orographiccyclogenesis-fall2017steenburgh/classes/5210/lecture_notes… · orographic cyclogenesis...

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OrographicCyclogenesisandtheInfluenceofMountainsonExtratropical Cyclones

JimSteenburghDepartmentofAtmosphericSciences

[email protected]

EUMETSAT

LearningObjectives

• Afterthislecture,studentswill– Recognizeandunderstandhowmountainsaffecttheclimatologyandlife-cycleofextratropical cyclones

– Beabletodiagnosepast,current,andfuturecycloneevolutioninareasofcomplexterrain

– Haveanimprovedabilitytocriticallyevaluatescientificliteratureexaminingorographiccyclogenesis

Outline

• ClimatologicalContext• DynamicalMechanisms• AlbertaCyclogenesis• AlpineLeeCyclogenesis• IntermountainCyclogenesis

ClimatologicalContext

GlobalTopography

HimalayaAlps

Greenland

Australia/NZ

Antarctica

JanuaryClimo (1981–2010)

250-mbwindspeed

DataSource:NCEP/NCARReanalysis,ESRL

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JanuaryClimo (1981–2010)


250-mbwindspeedand850-mbtransientmeridional momentumflux

Julyclimo (1981–2010)

250-mbwindspeed


Julyclimo (1981–2010)

250-mbwindspeedand850-mbtransientmeridional momentumflux


NHCyclogenesis/Lysis Density

Alps/Genoa

HimalayaAlbertaColorado

Intermountain?Greenland

HoskinsandHodges(2002),ERA-15&ECMWFOperationalAnalyses1979–2000(T42~300km)Annualstatisticsfor“feature-tracked”MSLPcyclones

SHCyclogenesis/Lysis Density

HoskinsandHodges(2005),ERA-401958–2002(T159~125km)JJAstatisticsforξ850 cyclones

GenesisLysis

Andes

NZAlps GreatDividingRange

AntarcticPeninsula

JanuaryNAClimo

Zishka andSmith(1980),NOAAmonthlycyclonetrackmaps1950–1977

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JulyNAClimo

Zishka andSmith(1980),NOAAmonthlycyclonetrackmaps1950–1977

Mesoscale Effects

Chungetal.(1976),Cyclogenesis Frequencyduring1958

ClassDiscussion

Petterssen (1956)WinterCycloneFrequency Zishka andSmith(1980)JanCycloneEvents

Reitan (1974)AprCycloneFrequencyWernli andSchwierz (2006)DJFCycloneFrequency

AreIntermountaincyclonesastatisticalphantomorartifactofMSLPreduction?Howdoweexplainthesedisparateresults?

CycloneDensity IC-Genesis

ERA

NARR

IntermountainCyclones

Jeglum etal.(2010)

Seasonality

Jeglum etal.(2010)

Seasonality

Jeglum etal.(2010)

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ClimateSummary

• Mountainshaveaprofoundinfluenceonstormtracksandcyclonestatistics– Frequentleecyclogenesis– Frequentwindwardcyclolysis– Apparent“discontinuous”or“masked”stormtracksacrossbarriers

• Statisticsvarydependingonreanalysischaracteristics(e.g.,gridspacing),identificationtechniques,andseason

DynamicalMechanisms

OrographyandCyclones• Windwardcolumncompressioncontributestoacquisitionof

anticyclonic absolutevorticity

• Leewardcolumnstretchingcontributestoacquisitionofcyclonicabsolutevorticity

• Theseeffectsare“superimposed” onthelarge-scaleforcing– Bestcaseforleecyclogenesis iswhenmountain-inducedcolumn

stretchingoccursinconcertwithsynopticconditionsfavorableforcyclogenesis• e.g.,500mb CVA,localmaximuminwarmadvection,condensationalheating

– Almostallcasesofleecyclogenesis areassociatedwithapre-existingsynoptic-scaletroughorcyclone

TheoreticalModels• Viewleecyclogenesis astheresultoftheinteractionofasynoptic-scale

troughorcyclonewithamountainridge(e.g.,Tibaldi etal.1990;Bannon1992)

• Observedcycloneevolutionresultsfromsuperpositionof– Agrowingbaroclinic wave(a.k.a.,theprimarybaroclinic wave)

– Secondarytopographiceddiesproducedbytheinteractionoftheprimarybaroclinic wavewiththetopography

• Theprimarybaroclinic wavewouldexistandgrowevenintheabsenceoftopography

• Secondaryeddiesalterthestructure,growth,andtrackoftheprimarybaroclinic wave

Conceptualmodel

PrimaryCycloneTopographicEddies

Bannon (1992)

QGCycloneEvolution

L

L

L

L

L

L

L

L

L

L

L

L

Superpositionofprimarycycloneandtopographiceddiesresultsin“amoeba-like”movementofcycloneacrossamountainridge

Contourintervalsnormalizedbymaximumpressureanomalyateachtime

Bannon (1992)

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QGCycloneEvolution

• Lowdeflectedtonortheastasitapproachesmountains• Growthrateslowsandbecomesnegative(mountainiscyclolytic)• Leetroughdevelopsascycloneimpingesonmountain• Lowappearstoleeofmountains,equatorward ofitsoriginallatitude,andisnot

traceableupstream(discontinuousprogression)• Enhancedgrowthratetothelee• Lowbrieflymovessoutheastbeforemovingnortheast

CycloneTrack

MountainProfile

GrowthRate

Topo/NoTopo

Bannon (1992)

ClassDiscussion

L L L

Whathappensasaprimarycyclonetraversesanisolated,“Alps-like”zonallyorientedbarrier?

Note:Barrierishighestinthemiddle

Eady ModelSolution

• Upstreamflowunperturbed• Pressuredipoleismaximizednearmountain• Low/highpressurecenterssplitaroundmountain;poleward centerstrongest

initially,thenequatorward centerbecomesdominant

Speranza etal.(1985),Buzzi andSperanza (1986)

OrographicStreamfunction TotalStreamfunction

MechanismsSummary• Orographiccycloneevolutioncanbeviewedasthesuperpositionofaparent

cycloneandtopographicpressureperturbationsgeneratedbyitsinteractionwithorography

• Thissuperpositionresultsinthe“amoeba-like”movementofcyclonesacrosstheRockies

• ForanisolatedbarrierliketheAlps,growthrateislessstronglyinfluenced,butcyclonestructureisdistorted

• Advantage:theconceptualizationcanbegeneralizedtoanumberofdifferentflowconditionsandmountaingeometries

• Caveats• ActualgrowthratesaremuchstrongerthansimulatedbyQG(Eady-type)models• Theorydoesnotfullyaccountforsteeporography,diabatic effects,nonlinearities,etc.

AlbertaCyclones

SynopticCharacteristics

Tracks LowestCentralMSLP

ThecentralMSLPis≤990mb only7%ofthetimeOnly28%reach≤992mb [arbitrary“strongcyclone”threshold(AngelandIsard 1997)]

ThomasandMartin(2007)

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CompositeHr -12

Palmén andNewton(1969),ThomasandMartin(2007),SteenburghandMass(1994)

Hr 0

L L

X X

FrontalcycloneapproachescoastStationaryleetroughdevelopsdueto

tocrossbarrierflow&vortexstretching

Leetroughandwarmanomalyamplify&broaden

Cyclogenesis asCVAmovesintolee

CompositeHr 12 Hr 24

L L

X X

CessationofdownslopeflowOngoingdeepeningduetosynopticforcing

Leetroughbroadensinscaleasairwarmedbydescentspreadseastward

Northernportionofleetroughmoveseastward

SouthernportionremainsfixedtoRockies

Palmén andNewton(1969),ThomasandMartin(2007),SteenburghandMass(1994)

CaseStudy

L0000UTC4March1986 1800UTC4March1986

SteenburghandMass(1994)

Confluentfrontogenesisintensifiespre-lee-troughbaroclinic zonethatseparatesdescendedPacificairfromcoldercontinentalair

LL

CaseStudy0600UTC5March1986 1200UTC5March1986

SteenburghandMass(1994)

Cold-advectionoverrunsleetrough,formingathermalridgeresembling

awarmocclusion

Resultingfrontalstructureis“nonclassical”

AlpineLeeCyclogenesis

Europeangeography

Map:WikipediaCommons

Alps

Pyrenees

Apennines

DinaricAlps

GulfofGenoa

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EuropeanGeographyTerrainmapcourtesy:RonSmith,YaleUniv.

Geneva

Zurich

Milan

TurinVenice

Innsbruck

Mt.Blanc(4807m)

Mt.Rosa(4634m)

Jungfrau(4368m)Wildspitze(3772m)

Grossglockner(3,798m)

PizBernina(4049m)

100km

ClimatologyoftheMediteranean

• Duringwinter(Jan),MediterraneanSSTsrangefrom12-20˚C,roughly2-4˚Cwarmerthanthemeanairtemperature

• Mediterraneanrepresentsatime-averagedheatsourceduringthecoolseason,withthesurroundingregionexperiencingatemperateclimate

• MeanlowpressureovertheMediterraneanwithanestimatedamplitudeof5mb

NavalEuropeanMeteorologyandOceanographyCenter

WintermeanSLP(Reiter1975)

H H

HH L LL L

L

NavalEuropeanMeteorologyandOceanographyCenter,Reiter(1975),Buzzi andSperanza (1983)

CycloneClimatology

Nissen etal.(2010),ERA-40(1.25ºx1.25º)MSLPCyclonesOct–Mar1957–2002

TrackDensity(allcyclones)

Highwindeventsonly

Cyclogenesis

Highwindeventsonly

CycloneClimatology

• Mostare“AlpineLeeCyclones”areshallowthermaland/ororographiclows;

• Buzzi andSperanza (1983)claimonly5-6/yeardevelopintodeepcyclones• Tibaldi etal.(1990)claim10-20moderatetostrongeventsayear• Explosivedeepening(e.g.,SandersandGyakum 1980)israre(Tibaldi etal.

1990)• Pichler andSteinacker (1987)describetwotypesofAlpineleecyclones

• Vorderseiten type,associatedwithsouthwesterlyupper-levelflow• Überströmungs type,associatedwithnorthwesterlyupper-levelflow

• Bothtypesassociatedwithblockinganddistortionoflow-levelcoldfront

Vorderseiten type Überströmungs type

PichlerandSteinacker(1987)

GeneralCharacteristics• Mosteventsarenotpurelyorographic

• Cyclogenesis occursoccurswhen:• Anupper-leveltroughisupstreamoftheAlps• Alow-levelfrontalsystemimpingesontheAlps• Anupper-level“forcing”(e.g.,CVA,coherenttropopause disturbance,leftexitregion,etc.)movesoverthenorthernMediterranean

• Alpineleecyclonesaretypicallysmallerinscalethantraditionalmidlatitude cyclones

• CanbeaccompaniedbytheMistral(France),Foehn(Austria),orBora(Italy,Slovenia,Croatia),andcanbefollowedbyaMediterraneancold-airoutbreak

ClassicView:AntecedentStage

• Cyclone &upper-leveltrough/jetstreakintocentralEurope• TheaccompanyingcoldfrontapproachestheAlps• Cyclogenesis isinhibitedbeneaththeupper-leveltroughbycoldadvection

(weakQGverticalmotionatmidlevels)

SFC 500mb

Jet?

SFC:1200UTC2Apr1973 500mb:1200UTC2Apr1973

Buzzi andTibaldi (1978)

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ClassicView:AntecedentStage


SFC:1200UTC2Apr1973

ClassicView:TriggerStage

• ColdfrontisblockedanddistortedbyAlps,resultinginfrontogenesisandsteepeningofthefrontalslope

• Positivethermalanomalyformsinleeatlowlevels(adiabaticwarming)• InitialdepressionappearsinleeofAlpsaheadof,noton,thecoldfront

SFC

SFC:0000UTC3Apr1973 SectionCD:0000UTC3Apr1973


ClassicView:TriggerStage

• Cyclonedeepenswhileremainingquasistationary• Upper-leveltroughfillsnorthofalps,butdeepenstothe

south• JetsplitsnorthwestofAlps

SFC 500mb

SFC:1200UTC3Apr1973 500mb:1200UTC3Apr1973


ClassicView:Baroclinic Stage

• Frontandverticalcirculation“reunite”andbecomestructurallycontinuous(note:nocrosssectionavailablefor0000UTC4Apr)

• Moretraditionalbaroclinic developmentoccurs• CyclonemovesawayfromAlpswithacharactersimilartoatraditional

frontalcyclone

SectionEF:1200UTC3Apr1973 SFC:0000UTC4Apr1973


PVView

• Upper-leveltroughandembeddedCTDmoveacrossandinitiatecyclogenesis overGulfofGenoa– C=CTD– P=PredecessorCyclone– S=PVBanner

McTaggert-Cowanetal.(2010a,b)

DTThetaandWind925–850-mbmeanrelativevorticity

LMistral

BlockedColdFront

PVView

McTaggert-Cowanetal.(2010a,b)

Leewardthetaanomalygeneratedbycold-frontalretardation,heavyprecipitationanddiabatic heatingonwindwardsideofAlps,andadiabaticdescent

EventuallyPVbannerprovidesanadditionalcontribution

LMistral

BlockedColdFront

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IntermountainCyclones

Composite SW Flow Event

Intermountaincyclogenesis isprecededbythedevelopmentoftheGBCZdownstreamofHighSierra

Jeglum etal(2010)

Composite SW Flow Event

GBCZservesaslocusforcyclogenesisasascentassociatedwithupper-level

troughmovesintoleeofSierra

Jeglum etal(2010)

Tax Day Storm

WestandSteenburgh(2010)

Tax Day Storm


Tax Day Storm


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Tax Day Storm


Summary• Mountains have a profound effect on the genesis, lysis, track, and

evolution of cyclones

• Geographic variations in the structure and evolution of orographic cyclones arise from unique regional climate and topographic characteristics

• Itcanbehelpfultovieworographiccycloneevolutionasthesuperpositionofaparentcycloneandtopographicpressureperturbationsgeneratedbyitsinteractionwithorography

• PV thinking may be applicable to a wider range of events given the broad spectrum of large-scale environments in which orographic cyclogenesis occurs, but has only been utilized by a few authors (e.g. McTaggert-Cowen et al. 2010a,b)

ClassDiscussion

• WhyareIntermountaincyclonesdeeperandmorefrequentinthespring?• WhatdynamicalprocessesmightcontributetothedevelopmentoftheGBCZ?• TheSierramatter,buthowmightotherterrainfeaturesinandsurroundingthe

GreatBasincontributetoIntermountaincyclonedevelopmentandevolution?• HowcouldweusePVthinkingtobetterunderstandIntermountaincyclogenesis?

Jeglum etal.(2010)

ReferencesBannon,P.R.,1992:AmodelofRockyMountainleecyclogenesis.J.Atmos.Sci.,49,1510-1522.

Bluestein,H.B.,1993:Synoptic-DynamicMeteorologyinMidlatitudes,Vol.II.OxfordUniversityPress,608pp.

Buzzi,A.,andA.Speranza,1983:Cyclogenesis intheleeoftheAlps.Mesoscale Meteorology:Theories,Observations,andModels,D.K.LillyandT.Gal-Chen,Eds.NATOASISeries,Reidel,55–142.

Buzzi,A.,andA.Speranza,1986:Atheoryofdeepcyclogenesis intheleeoftheAlps.PartII:Effectsoffinitetopographicslopeandheight.J.Atmos.Sci.,43,2826-2837.

Buzzi,A.,andS.Tibaldi,1978:AcasestudyofAlpineleecyclogenesis.Quart.J.Roy.Meteor.Soc.,104,271-287.

Chung,Y.-S.,K.D.Hage,andE.R.Reinelt,1976:Onleecyclogenesis andairflowintheCanadianRockyMountainsandtheEastAsianMountains.Mon.Wea.Rev.,104,879-891.

Hoskins,B.J.,andK.I.Hodges,2002:NewperspectivesontheNorthernHemispherewinterstormtracks.J.Atmos.Sci.,59,1041-1061.

Hoskins,B.J.,andK.I.Hodges,2005:AnewperspectiveonSouthernHemispherestormtracks.J.Climate,18,4108-4129.

Jeglum,M.E.,W.J.Steenburgh,T.P.Lee,andL.F.Bosart,2010:Multi-reanalysisclimatologyofIntermountaincyclones.Mon.Wea.Rev.,138,4035-4053.

Mattocks,C.,andR.Bleck,1986:Jetstreakdynamicsandgeostrophicadjustmentprocessesduringtheinitialstagesofleecyclogenesis.Mon.Wea.Rev.,114,2033-2056.

McGinley,J.,1982:AdiagnosisofAlpineleecyclogenesis.Mon.Wea.Rev.,110,1271-1287.

McTaggart-Cowan,R.,T.J.Galarneau,Jr.,L.F.Bosart,andJ.A.Milbrandt,2010a:DevelopmentandtropicaltransitionofanAlpineleecyclone.PartI:Caseanalysisandevaluationofnumericalguidance.Mon.Wea.Rev.,138,2281-2307.

McTaggart-Cowan,R.,T.J.Galarneau,Jr.,L.F.Bosart,andJ.A.Milbrandt,2010b:DevelopmentandtropicaltransitionofanAlpineleecyclone.PartII:Orographicinfluenceonthedevelopmentpathway.Mon.Wea.Rev.,138,2308-2326.

ReferencesNissen,K.M.,andCoauthors,2010:CyclonescausingwindstormsintheMediterranean:Characteristics,trends,andlinkstolarge-scalepatterns.Nat.HazardsEarthSyst.Sci.,10,1379-1391.

Palmén,E.,andC.W.Newton,1969:AtmosphericCirculationSystems.AcademicPress,603pp.

Pichler,H.andR.Steinacker,1987:Onthesynoptics anddynamicsoforographically inducedcyclonesintheMediterranean.Met.Atmos.Phys.,36,108-117.

Reiter,E.R.,1975:HandbookforforecastersintheMediterranean.Tech.PaperNo.5–75,NavalPostgraduateSchool,Monterey,CA,344pp.

Sanders,F.,andJ.R.Gyakum,1980:Synoptic-dynamicclimatologyofthe“bomb”.Mon.Wea.Rev.,108,1589-1606.

Speranza,A.,A.Buzzi,A.Trevisan,andP.Malguzzi,1985:Atheoryofdeepcyclogenesis intheleeoftheAlps.PartI:Modificationsofbaroclinic instabilitybylocalizedtopography.J.Atmos.Sci.,42,1521-1535.

Steenburgh,W.J.,andC.F.Mass,1994:ThestructureandevolutionofasimulatedRockyMountainleetrough.Mon.Wea.Rev.,122,2740-2761.

Thomas,B.C.,andJ.E.Martin,2007:AsynopticclimatologyandcompositeanalysisoftheAlbertaClipper.Wea.Forecasting,22,315-333.

Tibaldi,S.,A.Buzzi,andA.Speranza,1990:Orographiccyclogenesis.Extratropical Cyclones:TheErikPalmén MemorialVolume.C.W.NewtonandE.O.Holopainen,Eds.,Amer.Meteor.Soc.,107-127.

Wernli,H.,andC.Schwierz,2006:SurfacecyclonesintheERA-40dataset(1958–2001):PartI:Novelidentificationmethodandglobalclimatology.J.Atmos.Sci.,63,2486-2507.

West,G.L.,andW.J.Steenburgh,2010:Lifecycleandmesoscale frontalstructureofanIntermountaincyclone.Mon.Wea.Rev.,138,2528-2545.

Zishka,K.M.,andP.J.Smith,1980:TheclimatologyofcyclonesandanticyclonesoverNorthAmericaandsurroundingoceanenvironsforJanuaryandJuly,1950–77.Mon.Wea.Rev.,108,387-401.

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