DTIC ADA514106: Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby Wave Energy Dispersion

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42 MONTHLY WEATHER REVIEW VOLUME138
Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby
Wave Energy Dispersion*
XUYANGGEANDTIMLI
DepartmentofMeteorology,andInternationalPacificResearchCenter,
UniversityofHawaiiatManoa,Honolulu,Hawaii
MELINDAS.PENG
NavalResearchLaboratory,Monterey,California
(Manuscriptreceived16March2009,infinalform27July2009)
ABSTRACT
ThegenesisofTyphoonPrapiroon(2000),inthewesternNorthPacific,issimulatedtounderstandtherole
ofRossbywaveenergydispersionofapreexistingtropicalcyclone(TC)inthesubsequentgenesisevent.Two
experimentsareconducted.Inthecontrolexperiment(CTL),theauthorsretainboththeprevioustyphoon,
TyphoonBilis,anditswavetrainintheinitialcondition.Inthesensitivityexperiment(EXP),thecirculation
ofTyphoonBiliswasremovedbasedonaspatialfilteringtechniqueofKuriharaetal.,whilethewavetrainin
thewakeiskept.Thecomparisonbetweenthesetwonumericalsimulationsdemonstratesthatthepreexisting
TCimpactsthesubsequentTCgenesisthroughbothadirectandanindirectprocess.Thedirectprocessis
throughthe conventional barotropicRossbywave energydispersion,whichenhancesthe low-levelwave
train, the boundary layer convergence, and the convection–circulation feedback. The indirect process is
throughtheupper-leveloutflowjet.Theasymmetricoutflowjetinducesasecondarycirculationwithastrong
divergencetendencytotheleft-exitsideoftheoutflowjet.Theupper-leveldivergenceboostslarge-scale
ascendingmotionandpromotesfavorableenvironmentalconditionsforaTC-scalevortexdevelopment.In
addition,theoutflowjetinducesawell-organizedcycloniceddyangularmomentumflux,whichactsasa
momentumforcingthatenhancestheupper-leveloutflowandlow-levelinflowandfavorsthegrowthofthe
newTC.
1. Introduction 1999) and confirmed by satellite observations in more
recent studies (Li and Fu 2006). While the TC moves
Amongdifferenttropicalcyclone(TC)genesismech-
northwestward due to the beta effect and mean flow
anismsinthewesternNorthPacific(WNP),theRossby
steering, it emits energy southeastward and results in
waveenergydispersionfromapreviousTCisanimpor-
a synoptic wave train with alternating anticyclonic and
tant process (Fu et al. 2007). The connection between
cycloniccirculationsinitswake(ChanandWilliams1987;
tropical cyclogenesis and the TC energy dispersion
CarrandElsberry1995;Holland1995).Underfavorable
(TCED)wassuggestedearlier,basedonlimitedobser-
large-scaleenvironmentalconditions,theTCED-induced
vational data (e.g., Frank 1982; Davidson and Hendon
RossbywavetrainmayleadtonewTCgenesis(Ritchie
1989; Briegel and Frank 1997; Ritchie and Holland
andHolland1999;LiandFu2006;Lietal.2006).
MostofthepreviousTCEDstudies(ChanandWilliams
1987; Carr and Elsberry 1995) were confined to a two-
*SchoolofOceanandEarthScienceandTechnologyContri-
dimensional barotropic dynamic framework. A recent
bution Number 7845 and International Pacific Research Center
three-dimensional (3D) baroclinic modeling study by
ContributionNumber623.
Ge et al. (2008), investigated the vertical structure of
the TCED-induced waves. Due to the vertical differ-
Correspondingauthoraddress:Dr.XuyangGe,Dept.ofMeteo-
ential inertial stability, the upper-level wave train is
rology,InternationalPacificResearchCenter,UniversityofHawaii
characterized by an asymmetric outflow jet that de-
atManoa,2525CorreaRd.,Honolulu,HI96822.
E-mail:[email protected] velopsmuchfasterthanitslower-levelcounterpart.This
DOI:10.1175/2009MWR3005.1
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Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with
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Rossby Wave Energy Dispersion
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JANUARY2010 GE ET AL. 43
FIG.1.Timesequencesofsynoptic-scalesurfacewindpatternsassociatedwiththeRossbywaveenergydispersionofTyphoonBilis;
‘‘A’’representsthecenterlocationofBilis,and‘‘B’’representsthecenterlocationofTCPrapirooninthewakeofBilis.TheQuikSCAT
surfacewindproductisused.
betaeffect–inducedupper-levelasymmetrymayinfluence and two numerical simulations are conducted. In the
the lower-level Rossby wave train by changing the in- controlexperiment(CTL),boththestructureoftheprior
tensity and structure of the TC. Moreover, an easterly typhoon,TyphoonBilis,anditswavetrainareretained.In
(westerly)shearofthemeanflowmayfurtherenhance the sensitivity experiment (EXP), the prior typhoon is
(weaken) the lower-level Rossby wave train (Ge et al. removed but the wave train in its wake is kept. The
2007). The idealized numerical simulations of Li et al. disturbanceinthewavetrainisneededinEXP,serving
(2006) demonstrated the important role of TC energy as the precursor of the subsequent TC. Through this
dispersion in the wave train development and cyclo- sensitivityexperiment,weinvestigatetheroleofthepre-
genesis. The objective of the present study is to in- vious TC in the subsequent cyclogenesis. The numerical
vestigate the structure of the wave train in the reality results are presented in section 4. The physical inter-
andtheextenttowhichapreviousTCcontributestothe pretationsontheroleoftheupperoutflowjetassociated
large-scaleenvironmentalconditionsinitswake.Inpar- withthepreviousTCarediscussedinsection5.Finally,
ticular,weattempttorevealtheeffectoftheRossbywave theconclusionsanddiscussionaregiveninsection6.
energy dispersion and the upper-tropospheric influence
on the TC genesis, utilizing a real-case simulation for
2. WavetrainpatternspriortoPrapiroon
TyphoonPrapiroon(2000)inthewesternNorthPacific.
formation
The paper is organized as follows. In section 2, the
observedsynoptic-scalewavetrainpatterns,priortothe TyphoonPrapiroonformedovertheWNPon25August
cyclogenesis of Typhoon Prapiroon, are presented. In 2000.Thesynoptic-scalewindpatterns,priortothecy-
section3,themodelandexperimentdesignsaredescribed clogenesis,areshowninFig.1forthebandpass-filtered
44 MONTHLY WEATHER REVIEW VOLUME138
(3–8 day)surfacewindfields,followingLiandFu(2006).
The filtering scheme is that of Murakami (1979). The
response function for this filter is a Gaussian function.
Inthe3–8-dayband,themaximumresponseisatape-
riod of about 5 days. A Rossby wave train with alter-
nating cyclonic–anticyclonic circulation can be clearly
identified in the wake of the previous super Typhoon
Bilis, whichformedon18August.This wavetrainwas
orientated in a northwest–southeast direction with a
wavelength of about 2500 km, resembling a typical
TCED-inducedRossbywavetrain(LiandFu2006;Ge
et al. 2008). During the course of the wave train de-
velopment,analternatingrainy–clear-sky–rainypattern
occursinthewakeregion(Fig.2).Rigorousconvective
cells are primarily confined in the cyclonic circulation
region of the wake. The convective cells merge into
largermesoscaleconvectivesystems(MCSs),eventually
evolving into a tropical cyclone with a well-organized
structure(bottompanel).
The cyclonic circulation in thewave train intensified
and eventually led to the formation of a new TC Pra-
piroon to the northwest of Yap Island. Prapiroon was
locatedabout208totheeastofTCBilis,consistentwith
previousstudies(e.g.,Frank1982;RitchieandHolland
1999; Li and Fu 2006). Frank (1982) noted that WNP
typhoonsoftenformed158–208totheeastofapreexist-
ing storm. This implies a preferred wavelength of the
TCED-inducedRossbywavetrain.
The 150-hPa wind vector and relative vorticity on
22 August2000arepresentedin Fig.3a.Anelongated
outflowjet,withamaximumvelocityexceeding35 m s21,
extends in a clockwise direction from the northeast to
thesouthwest.Acyclonicshearvorticityoccurred out-
sideofthejetcore.Thisasymmetrichorizontalpattern
resembledbothobservational(BlackandAnthes1971;
Frank 1977; McBride 1981) and numerical simulation
results(Shietal.1990;WangandHolland1996;Geetal.
2008).Theasymmetricstructureremainedbasicallyun-
changedinthefollowingdays,untilTyphoonBilismade
FIG. 2. The evolution characteristics of TRMM rainfall rate
landfall. Figure 3b shows the vertical-radius cross sec-
(mmh21)andlower-levelwindfieldon(top)23,(middle)25,and
tion of the relative vorticity field along the axis of the
(bottom)26Aug.
wavetrain(dashedlineinFig.3a).Theverticalstructure
shows a noticeable baroclinic structure—that is, a cy-
clonic vorticity centered near 750 hPa with an anticy- clear evidence of southeastward Rossby wave group
clonicvorticitynear200 hPa,sothatthewavetraintilts velocity from the center of Typhoon Bilis. Thus, the
northwestwardwithheight.Theverticalstructureagrees observed diagnosis supports the notion that the TC
withournumericalsimulation(Geetal.2008). PrapiroongenesisisindeedassociatedwiththeRossby
TodemonstratetheexistenceofRossbywaveenergy waveenergydispersionfromthepreviousTCBilis.
dispersion,anEvector(Trenberth1986;LiandFu2006)
isusedtodiagnosetheenergypropagation.TheEvector
3. Themodelandexperimentsdesign
iscalculatedbasedonsynoptic-scalewindfieldsduring
an11-dayperiodcenteredon20August2000.Asseen The fifth-generation Pennsylvania State University–
fromFig.4,priortotheformationofPrapiroon,thereis National Center for Atmospheric Research Mesoscale
JANUARY2010 GE ET AL. 45
FIG.3.(a)Theobserved150-hPawindvector(ms21)andrelativevorticity(1025s21)fields
at1200UTC22Aug2000and(b)thevertical-radiuscrosssectionofrelativevorticityfields
alonganorthwest–southeast-orientedaxis(thedashedlineatthetop).Thehurricanesymbol
representsthecenterofBilis.
Model(MM5)isusedtosimulatethiscyclogenesisevent. Center for Atmospheric Research (NCEP–NCAR) re-
Themodelhasadomaincenteredat(208N,1458E)with analysisdatasets.
a grid size of 271 3 201 and a horizontal resolution of Two experiments are designed to elucidate the role
30 km. There are 23 vertical sigma levels. The Betts– ofcontinuousRossbywaveenergydispersionfromthe
Millercumulusparameterizationschemeisused.Theini- previous TC. In CTL, the preexisting Typhoon Bilis
tialandboundaryconditionsareobtainedfrom 6-hourly and its Rossbywavetrainareretained.In EXP,aspa-
NationalCentersforEnvironmentalPrediction–National tial filtering scheme based on Kurihara et al. (1993) is
46 MONTHLY WEATHER REVIEW VOLUME138
FIG.4.HorizontalmapofEvectorsobtainedbyan11-dayperiod
centeredon22Aug2000.ThedotrepresentsthecenterofTyphoon
Bilis.
used to remove the preexisting Typhoon Bilis and TC
Kaemi.Thesuccessiveapplicationofthissimplesmooth-
ing operator was applied to wind, geopotential height,
temperature,andspecifichumidityfieldsoftheNCEP–
NCAR reanalysis at a given time. Figure 5 shows that
the method successfully separates the basic field and
thedisturbance field.InEXP,we usethebasic fieldto
replace the total field, only in the box area shown in
Fig.5b.Outoftheboxarea,thetotalfieldisusedandis
the same as that in CTL. By doing so, we intentionally
remove the previous TCs (Bilis and Kaemi) but keep
theinitialwavetrain.Throughthecomparisonbetween
these two simulations, we intend to demonstrate how
thepreexistingTCmayinfluencethesubsequentgenesis
of a new storm. The model is integrated for 5 days,
foreachexperiment,withtheinitialtimeat0000UTC
22August.
4. Numericalresults
In CTL, the simulated track of the newly developed FIG. 5. (a) The separation of the large-scale analysis surface
storm,TCPrapiroon,isshowninFig.6aalongwiththe pressurefield(hPa)andwindfields(ms21)into(b)theenviron-
mentalfieldand(c)thedisturbancefieldatinitialtime0000UTC
observed best track from the Joint Typhoon Warning
22Aug.Thereddotin(c)representsthecenterofTCBilis.
Center(JTWC).Sincethereisonlyaweaktropicalde-
pression in EXP, we do not show its track here. The
simulatedtrackisdeterminedbasedonthecentroidof
wherepisthesealevelpressure(SLP),andthedouble
asolidbodydefinedas
integrals are calculated over an area with a radius of
750 km, centered on the minimum sea level pressure
1
(xc, yc)5 ðð (MSLP).InCTL,thenewlydevelopedTCPrapiroonis
p(x, y)dxdy located slightly to the north of the observed location.
Despite this discrepancy, the track of the simulated
(cid:2)ðð ðð (cid:3)
3 xp(x, y)dxdy, yp(x, y)dxdy , storm agrees reasonably well with the observation, be-
causeitcapturesmajorfeatures,suchasinitialwestward
JANUARY2010 GE ET AL. 47
To further illustrate their difference, we display the
evolutionoftheSLPpatternin CTL(Fig.7)andEXP
(Fig.8).Aweaktropicaldepressioninitiallydominates
around the genesis area in both CTL and EXP. At
0000UTC23August,severalisolatedmesoscalevorti-
ces,withrelativelyhighvaluesofvorticity,areembed-
dedinthegenesisareainCTL.Thesevorticesgradually
intensify, move closer, and eventually merge together.
The results suggest that vigorous convective cells de-
velopduringthisperiodandaremarkedbymergingand
theinteractionofanumberofMCSswithinthegenesis
region.Thesimulatedprecipitationsareprimarilycon-
centratedinthecycloniccirculationregions(notshown).
SuchamesoscaleprocessisevidentintheTropicalRain-
fallMeasuringMission(TRMM)rainfallpatternduring
thegenesisperiod,asillustratedinFig.2.Thesimulation
inCTL(Fig.7)bearsacrudesimilaritywiththeTRMM
rainfallpattern.
ThedevelopmentandintensificationofMCSsarean
integralcomponentofthetropicalcyclogenesis.Figure9
displays the evolution of the relative vorticity field in
thegenesisareaofPrapiroonduringa24-hperiodfrom
1200 UTC 25 to 1200 UTC 26 August. In CTL, two
distinct cyclonicvortices, identifiedas vortexA andB,
respectively, rotate cyclonically about each other. As
vortex A becomes dominant and subsequently evolves
FIG.6.(a)BesttrackfortyphoonPrapiroonfromJTWC(with intothecoreoftheTC,vortexBisshearedintoamajor
hurricanesymbol)andsimulatedtrackinCTL.Thetracksstartat rainband;thisprocesshasbeenextensivelyinvestigated
1800UTC24Augwith6-htimeinterval.(b)Timeevolutionsof (e.g.,LanderandHolland1993;Kuoetal.2004).Itoc-
MSLP(hPa)inobservation(thickdashedline),CTL(solidline),
cursbyareversecascadeinwhichalarger,moreintense
andEXP(thindot–dashedline)areshown.Thehurricanesymbol
vortexexertsashearingdeformationonnearbypertur-
representsthewarningtime.
bations, which are eventually absorbed or merged to
producealargersystem.
moving and then northward turning. The simulated Inasharpcontrast,muchweakerconvectiveactivities
TC intensity change, in CTL, also agrees reasonably occurinthecyclonicwakeregioninEXP(Fig.8)andno
wellwiththeobservation(Fig.6b).Thisdifferssignif- clearvortexinteractionormergingcanbeidentified(see
icantly from that in EXP. The central pressure drops bottom panel in Fig. 9). Consequently, the simulated
much more slowly in EXP than in CTL. For instance, stormintensityinEXPismuchweakerthanthatinCTL
at 1200 UTC 26 August, the MSLP is only about (Fig.6).Giventheremarkabledifferenceinthestrength
1004 hPa in EXP, which is about 15 hPa weaker than ofconvectiveactivityinthewakeregionbetweenCTL
thatinCTL. andEXP,howisitthattheexistenceofthepreviousTC
Previousstudies(e.g.,LiandFu2006;Geetal.2008) modulatesthelarge-scaleenvironmentalconditionand
showed thatTC energydispersionissensitive toits in- convective activities in the wake? We will address this
tensity.WhenaTCisweak,theassociatedRossbywave questioninthefollowingsection.
trainishardlydiscerned.AftertheTCisstrengthened,
theRossbywavetrainbecomesmuchclearerandcanbe
5. RoleofapriorTConthedevelopment
well detected. Note that during the period concerned,
ofanewTC
themaximumwindofTSKaemiisonly45kt(23 m s21),
whileBilisreaches140 kt(72 m s21).TheEvectorsin The influence of a previous typhoon on the sub-
Fig. 4 also show that the wave energy propagation is sequentTCgenesismaybearesultofbothadirectand
mainlyfromTCBilis.Therefore,thedifferencebetween anindirectprocess.Thedirectinfluencestemsfromthe
CTLandEXPisprimarilyattributedtotheinfluenceof continuous energy dispersion of the previous typhoon,
TCBilis. which strengthens the low-level Rossby wave train so
48 MONTHLY WEATHER REVIEW VOLUME138
FIG.7.Theevolutionofsurfacepressurefields(contour;hPa)and850-hParelativevorticity(z$231025s21;
shaded)inCTL.
thatthecyclonicvortexinthewakemaybeservedasthe trainbecauseofthecuttingoffoftheenergysource,which
precursor of the subsequent typhoon. Both numerical weakens the cyclonic circulation–moisture–convection
simulations (Li et al. 2006; Ge et al. 2008) and obser- feedbackinthewakeandthusslowsdownthesubsequent
vational analyses (e.g., Li and Fu 2006) reveal a close cyclogenesisprocess.
relationship between cloud liquid water and vorticity Theindirectinfluenceispossiblythroughtheeffectof
inthewake—thatis,thecyclonic–anticyclonic–cyclonic theintenseoutflowjetoftheprevioustyphoon.Thein-
wavetrainpatternthatcorrespondswelltothecloudy– tense asymmetric outflow jet results from the Rossby
clear-sky–cloudypattern.Thisfeatureisclearlyseenin wave energy dispersion in the upper level and is a key
Fig. 2. This in-phase relationship implies a positive component of the upper wave train (Ge et al. 2008).
feedback between the low-level cyclonic flow and the Because of the weaker inertial instability in the upper
diabaticheating.The cyclonicvorticity strengthensthe troposphere, the outflow jet favors a larger horizontal
boundarylayerconvergencethroughtheEkman-pumping extension.Asaresult,theareaextentofthehurricane’s
effect,whichfurtherleadstotheincreaseintheboundary influence is much larger in the upper level than in the
layer moisture anda convectively unstablestratification. lowerlevel(RossandKurihara1995;WuandKurihara
Theremovalofthepreexistingtyphoon,TyphoonBilis,in 1996).Analogoustothedynamicsofthemidlatitudejet
EXP,slowsdowntheintensificationoftheRossbywave streak,anintenseTCoutflowjetmayinduceasecondary
JANUARY2010 GE ET AL. 49
FIG.8.AsinFig.7,butforEXP.
circulation, with an enhanced ascending (descending) clearintheverticalcrosssectionofDNBE(Fig.10b),with
motion occurring to the left (right) side of the jet-exit alargeareaofpositive(negative)valuesbeinglocatedin
region(Shietal.1990).Toillustratethismechanism,we the cyclonic (anticyclonic) shear side. For comparison,
calculate the residual term of the nonlinear balance wealsocalculateDNBEinEXPandfindnosignificant
equationas(Zhang2004)DNBE52J(u,y)1fz2=2F2 divergencetendency.Here,DNBEappearsintheupper
bu,whereu,y,f,z,andFrepresentthezonalandme- troposphere,whichisconsistentwiththefactthatthere
ridionalvelocities,Coriolisparameter,relativevorticity, isnoclearlydefinedoutflowjetduetotheremovalofTC
andgeopotentialheight,respectively.Theresidualterm Bilis(Fig.11).
DNBE, to the first order of approximation, may be re- The model-simulated 150-hPa divergence fields on
gardedasthetendencyofthehorizontaldivergence,and 24 August are presented in Fig. 12. In CTL, much
its magnitude quantifies thedeviation of the flowfrom strongerupper-leveldivergenceactivitiesprevailtothe
anonlinearbalancestate.AsshowninFig.10a,aposi- leftofthejet-exitregion(i.e.,theregionwithintheblack
tive DNBE (divergent tendency) is located at the cy- circle where the subsequent TC genesis occurs). Con-
clonicshearsideofawell-developedupper-leveloutflow versely,theupper-leveldivergenceismuchweakerand
jetcoreandanegativeDNBE(convergenttendency)isat lessvigorousconvectiveactivitiesappearinthegenesis
the anticyclonic shear side. This asymmetric pattern is region in EXP. The disparities are also evident in the
50 MONTHLY WEATHER REVIEW VOLUME138
FIG.9.Theevolutionof850-hPavorticityfields(131025s21andz$131024s21;shaded)in(top)CTLand(bottom)
EXPduringa24-hperiod.
midtropospheric vertical motion and moisture fields found that an upper-level intense and organized eddy
(figurenotshown).ThedifferencebetweentheCTLand fluxconvergence(EFC)ofrelativeangularmomentum
EXPexperimentsislikelyascribedtothefollowingtwo is crucial for rapid intensification of storms. Molinari
processes. The first process is through the use of the andVollaro(1989,1990)alsofoundtheroleofEFCin
conventional barotropic Rossby wave energy disper- the reintensification of Hurricane Elena (1985). These
sion, by which the low-level Rossby wave train is en- previousresultsmotivateustoinvestigatetheEFCterm
hanced as well as the boundary layer convergence and (cid:2)(1/r2)›(r2u9y9)/›rinbothexperiments,whereu isthe
L L L
the convection–circulation feedback. The second pro- storm-relative radial velocity, y is the storm-relative
L
cess is through the upper-level divergence associated tangentialvelocity, ris theradius, the overbardenotes
withtheimbalanceoftheoutflowjet.Theso-generated theazimuthalmean,andtheprimeindicatesthedevia-
large-scale ascending motion and low-level moisture tionfromtheazimuthalmean.
convergencesetupafavorableenvironmentalcondition Figure13showstheevolutionoftheupper-levelEFC
forthemesoscaleconvectiveactivity.Theconvectively fromthesetwoexperiments.InCTL,therewasageneral
generatedmesoscalevorticesmaymergeintoaTC-scale trendofaninwardshiftofanareawithEFCgreaterthan4
vortex.Thus,boththelower-levelRossbywaveenergy ms21day21.AmaximumEFCoccurredintheouterre-
dispersion and the upper-level outflow jet may signifi- gion(about1000kmfromthenewTCcenter)at0000UTC
cantlyaffectcyclogenesisinthewakeofapriorTC. 24Augustanditsvaluereachedabout24 m s21 day21.
BycomparingthecompositeAtlanticdevelopingand ThemaximumEFCgraduallyshiftedinward,andreached
nondeveloping tropical disturbances, McBride (1981) itsmaximumof32 ms21 day21at1800UTC24August.