m'• and then decreasedas the T-28 crossedthe
•[•IF•[•BF•rH•HkL•rLr•H•r•L•[r•LH•I••r•r•i•I•L•r•
Buffer 447 2D-P Start Time = 184757 80 Duration = 1 10 secs
fringeof the updraftand approached
the northwestern
edgeof the
•l•l•a•B•••l••••
IIIIIIIIIIIIIIIIlI!11111•11111111111111llllll
cloud.
Buffer 444 2D-P Start Time = 184750 85 Duration = 3 49 secs
Buffer 450 2D-P Start Time = 1848O1.55 Duration = 0 82 secs
[I.ILItlII•IiI'I.I[LILlII'I•IJlHI•IIFLLlI[IIILIIIIrUrlIII
t1_
11L'tl•MllrII'I•IlIrI•III'LI-----IIIII.
II-LI'1111
ILIIFIIIrLlll.
Much morevariationin the electricfield wasmeasuredby the
T-28
duringthis penetrationthan in the two earlierpenetrations.
•LI--!•IrFI_-HIIIIl!!!ltH-•II'III•111111111BIII
II-IIIIIIIIIIII.
IilII.
I•!111tlIIIIItlilIII!rLIHIIIIIII'11•I'
When the vertical componentreachedits most negativevalue
HILLI'LI!IH--q--11I---•--.....!1•!.......II_!11tllLllllllll'rllLIPL•lli'lLrlr•tlltllrllll•l!llliHr'llbLl[
(1848:18), the T-28 was in the mostsignificantdowndraftmeasBuffer 460 2D-P Start Time = 184811 55 Duration -- 0 45 $ecs
I'B•.1111111-•!!!!!!H•!!!
!!!!!!!_rrluI.JHHrltllLl'111,1111-111111lll
I-LI-HIII_11IIItlI,
IIII.
IIIIit
Illl[11HIirlllll
ured
duringthat penetration.Yet, the particlephases,sizes,and
Buffer 462 2D-P Start Time = 184813 55 Duration -- 0 71 secs
duringthis time were similarto thosesampledbelllelIL[LIItI-tl-.-I_
LIIIPllJll!l!!lailllll[tlJl-lllllll•llrHlllllrlllll
IIIIIIBIIBtlilIII,,,1111It[MII-I-11111114111Iilll
Ilfitll concentrations
Buffer464 2D-P Start Time = 184815 55 Duration = 0 08 secs
fore and after this downdraftwas encountered.One interpretaLltitlI!I!-IIIFI!ilI'IIiI!II•I!!!!!•!IILIIJI!Ir!tlIItLIILIIIIIItlII-IIFrlIi•I
- - i-IFFI
IlL
tion of these observations is that the T-28 was close in altitude to
Buffer 453 2D-P Start Time = 184804 55 Duration = 0 25 secs
Bufter 466 2D-P Slart Time = 184817 55 Dural•on = 0 58 $ec$
•!11111-11•1111BII.I.IILI•KL[IBItle•IIIiI'IIItlrlItl
tHrl-II
F' ritli[11LI
- ttl•l•l-I,
lallt
the centerof a negativec!•argelayer, and that the centeraltitude
of this regionwasbelowthe aircraftin the downdraftand above
it elsewhere.
Buffer467 2D-P Start Time = 184818 55 Duration-- 0 30 secs
i•;•111'tJJlllellrJllJ.
l'_l-•...._IF'•
._11-_•-!-!4!•!--lkllllltllllllf•
;'c;"-IIL:II-IIt[ILplILlII/[IrI'I,
Buffer 469 2D-P Start Time = 184820 55 Duration= 0 22 secs
4. Summary of Observations
tI.1111elI,•LP!I,UL!
I-'---! l!F-•! IlllIII,
IIt[IIL•IIllI-II•tlLI-LIItlII"ILIdlilI'PI[ILIt-[
Buffer 471 2D-P Start Time = 184822 55 Duration= 0 19 secs
IIIilIIliL_I411H_F-!bl
Itt---t I• III,
IIIII1'II
The early electricalevolutionof this stormwas stronglycouBuffer472 2D-P Slart Time = 184823 55 Duralion = 0 41 secs
pled with the evolutionof the stonn's microphysics.For this
I•11tllllll•LiiillrlttlHllllalllrl•1itllltll
Hltl--ILdlIIIllIIII!I"'I-HI[III•IIIHIIIII!1111ttl--IIIII1•111'1"11-1
Buffer473 2D-P Start Time = 184824.55 Duration = 0 02 secs
:llll•L•r••-•F•:•-•`•f••F•-•p•`--.
-----•'-•F
--
reason, we sununarizehere the evolution of the storm as a whole
and do not breakit into electricalandnonelectricalcomponents.
The P-3 penetratedthe storm near the time and level of the
Buffer477 2D-P $1artTime = 184828 55 Duralion= 0 10 secs
first radar echo. It detecteda t•w small precipitation-size
ice
._ - I-IL!IL•ILAI•HI-_I------It-IFI
Buffer 479 2D-P Slad Time = 184830 55 Duration = 0 14 sees
particlesat the -10øC level, but no significantelectric fields.
---•"-*,•- --•.....-_--:.=•4•-•.•--:•---•l--iil[ltl•11•11tltllllllllll'111Ill•1111•11111
FI
Eight minuteslater, the T-28 penetratedat the -3øC level. It
detectedsignificantconcentrations
of precipitation-size
particles
in the dmvndraft(mostlyice) and only a fexvprecipitation-size
Buffer 481 2D-P Start Time ,= 184832 55 Duration = 0 38 secs
up to this
l-N]IaI'LLIEL.,PlttlIH
!!!11"[[I,I[t[IIt-LI[IL
I-[1tMI-LIFIIIIIIIIII'-LIII-r[Hparticles(mostlyliquid) in the updraft. Electrification
pointhadremainedrelativelyxveak.
Buffer
483
2D-P
Start
Time
=184834
55
Duiitlon
=085
secs
!!
_l-t!'
l[IJ.-i
. ...__!!!!!-LI-l_
_l•tk_!-I!!11_
_1-1•11•!-IL-IIFIIlalLI•__I_IiI-LLll
Five minutesafter the T-28's first penetration,it again flew
Buffer 485 2D-P Start Time = 184837 05 Duration = 0 74 secs
-IIIII1-1II
I-tlI•111LLIilIilI!.IlIIIIII-PH.I-LllIFIY•ILLJJLII-I
PI---ILItPIHII,l!-[PI tttk-!IIIIIIIllI•ILFI. tlu'oughthe stormat the-3øC level. It encountered
significant
concentrations
of
precipitation-size
particles.
Moderate
updrafts
__11lPtLI,,I•h
were encountered,which containedthe lowest concentrationof
Buffer 493 2D-P Start Time = 184847 30 Duration = 2 01 secs
precipitation-size
particles. The observations
are consistent
with
-t-I!rI[IiltI[LLII-H
11tlIIlt--11LI-••:•.•IItll•l•LIIIIIllatllllltltll
!11rli
i'111t•rlErlli'11
ilel
liquid particlesin the updraft growingby collision/coalescence
Buffer 497 2D-P Stad Time = 184852 75 Duration -- 0 34 secs
& IIIJlfi!llqllillillllllL
Ilrl•ltlrllllLItlll!lll!llUll
l--71tllllr-iiiii
I-Ill,IlL
IIt--IlllE•L IlLIII, I1.• 1 I'LIIHIIIIIIIIIand then beingcarriedupward,where they would freezeat temperaturesbetween-5 ø and -10øC. This interpretationis sup-11tl--rL[11-•__
i1_P1111
•1•-•t•-•L•.•'•L••bP•H•L•P•'•
Buffer 501 2D-P Slart Time = '184856 75 Duralion = 044secs
portedby the ZD• profile. Also, high concentrations
of ice partiIIIIItlIILI'I!IH
LHIl-•ll"tllfl!,llll.
lflTItl!111111111J"lltllllll
II-II,IllIllIII[11,[tI-IIIIILiltlIHIIIItllIIIIiilIII[tttlIIIILirl
[1•11
cles (graupel)were detectedin the douq•draft. These particles
Buffer475 2D-P Slad Time = 184826 55 .Duration= 0 28 secs
Le!IIIlllILF!
•ll•l[!Frr!_!!•!,_
•!•.•1•L•r-•e•i••[••H•.k•L•r•-•I•h
Buffer 499
2D-P
Start Time = 184854 75 Duration = 0 44 secs
Figure 14. As in Figure6, exceptlbr duringthe (a) doxvndraft,
(b) transitionzone,and(c) updratlin thethirdpenetration.
2O
Ver' t Lco I
Hor- tzon tal
....
-10 kV m'• attheedgeofthedowndraft
region
at 1848:18.The
vertical componentof the electric field then recovered to
13 kV m'• and then decreasedto zero as the T-28 crossedthe
fringe of the updraft region and approachedthe northwestern
edgeof the cloud.
The measuredhorizontalcomponentof the electricfield was
oppositein signfrom that measuredduringthe secondpenetration. This is consistentxviththe secondpenetration,sincethe
T-28 was flying in the oppositedirection. The horizontalcomponentdecreased
fromzeroat the southeastern
edgeof the cloud
to a minimum
of-20 kV m'• about40 s intothepenetration,
coincident with the n•aximum in the vertical componentin the
downdraftregion. The T-28 then passedthroughan area at
1848:30 where the horizontalcomponentwas a relative maxi-
mum(0 kVm'•). Thislocation
corresponded
to theboundary
betweenthe transitionregionandthe updraft,anda shiftin sign
of the verticalcomponent,indicatinga localizedregionof relatively high positivechargedensityaboveand to the left (south),
or negativechargebelowandto the riglit(north),of theposition
E 1•
.......
0 -•5
•"x
/1
\
••
\
\
(-
x
•_• -10
20
x
i
,
,
/
i
i
i
i
I
i
i
18•7.5
T
]-
P1E
Figure 15. As in Figure7, exceptfor the third penetration.
FRENCHET AL.:OBSERVATIONS
OFEVOLUTIONOFA THUNDERSTORM
18,975
thattileaircraft
wasnotontilemainelectrical
didnotmeltuntiltheywere•vellbelowthelevelof theaircraft, large,indicating
of lightning
andof a second
as wasevidentfromsubsequent
P-3 observations
about1 km axiseither.Withthedevelopment
convective
region(or themovement
andreinvigoration
of the
lower in altitude.
one)thedistribution
ofcharge
withinthestorm
hasbeThe electricalcharacter
of the stormhadchanged
significantly original
duringthe5-raininterval
betxveen
thetimesof the firsttwo comemorecomplexby thistime.
penetrations.
Such
rapiddevelopment
ofelectrification
hasbeen
notedin studiesof otherCaPE storms[e.g., l¾illiset al., 1994;
Ramachandran
et al., 1996]andill similarstudies
of thunderstorms
elsewhere
[e.g.,Dyeetal. 1986,1988].Duringthesec-
5. Discussion
The evolution of this storm can be divided into two main
priorto anyindications
of
ondpenetration,
vertical
electric
fieldsof theorderof several parts. Firstis the earlyevolution
kVm 4 were measuredat the level of the T-28. From these
measurements
andonesmadeduringearlierpenetrations,
it is
obvious
thattherewasa definitelagbetween
theinitiation
of
electrification,
includingthe time t¾omfirst echotN-oughthe
next12to 15min. Thefirstpenetrations
of boththeP-3andthe
T-28 were madeduringthis time. The secondpart of this
precipitation-size
particles
andtherapidbuildup
oftheelectric storm'sevolutionconsistsof the following10 to 15 rain of its
field. Thisdelayis notsuq,
rising;it hasbeendocumented
sev- lifetime. Duringthisperiod,the T-28 madetwo penetrations,
andelectricalactivitybecame
eraltimes[e.g.,Reynolds
andBrook,1956;Dyeetal., 1986].h• theP-3 madeits lastpenetration,
bothof theseearlierstudies
thedelaywasof theorderof 8 to 10
evident. After this second"evolution
period"the stormbeganto
stage(microphysically
anddynamically).
We
min,whilewefound
a delayof 11to 17min. Whether
thisdif- enterits decaying
ference
is significant
is opento debate,
butonepossible
expla- didnotanalyzethisfinaltimeperiodin detail,above.
nationis thatill the earlierstudies,
precipitation
wasinitiated
ttu-ough
iceprocesses.
In ourcase,
precipitation
wasinitiated
at
higher
temperatures
through
collision/coalescence.
If iceplays
a 5.1. First Stage
significant
rolein tileelectrification
of thunderstorms
(assugThe first 12 to 15 min in the evolutionof this stormwas chargested
by precipitation-based
electrification
hypotheses),
we acterized
bybroad
updralls
reaching
14m s'• andcontaining
would
expect
a longer
delayincoalescence-dominated
clouds. CLWCranging
uptoatleast
4 gin-3initially
(asmeasured
bythe
Aninteresting
feature
found
in theelectric
fieldprofileduring P-3 KingProbe;theT-28 JW readings
laterin thisperiodare
thesecond
T-28 penetration
is thattilemaximumseems
to occur probably
anunderestimate
ofthetrueCLWC).Concentrations
of
between
thetransition
anddox•q•draft
regions
andis nearlycoin- mediumto smallraindrops
(0.4 < d _<1.5 ram)rangedtip to alcident
withtheregionof highest
reflectivity.
Thistoohasbeen most1000m-3. Downdrafts
duringthisperiodxverebroadand
documented
in earlierstudies
[Dyeet al., 1986,1988;IFilliset ranged
to-10 m s-1. Thedovmdrall
region
wasdevoid
ofcloud
al.,1994].
Thisfinding
supports
tileideaofa precipitation-based
liquidwater.Them•jorityoftheparticles
in thisregion
•vereice
charging
mechanism
active
in thiscloud.It is ill thetransition(d _<2 nm•)yeta fewraindrops
•veredetected
in thetransition
zonewhereonewouldexpect
to findthegreatest
number
of dif- zonenearthe edgeof thedox•q•draft
withina kilometerof the
ferent
particle
types
(graupel,
snow,
cloud
ice,etc.).Also,inthis updraftregion.
region,
CLWCwouldbelargeenough
to allowtimingof tile
Ouranalysis
is consistent
witha coalescence-dominated
congraupel
particles
(averyimportant
requirement
lbrnon-inductive
vectivestormin whichraindrops
tbnnandgrowin an updraft
charging
totakeplace).Without
measurements
ofactual
particle regionthatis richin cloudliquidwater. Theseraindrops
are
charge
at thisandotherlevelsin thisstorm,
it is impossible
to carriedvia thetipdrafts
to levelsabovethe0øCisotherm.The
determine
towhatextentcharge
separation
mayhavebeentaking Zm column(collocated
withthetipdraftandindicative
of larger
placein thetransition
zone.However,
theideaof charging
in raindrops)
extended
toabout
6 kmthroughout
thisportion
ofthe
thisregionis consistent
•vithourmeasurements.
stonn'sevolution.Thisheightcorresponds
to approximately
The P-3 madeits second
penetration
about3 rain aftertile -10øC. At thislevel,eventhe smallerraindrops
wouldbeginto
T-28 exitedthecloudl¾om
its second
pass.TheP-3 flewsignili- freezeandthenewlyformedgraupcl,withdiameters
of 3 nunor
cantly
lowerthanintheearlier
penetration
(4.1kin,•-3øC).The less,wouldcontinue
to growthrough
accretion
of cloudandrain
water.
electricfield profilemeasured
by tile P-3 wasconsiderably
smaller
in magnitude
butconsistent
in signwiththatmeasured It is alsoprobable
thatcloudicewouldbcIonnedthrough
the
Hallet-Mossop
process
[Hailerand•1ossop,
1974]nearthislevel
bytheT-28duringits second
penetration.
Thesemeasurements
are consistent
with a largeregionof negativechargelocated in theupdraft,
giventhetemperature,
highCLWC,broadcloud
abovetheflightlevelof tileT-28. Tilegreater
distance
betxveendroplet
sizedistribution
characteristic
of warm-base
clouds,
and
theP-3andthecharge
region,
andpossible
rearrangement
of tile the existence
of graupel. Thesecrystalswouldgrow l•airly
charge
distribution
dueto tilelightning
discharge,
couldaccount quicklythrough
deposition,
sincethe enviromnent
shouldbe
fortheweakerfieldsobserved
bytheP-3. It is alsopossible
that highlysupersaturated
•vithrespect
to ice. Aggregation
of these
should
alsoleadtotheproduction
of snowthatcould
someof theredtiction
in tile strength
of tile electricfield (with icecrystals
anothersource
of graupel.
decreasing
altitude)
•vasduetoa region
of weakpositive
charge thenrimeandrepresent
At higherlevels,someoftherecently
lbnnedgraupel
beganto
located
between
thetwoflightlevels(4.1 and5.25kin, +3ø and
intothedo•ldraft. Loading
of thetipdraft
-3øC);however,
it is ilnpossiblc
to tollif thisis thecasel¾om falloutofthetipdraft,
the data.
mayin facthavehelped
to produce
theupper-level
downdraft
at
the
Thethirdpenetration
madeby theT-28beganabout2 rain aboutthe sametimerainbegallto freeze.As Illis occurred,
portion
of theupdraft
regionpropagated
northxvestward
aftertheP-3'ssecond
penetration.
Again,theT-28 flewat 5.25 leading
regionwas
km. Thispenetration
wasoffset
œrom
themostactive
convectionat lowerlevels.Muchof tilegraupelin tiledoxvndraft
withinthestorm(south
of tilehighZD•column).Themeasuredless than 2 nnn in diameter. Particle concentrationsin this
regionweremuchhigherthanin theupdraft,
withthe
horizontal
component
of tile electricfieldwasalsorelatively downdraft
18,976
FRENCH ET AL.: OBSERVATIONS OF EVOLUTION OF A THUNDERSTORM
highest
concentrations
(nearly1000m'3)foundin thatportion
of
the downdraftnearestthe updraft. This, coupledwith the low
concentrations
in the tipdraft, explainsthe patternof moderate
reflectivitythroughoutmuch of the downdraft•vith the highest
refiectivities in the transition zone, between the updraft and
downdraft,on the edgeof the ZDRcolunto. The weakestrefiectivities were collocatedwith the updraftandthe ZDRcohmm.
Electrification,throughoutthis early stagein the storm'sevolution, was extremelyweak. This is to be expectedif charge
transferandseparation
were basedon sometypeof precipitation
mechanisminvolvingice-phaseparticles. Throughoutthis period, concentrations
of ice particlesremainedrelatively low.
Also, it was not until the end of this periodthat significantrefiectivities, implying higher concentrations
of graupel, were
found to exist at temperaturesof-20øC. Significantcharge
transfervia a noninductivemechanis•nrequiresreasonablyhigh
concentrations
of graupelandsnowand/orcloudice in the-20øC
region[e.g.,Takahashi,1978a;Satmde•'s,
1993]. If the concentrationsare not high enough,there will be too tkw collisions
betweenthese particlesto lead to strongelectrification. Althoughsomechargetransfermayhavetakenplaceat and around
-10øC with graupeland cloudice (tbnnedthroughthe HalletMossopmechanism),
it is not clear from the observations
what
effectthismayhavehadon the large-scale
electrification
of this
storm. This questionwill be addressed
fitrtherin the companion
modelingpaper.
5.2. Second Stage
The nextportionof tile stonn'sevolutionwascharacterized
by
a narrowerupdraftanda morebroad,diffilse,doxxqldraft.
The regionswereseparated
by a 1- to 2-kin-widetransitionzonev,'hich
hadweak to moderatetipdraftsanddo,aqldrafis.The tipdraftwas
During this portionof the stonn'sevolution,it went througha
rapidincreasein electrification,as evidencedby the observedincreasein electricfield strengthat the aircraftpenetrationlevels.
This is consistentwith a precipitation-based
mechanismbeing
responsible
for the electrification.As graupelandcloudice both
beganto appearin significantconcentrations
at low temperatures
(-20øC), appreciablechargetransferand separationwould be
expectedto occuraccordingto the noninductivecharge-transfer
processes
studiedin the laboratory[Takahashi,1978a;Saunders
et al., 1991]. When particles (graupel/cloudice and/or graupel/snow)collidein regionsof this temperatureand in the presenceof cloudliquid xvater,the graupelacquiresnegativecharge
and cloudice and/or snowacquirespositivecharge. If theseinteractionswere taking place in the transitionregion(where all
the necessary
components
were present),we wouldexpectgraupel to be chargednegativelyand snoxvand cloud ice to be
chargedpositively.
As the graupelparticlest•qllout, the negativechargesare carried dovmxvard,
away t¾omthe regionof production.This leads
to large-scalechargeseparation. We envisionthis processcontinuing,leadingto a regionof negativechargeabovethe aircraft
penetrationlevels, and positivechargeat even higher altitudes.
This processcontinuesand electricfields build tip to the point,
near 1842:00,when lighmingwasdetected. Furtherincreasesin
electricfield strength,as measuredby the T-28, mostlikely are
the restilt of two phenomena. First, as chargecontinuesto be
separatedwell abovethe 0øC level, electric fields will continue
to increase.Second,sincethe negativechargeis associated
with
graupeldescending
in tile doxxqldrafi,
we might expecttile lnain
negative charge region to descendwith time. Although the
magnitudeof tile chargein this regionmaynot be increasing,
the
electric fields at the level of the T-28 would continue to increase
as the descendingcenter of mass of the chargedgraupel ap-
richin cloudliquidwater(peakJWreadings
of2 gnf3)butwas proachedthe aircraftaltitude t¾omabove. As the centerof this
relatively
freeofprecipitation-size
particles
(hundreds
m'3).The charge layer passesthroughthe aircraft penetrationlevels in
downdraftwascharacterized
by verylittle cloudliquid waterand
downdraftregions,the sign of the vertical field componentre-
highconcentrations
(many
thousand
m'3)ofgraupel.
Thetransi- verses.
tion zone sharedcharacteristics
of both the updraft and dmvn-
draft.In thisregion,
CLWCwasmoderate
(-1 g nf3)andprecipitationparticleconcentrations
xverealsomoderate(manyhun-
dredto a fewthousand
m-3).Someparticles
detected
in thisre-
ConcludingRemarks
Althoughwe cannotbe sureof all of the elementsenvisioned
gionwere ice, while othersxveremixedphaseor liquid.
in this discussion
of tile evolutionof tile August9 storm,we have
Measurementsmade duringthis portionof the stonn's life- presenteda "story" that is consistentwith the measurements.
time seeinto presenta consistent
view of howthe stormevolved We alsoquantitativelydescribedhow the stormevolvedbothmimicrophysically
from being dominatedby liquid coalescence crophysically
and electrically. In a companionpaper,we attempt
processes
to beingdominated
by ice-phase
processes.
Raindrops to capturethisevolutionin a numericalsimulationusingthe txvowould form and grow in the updraftregionand be carriedto dimensionalStormElectrificationModel (2-D SEM) and quantihigherregionsin the storm,wherethey wouldfreezeforming tativelycomparethe simulatedto the observed
storm.
graupel.Also,duringthisstage,significant
mnounts
of cloudice
weremostlikely beingproduced
throughheterogeneous
l¾cczing
Acknowledgments. We wish to thank V. N. Bringi and J.
of clouddropletsat higherlevelsin the storm(althoughxvcob- Turk for radaranalysissoftwareusedin generatingseveralof the
tainedno in situ observations
at theselevelsto vcril}' this conjec- figures; K. Hartman for generating the particle images; D.
ture). Ice crystalsproducedthroughthis and othermechanisms Priegnitzfor help in creatingsomeof the radarfigures;andC. P.
would grow by diffilsionand aggregateto lbrm snow,which R. Saunders,I. Brooks, and R. Black for providingthe NOAA
couldthen serveas graupclembryos,althoughraindropt?eezing P-3 data. This researchwas supportedby the National Science
is probablythe primary graupelproductionmechanism. We Foundationunder grantsATM-9104474, ATM-9401117, ATMwouldexpectthe graupelto continueto growby accretion
in the 9022846, ATM-9509810, and ATM-9200698.
updraftandtransitionzoneuntil theseparticlesbecamepart of a
downdraft. Microphysically,this is the same processas de- References
scribedfor the early portionof the stormevolution. The main
Aufdermaur,A. N., and D. A. Johnson,Chargeseparationdueto riming in
differenceis that, in the maturephase,the processtakes place
an electricfield, Q. d. R. Meteorol. Soc.,98, 369-382, 1972.
over a largerdepthand thereforemoreand largerparticlesare Black, R. A., and J. Hallet, Observationsof the distributionof ice in hu•Ticanes,d. Atmos. Set., 43, 802-822, 1986.
produced.
FRENCH ET AL.: OBSERVATIONS
OF EVOLUTION
OF A TItUNDERSTORM
18,977
ning.Ph.D. thesis,400 pp., Inst.of Sci. andTechnol.,Univ. of Manchester, Manchester,England, 1981.
Krehbiel, P. R., Electrical structureof thunderstorms,in The Earth's Electrical Environment, pp. 90-113, Natl. Acad. Press,Washington,D.C.,
Bringi,V. N., andA. Hendry,Technology
of polarization
diversityradarsfor
meteorology,
in Radar Meteorology,editedby D. Atlas,pp. 153-190,
Am. Meteorol. Soc., Boston,Mass., 1990.
Bringi,V. N., A. Detwiler,V. Chandrasekar,
P. L. Smith,L. Liu, I. J. Cay!or,andD. Musil,Multiparameter
radarandaircraftstudyof tile transition from earlyto maturestormduringCAPE:The caseof 9 August
1991, in 26th InternationalConferenceon Radar Meteorology,Norman,OK, pp. 318-320, Dan.Meteorol.Soc.,Boston,Mass.,1993.
Brooks,I. M., Laboratorystudiesof thunderstorm
chargingprocesses,
Ph.D.
thesis,164pp.,Univ. of Manchester,
Manchester,
England,1993.
Brooks,I. M., and C. P. R. Saunders,
An experimental
investigation
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