norwegian cyclone model - university of...
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TheNorwegianCycloneModelandExtensions
JimSteenburghUniversityofUtah
Reading:Lackmann Section5.4;Bjerknes andSolberg(1922),Godske etal.(1957,p.526-537),
Reed(1990)
WhatistheNorwegianCycloneModel?
• Conceptualmodeldescribingthelifecycleanddynamics ofextratropical cyclones
• DevelopedbytheBergenSchoolofMeteorologyafterWorldWarI
• Definedmodernmeteorologicalanalysis
• Stillwidelyusedtoday
Bergen,Norway
Sources:GoogleMaps,WikipediaCommons
BergenSchool
Sources:NOAA,AGU,http://www.snl.no/meteorologi,Time,WikipediaCommons
Vilhelm Bjerknes1862-1951
JacobBjerknes1897-1975
Halvor Solberg1895-1974
TorBergeron1891-1977
Carl-GustavRossby1898-1957
Sverre Petterssen1898-1974
ErikPalmén1898-1985
Svien RosslandAstrophysicist1894-1985
CarlGodske1897-1975
JohanSandströmOceanographer1874-1947
AlsoErikBjörkdal
Vilhelm Bjerknes andAssistants1917-1926
BergenSchool
Source:Shapiro(1994),TheLifeCyclesofExtratropical CyclonesCommemorativePhotoAlbum Source:www.snl.no/meteorologi
LefttoRight:TorBergeron,Halvor Solberg,Vilhelm Bjerknes,Harald SverdrupJacobBjerknes,Sverre Petterssen,CarlGodske
BergenSchool
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BergenSchool
Source:Shapiro(1994),TheLifeCyclesofExtratropical CyclonesCommemorativePhotoAlbum
TorBergeronandJacobBjerknes
RecommendedReading
Bjerknes andSolberg(1922) Godske etal.(1957) Reed(1990)
InitialDescription:IdealCyclone• Twoairmasses (warmand
cold)separatedbyafairlydistinctboundarysurfacethatrunsthroughthecenterofthesystem
• Theboundarysurfaceisimaginedtocontinuethroughagreaterpartofthetroposphereatasmallangletothehorizon
ColdSectorOrPolarAirmass/Airstream
ColdSectorOrPolarAirmass/Airstream
Zonalsectionthroughcoldsectornorthoflowcenter
Zonalsectionthroughwarmsectornorthoflowcenter
Source:Bjerknes (1919),Bjerknes andSolberg(1922)
InitialDescription:IdealCyclone• Warmairinthewarm
sectorisconveyedbyaSWorWcurrent&ascendsthewedgeofcoldairaheadofthewarmfront,producingwarm-frontalprecipitation
• Theintrusionofcoldairfrombehindthesystemintothewarmsectorliftsthewarmairmass,producingcold-frontalprecipitation
ColdSectorOrPolarAirmass/Airstream
ColdSectorOrPolarAirmass/Airstream
Zonalsectionthroughcoldsectornorthoflowcenter
Zonalsectionthroughwarmsectornorthoflowcenter
Source:Bjerknes (1919),Bjerknes andSolberg(1922)
IdealizedCycloneLifeCycle• InitialPhase
– Twooppositelydirectedcurrentsofdifferenttemperatureareseparatedbyanearlystraightboundary
– Theboundarybeginstobulgetowardthecoldairattheplacewherethecyclonewillform
• OpenWave– Theamplitudeofthewarm
waveincreases– Coldairmovescyclonically
aroundthelowcenter– Warmsectornarrows
Source:Bjerknes andSolberg(1922)
InitialPhase
OpenWave
Seclusion Occluded
Death
IdealizedCycloneLifeCycle• SecludedPhase
– Coldfrontovertakeswarmfrontsouthoflowcenter
– Pieceofwarmsectorairiscutoff
• OccludedPhase– Remainingpartofwarmsector
isremovedfromsurface
• Maturity/Death– Occludedfrontdissipates– Cyclonebecomessymmetric
vortexofcoldair
Source:Bjerknes andSolberg(1922)
InitialPhase
OpenWave
Seclusion Occluded
Death
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VerticalEvolution
• OpenWavePhase– Twowedgesofcoldairapproacheachother
– Intermediatewarmsectorairislifted
– Transformspotentialtokineticenergy• Simplisticview
Source:Bjerknes andSolberg(1922)
VerticalEvolution
• OccludedPhase– Oncetwowedgeshavemetontheground,theupperwarmsectorislifteduntilwarmsectorhascooledadiabaticallytothetemperatureofitssurroundings
– Throughoutthisphase,cyclonegainskineticenergy
Source:Bjerknes andSolberg(1922)
EnergyTransformations• Essentialconditionforcycloneformationiscoexistence
ofwarm&coldairadjacenttoeachother
• Allcycloneswhicharenotyetoccludedhaveincreasingkineticenergy
• Soonafterocclusion,thecyclonebeginstofill
• Inlaterstages,cyclonebecomesahomogenousvortexofcoldairthatconsumesthepreviouslygeneratedkineticenergy
Source:Bjerknes andSolberg(1922)
OcclusionTypes
• ColdType– Formsifairbehindcoldfrontiscolderthanairaheadofwarmfront– Hascharacterofcoldfrontwithnarrowprecipitationzone
• WarmType– Formsifairbehindcoldfrontiswarmerthanairaheadofwarmfront– Hascharacterofwarmfrontwithbroadprecipitationzone
• Claimcoldtypeismostcommon
ColdType WarmType
Source:Bjerknes andSolberg(1922)
SecondaryColdFronts
Source:Bjerknes andSolberg(1922)
• Thecoldairmaycontainaseriesofsecondarycoldfrontsaccompaniedbyonlysmallcontrastsintemperatureandwind
• Theappearanceofastrongsecondarycoldfrontthatisstrongerthantheprimarycoldfrontindicatesareinforcementofthecyclone
SubsequentRefinements
Sources:NOAA,AGU,http://www.snl.no/meteorologi,Time,WikipediaCommons
Vilhelm Bjerknes1862-1951
JacobBjerknes1897-1975
Halvor Solberg1895-1974
TorBergeron1891-1977
Carl-GustavRossby1898-1957
Sverre Petterssen1898-1974
ErikPalmén1898-1985
Svien RosslandAstrophysicist1894-1985
CarlGodske1897-1975
JohanSandströmOceanographer1874-1947
AlsoErikBjörkdal
BergenSchoolmeteorologistsdidnotstopworkingin1922!
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OccludedCycloneRefinements
• Identifynewfeatures– Upper-coldfront
• Accompanieswarm-typeocclusions
– Bent-backocclusion• Extendsintopolarairstreambehindlow
– Falsewarmsector• Betweenbent-backocclusionandprimarycoldfront
Source:Godske etal.(1957)
LifeCycleRefinements
• AntecedentStage– SimilartoBjerknes andSolberg(1922)
• NascentStage– Newlyformedwavewithvelocitynearlyequaltothatofwarm-sectorairnearground
Source:Godske etal.(1957)
LifeCycleRefinements
• WaveCyclone– Furtherdevelopmentofcycloneandfrontalwave– Frontolysis occursalongcoldfrontnearlowcenter– Phaselagofupper-levelwaverelativetosurfacewave
Source:Godske etal.(1957)
LifeCycleRefinements
• Occludedcyclone– Coldfrontclimbswarmfrontandformsupper-coldfront– Pressuretroughformstorearofcycloneandrotatescyclonicallyaroundlowcenter
– Nearlow-center,abent-backocclusionmaycoincidewithtrough
Source:Godske etal.(1957)
LifeCycleRefinements
• Occludedcyclone– Moreremovedfromlowcenter,itmaybeanon-frontaltrough– Cycloneregenerationcanoccurifbent-backfrontislongerandstrongerthannormalandseparatespolarairmasses ofdifferingtemperature
Source:Godske etal.(1957)
MoreModificationsandExtensions
• FrontalStructure/Dynamics– Depictionofthepolarfrontasadiscontinuityseparatingtropicalandpolarairmasses isanoveridealization
– Upper-levelandsurface-basedfrontsmaybediscontinuousandhavedifferingdynamics
J
J
Source:KeyserandPecnick (1985),Reed(1990)
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MoreModificationsandExtensions
• FrontalStructure/Dynamics– Surface-basedfrontsmayhaveextremeintensityattheground,butweakenwithheight
Source:Sanders(1955),Reed(1990)
MoreModificationsandExtensions
• FrontalStructure/Dynamics– Frontalzonesarebetterregardedasregionsofactivefrontogenesisratherthansemi-permanentphenomenon
– Frontsareoftenaconsequenceofcyclogenesis ratherthanthecause
Source:Schar (1989),Reed(1990)
L L
L
L L
H H
H
MoreModificationsandExtensions
• CycloneDynamics– Cyclonedevelopmentmaybeviewedasaconsequenceofbaroclinic instabilityratherthanfrontalinstabilities
– Therearethreemajorbuildingblocksforobservedcyclogenesis (thankstodiscoveryofjetstreamanddevelopmentofPVthinking)• Upper-leveltrough/cyclonicPVanomaly• Surfacefront(surrogatecyclonicPVanomaly)• Diabatic heating
Source:Reed(1990)
MoreModificationsandExtensions
• CycloneDynamics– TherearepatternsofcyclonedevelopmentnotenvisionedbytheBergenSchool• e.g,cyclogenesis inpolarairstreams(a.k.a thepolarlow)
Source:Reed(1990)
L
PolarLow
ClassActivity• Divideintogroupsof3-4students
• Eachgroupanalyzesthelifecycleofselected(andindependent)frontalcyclonesidentifiedwiththeIDV“Global-10day”bundle
• Basefrontalanalysesonthe925-mbtemperatureanalysisanduseIDVsdrawingcontroloptionstoanalyzethefrontsevery12hours
• DrawingcontrolmaybeaccessedbyclickingonthepencilatthetopoftheIDVwindow
• Useadifferentdrawingcontrol(byclickingonthepencilagain)foreachanalysistime