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OrographicCyclogenesisandtheInfluenceofMountainsonExtratropical Cyclones
JimSteenburghDepartmentofAtmosphericSciences
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)
DataSource:NCEP/NCARReanalysis,ESRL
250-mbwindspeedand850-mbtransientmeridional momentumflux
Julyclimo (1981–2010)
250-mbwindspeed
DataSource:NCEP/NCARReanalysis,ESRL
Julyclimo (1981–2010)
250-mbwindspeedand850-mbtransientmeridional momentumflux
DataSource:NCEP/NCARReanalysis,ESRL
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
Buzzi andTibaldi (1978)
SFC:1200UTC2Apr1973
ClassicView:TriggerStage
• ColdfrontisblockedanddistortedbyAlps,resultinginfrontogenesisandsteepeningofthefrontalslope
• Positivethermalanomalyformsinleeatlowlevels(adiabaticwarming)• InitialdepressionappearsinleeofAlpsaheadof,noton,thecoldfront
SFC
SFC:0000UTC3Apr1973 SectionCD:0000UTC3Apr1973
Buzzi andTibaldi (1978)
ClassicView:TriggerStage
• Cyclonedeepenswhileremainingquasistationary• Upper-leveltroughfillsnorthofalps,butdeepenstothe
south• JetsplitsnorthwestofAlps
SFC 500mb
SFC:1200UTC3Apr1973 500mb:1200UTC3Apr1973
Buzzi andTibaldi (1978)
ClassicView:Baroclinic Stage
• Frontandverticalcirculation“reunite”andbecomestructurallycontinuous(note:nocrosssectionavailablefor0000UTC4Apr)
• Moretraditionalbaroclinic developmentoccurs• CyclonemovesawayfromAlpswithacharactersimilartoatraditional
frontalcyclone
SectionEF:1200UTC3Apr1973 SFC:0000UTC4Apr1973
Buzzi andTibaldi (1978)
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
WestandSteenburgh(2010)
Tax Day Storm
WestandSteenburgh(2010)
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Tax Day Storm
WestandSteenburgh(2010)
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.
