Spatial-TemporalChangesofOceanWavesGeneratedbyExplosiveCyclones

YukiKita*,Takuji Waseda,Adrean WebbTheUniversityofTokyo

GraduateSchoolofFrontierSciences*JSPSFellowship(DC1)

1stInternationalWorkshoponWaves,StormSurgesandCoastalHazards@Liverpool2017

SupportedbyJSPSKAKENHIGrantNumber17J07627

WavesunderExplosiveCyclonesnAnextratropicalcycloneisgeneratedinhighbaroclinicityareasinmid-latitudezones.Whenitisintensifiedrapidly,whichiscalled“ExplosiveCyclones(ExpC)” (e.g.SandersandGyukum,1972).

nLikeatropicalcyclone(TC),itcangenerateextremelyhighwaves,resultinginseveredisasters.

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OceanwavesgeneratedbyExpC atHokkaido,onmiddleDecember,2014(AsahiShimbun) Original:JapanMeteorologyAgency

Processedin“DigitalTyphoonDatabase”byNII

PreviousstudiesnForallquadrantsofthehurricane,withtheexceptionoftherightrearquadrant,thespectraaredominatedbyswell(Young2005).

nMori(2012)investigatedfreakwavesunderidealTCconditions.FreakwavesresultingfromnonlinearwaveinteractionarefavorableintherearrightquadrantofTC.

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Significantwaveheights(shade)andsurfacewind(contour)underanidealTCcondition(Fig.1,Mori2012)

TC

HighWave

NarrowSpectrum

SpecificationsofExtratropicalCyclonesnUnlikeTCs,extratropicalcycloneshaveanasymmetricstructureinwinds.Extratropicalcycloneshascoldandwarmfronts,whichtransformswiththeirdevelopments.

nExtratropicalcyclonesgenerallymovemorequicklythanTCs.

nExpCs developmorerapidlythannormalones(e.g.24hPa/hrs drop)

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Tounderstandspatial-temporalchangesofoceanwavesunderExpCs anditsdevelopmentmechanismcomparingwithTCcases,weimplementedandanalyzednumericaloceanwavesimulationaroundJapanduring1994-2014.

Methodsü WAVEWATCHIIIfor20yearsaroundJapanü CompositeanalysisofoceanwavesunderhundredsofExpCsü SpectralanalysisofoceanwavesinoneExpC onNov2005

OceanWavesHindcastSimulation:TodaiWW3nWAVEWATCHIIIhindcastsimulationaroundJapanfor21-yrs:1994-2014(Webbetal.2016;Waseda etal.2016).

n Itwasvalidatedwithcoastalobservations.

n DataAccess:http://www.todaiww3.k.u-tokyo.ac.jp/nedo_p/en/

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TodaiWW3 PacificModel OffshoreModel

Spatial Resolution lat0.6°×lon0.75° lat0.2°×lon0.25°

Domain 101°𝐸: 70°𝑊75°𝑆: 75°𝑁

110°𝐸: 170°𝐸10.2°𝑁: 60°𝑁

Spectral Resolution 35frequencies (0.04118~1.05Hz)36directions

Forcing Thewindandseaice(CFSR)Nooceancurrent

Wind-Wave Interaction ST4(Ardhuin etal.2010)

OffshoreModel

ExampleoftheSimulation:2013Jan

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CyclonesTrackingnAtmosphericReanalysisData:NCEPCFSR

lPeriod:1994~2014lArea:EasternAsia,NorthPacific(100°E:160°W,10°N:70°N)

nBasedonthemethodinKyushuUniversitycyclonedatabase*,alocalminimuminMSLP (MeanSeaLevelPressure)istracked.

nTCs:Cyclonesemergesinthesouthernof25°N

n DevelopmentrateofExpC 𝜀 > 1; 𝜀 = 9:;<=9:><?@

ABCDE°ABCF:

[hPa/hours]

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AnnualAverageNumber

AllCyclones： 539.0

TropicalCyclones： 27.2

ExplosiveCyclones： 38.6

ExplosiveCyclones(Japan)： 19.2

JapanArea

TrackingArea

*http://fujin.geo.kyushu-u.ac.jp/meteorol_bomb/index.php

𝑝:MSLP𝜙:latitude

CompositeAnalysis

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Zoneforcompositeanalysis

Aextratropicalcycloneiscompositedwhenithas980hPainthiszone

TodaiWW3OffshoreModel

PropagatingdirectionHowtocomposite

Radius:10°(lat,lon)

CompositeAnalysis

9

Zoneforcompositeanalysis

Aextratropicalcycloneiscompositedwhenithas980hPainthiszone

TodaiWW3OffshoreModel

PropagatingdirectionHowtocomposite

Radius:10°(lat,lon)

OceanWavesSpecificationsunderExpCs andTCs

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Hs

Windh=10m

TC ExpC

n ExpCs havelargerareaofhighHsthanTCs.

n PropagatingspeedofExpCsishigherintheirearlystagethanthatofTCs.

N=45 N=91

*+0h:thetimewhenMSLPisatthedeepest

0

5

10

15

20

-36h -30h -24h -18h -12h -6h +0h +6h +12h+18h

speed[m

/s]

PropagatingSpeed

ExpC TC

WindSpeedat10m[m/s]

Hs[m]

OceanWavesDevelopmentunderExpCs

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10° 10° 10°

10° 10° 10° 10°

10°

10° 10° 10°

10° 10° 10° 10°

10°

-24h -12h +0h +12hHs

Windh=10m

n AsanExpC develops,windsandoceanwavesalsoenhance.n AfteranExpC beginstoweaken,oceanwavesstillcontinuetogrow.

Hs[m]

WindSpeedat10m[m/s]

OceanWavesSpecificationsunderExpCs andTCs

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n Peakedness factor𝑄9 isdefinedas(Goda,1970)

𝑄9 =2𝑚K@ L𝜔𝐸

@ 𝜔 d𝜔�

�n Directionalspreadisastandarddeviationofthedirectionalspectrum.

n TherightandfrontareasofExpCs havenarrowspectral.

DirectionalSpread

𝑸𝒑

TC ExpC

N=45 N=91

DirectionalSpread[°]

Peakedness Parameter

CaseStudy:2005NovExpC

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SignificantWaveHeight[m]

OceanWaves:1800,12Nov2005nWhentheMSLPofthisExpC reachedthedeepest,twoareashavenarrowspectrum:1. RightoftheExpC2. Frontof thewarmfrontline

nTamuraetal.(2009)showedthatcoexistenceofswellandwind-seawasaprecursortothedevelopmentofthenarrowspectrum.

14DirectionalSpread Peakedness Parameter

WaveSpectraat18:00

15

A

BC

HighWave

NarrowSpectrum

Hs[m]

WavePropagatingDirection

North

wind-sea

A

Direction[°]

Freq

uency[Hs]

wind-sea

B

Direction[°]

Freq

uency[Hs]

wind-seaswellswell

C

Freq

uency[Hs]

Direction[°]

ConceptualDiagramofOceanWavesunderExpCs

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① ②

③ ④

HighWave

NarrowSpectrum

SummarynHighwavezonesofExpCs isgenerallylargerthanthatofTCs.Bothofthemhavehighwavezonesattheright tothecenter.

nAfteranExpC beginstoweaken,oceanwavesstillcontinuetogrow.

nWhenanExpC sufficientlydevelops,rightzonestothecenterandfrontzonesofthewarmfronthavenarrowspectrum,whichisfavorableforfreakwaves.

nForthenextstep,aoceanwavesimulationwithanidealExpCconditionshouldbeconducted.

17HighWave

NarrowSpectrum

TC ExpC

HighWave

NarrowSpectrum