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2/21/17 WhatIsaFront? SurfaceFronts Atmos 5210/6210 Synoptic–DynamicMeteorologyII JimSteenburgh UniversityofUtah [email protected] • Anelongatedzoneofstrongtemperaturegradient(>10°C/1000 km)andrelativelylargestaticstabilityandcyclonicvorticity (Bluestein1986) • Slopingzonesofpronouncedtransitioninthethermalandwind fields (Keyser1986) • Inthebroadestsense,itisaboundarybetweentwoairmasses (Bluestein1993) • Theinterfaceortransitionzonebetweentwoairmasses ofdifferent density (GlossaryofMeteorology2000) • Ican’tdefinefront,butIknowonewhenIseeone (Steenburgh) TypesofFronts TypesofFronts Surfacefront– Afrontwithgreatestintensityattheground Source:Sanders(1955) Coldfront– Afrontwithcoldairadvancing Source:Schultzetal.(1997) TypesofFronts Back-doorcoldfront– Acoldfrontthatmoveswestward,poleward, orwestwardandpoleward Source:Hakim(1992) TypesofFronts Warmfront– Afrontwithwarmairadvancing(or,alternatively, coldairretreating) Source:Korner andMartin(2000) 1 2/21/17 TypesofFronts 00 UTC 17 Jan 2004 TypesofFronts Caribou, ME Back-doorwarmfront– Awarmfrontthatisadvancingequatorward, westward,orequatorward andwestward Stationaryfront– Aquasistationary front.Maybecomeawarm orcoldfrontifitbeginstomove Source:HakimandUccelini (1992) TypesofFronts TypesofFronts Occludedfront– Atongueofwarmairthatextendsfromthe lowcentertothepeakofthewarmsector Bent-backfront– Afrontthatextendsintothepolarairstreambehind thelowcenterandmayhavethecharacterofacoldfront Source:MassandSchultz(1993) Source:Korner andMartin(2000) TypesofFronts Coastalfront– Ashallow,mesoscale boundaryseparatingwarmmarine airfromcoldcontinentalair Source:Bosart (1981) TypesofFronts Upper-levelfront– Azoneofstronghorizontaltemperaturegradient intheupperandmiddletropospherethatdoesnotnecessarily extendtotheground Source:Shapiro(1983) 2 2/21/17 TypesofFronts TypesofFronts Splitfront – Acoldfrontthathasbeen“overrun”bylow-θe airaloft (i.e.,BrowningandMonk’suppercoldfront) Coldfrontaloft – Acoldfrontthatislocatedaloftandaheadofa surfacepressuretrough Source:BrowningandMonk(1982) Source:Hobbsetal.(1990) VerticalStructure CharacteristicsofSurfaceColdFronts A • Frontalzoneislocatedon coldsideofwindshift • Windveersasonemoves acrossfronttowardcold air • Identifiedwithtriangles pointingtowardthe warmair A' • Stripofhighvorticity @ windshift(notshown) Source:Shapiro(1983) A A' • Temperaturegradientmostintense@groundandweakenswith height • Frontalzonemarkedbystrongstaticstabilityandverticalwind shear • Frontsteepestneartheground Source:Shapiro(1983) ExampleCold-FrontPassage (1) Wind Veers ColdFrontMovement • Speedofcoldfrontnotnecessarilydeterminedbythe differencebetweenpreandpost-frontalwindsnormalto thefront • Speedbettercorrelatedwithwindspeednormaltothe frontinthecoldair (2) Temp falls (may begin after wind veers) Pressure rise (trough may precede front) • Numericalmodelsgenerallyquitegoodatcold-frontal speedtodaysodon’trelyonoldrulesofthumb! • Situationswherethefrontpropagates(i.e.,movesfaster thanexpectedfromadvection)exist– beware! 3 2/21/17 TheFrontalNose NarrowCold-FrontalRainband (NCFR) NCFR From ESRL’s Fort Ross X-band Radar • Frontmayslope forwardatlowestlevels • Narrowplumeof intenseascent(>10 m/s)maybefoundat leadingedge – Sometimesproduces ropecloudornarrow cold-frontalrainband Source:Carbone(1982),Shapiro(1984) Source:ESRL Cold-FrontExample CoreandGapStructure Gap Core Source:Wakimoto andBosart (2000) Source:Wakimoto andBosart (2000) Vorticity andVerticalVelocity Frontal vorticity strip Source:Wakimoto andBosart (2000) ConceptualModel Core updrafts approach 5 m/s Reflectivity maxima downstream of cores Source:Wakimoto andBosart (2000) 4 2/21/17 CharacteristicsofWarmFronts • Frontalzoneisoncoldsideof warmfront • Windveersacrossfrontasone movestowardthewarmair • Stripofhighvorticity atwindshift (notshown) • Identifiedwithsemicircles pointingtowardcoldair WarmFrontExample Source:Wakimoto andBosart (2001) Mesoscale Structure VerticalStructure A B B • Weakwindshiftacrossfrontatlowlevels(800mAGL) • Precipitation(infered fromdBZ)strongestahead (poleward)ofwarmfront Source:Wakimoto andBosart (2001) A • Slopingregionofenhancedhorizontalandverticalθe gradient • Veeringwindswithheight • Nodistinctfrontaldiscontinuityatsurface(frontbestdefinedaloft) Source:Wakimoto andBosart (2001) VerticalStructure VerticalStructure • Front-relativewindsshow strongveeringwith height • Stripofhighvertical vorticity withlocalized maximainfrontalzone • Cross-frontθvgradient delineatesfrontalzone • Highestvorticity also foundaloft,notatthe surface – Weaknearsurface • Strongslopingregionof front-relativecrossfrontalflow – Warmsectorairascending underlyingcoldair Source:Wakimoto andBosart (2001) Source:Wakimoto andBosart (2001) 5 2/21/17 BasicFrontalDynamics FrontogenesisMathematically • Frontogenesis:Processofincreasingthe magnitudeofthehorizontaltemperature gradient(i.e.,creatingafront) !" = !! > 0! → Frontogenesis!! • Frontolysis:Processofdecreasingthe magnitudeofthehorizontaltemperature gradient(i.e.,destroyingafront) !" < 0! → Frontolysis!! ! BasicFrontalDynamics BasicFrontalDynamics • Differentialheating&cooling:Horizontal gradientsinheating/coolingstrengthenor weakenthetemperaturegradient ! • Considerazonallyorientedfrontwitha meridional temperaturegradient q q+Dq q+2Dq y ! ∇ θ !! !" ! 1! !" = ! ! !θ ∇ θ ∝ −! !" ! !" !" ! ! x • Inthiscase,frontogenesisis ! !"/!" ! !θ !θ !" !θ !" !" = ∇! θ = ! − − !" !"/!" !" !" !" !" !" !" Differential diabatic heating/cooling ! Confluence q ! q+2Dq x Tilting BasicFrontalDynamics BasicFrontalDynamics • Differentialheating&cooling:Horizontal gradientsinheating/coolingstrengthenor weakenthetemperaturegradient • Confluence:Horizontaldeformationactsto increaseordecreasethehorizontal temperaturegradient Frontogenesis! !" = q q+Dq q+2Dq q+3Dq y x q+Dq y ! ! !θ !" ∇ θ = −! − !" ! !" !" ! q q+Dq y q+2Dq x 6 2/21/17 BasicFrontalDynamics BasicFrontalDynamics • Confluence:Horizontaldeformationactsto increaseordecreasethehorizontal temperaturegradient • Tilting:Differentialverticalmotion strengthensorweakensthehorizontal temperaturegradient Frontogenesis! !" = ! !θ !" ∇ θ = −! − !" ! !" !" ! ! q q+Dq q+2Dq y q q+Dq z q+2Dq x y BasicFrontalDynamics RealWorldExample • Tilting:Differentialverticalmotion strengthensorweakensthehorizontal temperaturegradient Frontogenesis! q+2Dq q+Dq q z Time-height section showing frontal nose, potential temperature hypergradient, and vertical motion y Surface cold front passage near Boulder (BOU) Source:Shapiro(1984) RealWorldExample Confluence (<0 = frontogenesis) RealWorldExample Tilting (<0 = frontogenesis) • Horizontalconfluenceactstostrengthenfront • Tiltingweakensfron (above100m) Source:Shapiro(1984) Total (<0 = frontogenesis) • Intotal,confluencetermwinsandfrontis characterizedbyfrontogenesis • Gradientdoesn’tfurtherstrengthen,however, duetoturbulentmixing Source:Shapiro(1984) 7 2/21/17 Discussion HowDoesOrographyAffectFronts? • Movement – Low-levelflowblockingandchannelingmayretardoracceleratea front,resultinginadistortionofits“shape” HowDoesOrographyAffectFronts? • Frontogenesis/frontolysis – Terrain-inducedhorizontalflowfieldmaycontributetofrontogenesis orfrontolysis – Terrain-inducedverticalmotionpattern(andassociatedadiabatic warmingandcooling)maycontributetofrontogenesisorfrontolysis • Verticalstructure – Low-levelblockingmayacttodecouplesurface-basedandupper-level portionsoffront – Insomecases,entirelowerportionofafrontmaynotbeableto surmountamountainridgeorrange,leavingonlyupper-levelfront OrographicImpactsonFrontal Movement ExamplesofFrontal Deformation/Distortion • Themountain-inducedflowadvects anddistortsa front(EggerandHoinka 1992) • Examples – Pre-frontaldownslope(Foehn)andlow-levelblockingof thepost-frontalwindcanretardtheprogressionofafront onthewindwardsideofamountainrange – Amountain-inducedanticyclonecanacttorotateafront anticyclonically upwindofamountainrange – Terrain-channeledflowalongavalley,plain,orgapcan produceaccelerationofafront – Ageostrophic flowalongamountainbarriercanresultin equatorward surgesofcoldairknownascold-airsurges, coastallytrappeddisturbances,orcold-airdamming Seclusion NorwegianCycloneModel:Skagerak Mountainsretardawarmfront,resultingin theformationofawarm-coreseclusionandsecondarycyclogenesis Source:Bjerknes andSolberg(1922) ExamplesofFrontal Deformation/Distortion ImpactofPre-FrontalFoehn & Blocking • Windwardretardationof coldfrontdueto – CompetitionwithprefrontalFoehn – Blockingofpost-frontal wind,whichbecomes alongbarrierandalong front • Mostdramaticwithlow Froudenumbers (i.e.,U/NH<1) OrographicdistortionofacoldfrontbyEuropeantopographyincluding frontalretardationbyAlpsandaccelerationinRhoneGapbetween Alps/Pyrenees(Mistral)andeastofAlps Source:Bergeron(1928),alsoavailableinGodske etal.(1957) Source:Smith(1986),Hoinka andVolkert (1991) 8 2/21/17 ImpactofPre-FrontalFoehn & Blocking FrontalRotation Stronger Winds SLP Contours • • Decouplingofnear surfaceandupper-level portionsoffront Source:Buzzi andSperanza (1978),Steinacker (1982) AppalachianFrontalEvolution Frontsrotateanticyclonically asthey approachmountainbarrier Weaker Winds Faster Movement – Poleward portion • Resultofsuperpositionofmountain anticyclonewithlarge-scaleflow • Oppositeaffectequatorward portion Frontal Isocrones Slower Movement Source:Smith(1982),Hoinka andVolkert (1992) TerrainChanneling “Average” cold front • Stablelow-Froude numberpostfrontalflow becomesoriented alongvalleyaxis • Coldairadveced rapidlyupvalley Source:O’Handley andBosart (1996) Source:Smith(1986),SteenburghandBlazek(2001) TerrainChanneling TerrainChanneling • Frontmaydevelopgravitycurrentlikestructure • Terrain-paralleljetmaydevelopinpost-frontal environment • Contributestodevelopmentoffrontalsurgeinvalley Source:SteenburghandBlazek(2001) – Pronouncedfrontalnose – Low-levelrear-to-frontflow;prefrontalwarmairascendsnose – Rapidincreaseinpressureandfallintemperaturewithfropa Source:Simpson(1982),SmithandReeder(1988) 9 2/21/17 TerrainChanneling ColdSurges • Surgesofcoldairfrequentlymovealongamountainrange withcoldairtotherightintheNorthernHemisphereand leftintheSouthernHemisphere • Examples – SoutherlyBuster/CoolChange(Australia) – MarineSurge(U.S.WestCoast) – NorthAmericanColdSurges(eastofRockies) • Gravitycurrentstucture revealedoverStraitofJuan deFucabydual-Doppler analysis • AffectCanada,US,CentralAmerica • Mechanism – Topographydisruptsgeostrophicbalance,resultinginenhanced along-barrierflowandcoldadvection Source:Colle etal.(1999) ColdSurges Discussion Whatarethechallengesoffrontalanalysis incomplexterrain? WestCoastMarinePushorSurge NorthAmericanColdSurge Source:Colle andMass(1995),MassandSteenburgh(2000) ChallengesofFrontalAnalysisin ComplexTerrain? • Errorsarisingfromreductionofpressuretosealevel(affect correspondingpressureanalysis) • Difficulttodeterminestrengthofhorizontaltemperature gradientsduetovariationsinstationelevation • Conventionalobservingstationsbiasedtovalleylocations andcanbelowdensity • Diabatic effectsandboundarylayerprocessesobscure large-scaleairmass andwindchanges ImprovingFrontalAnalysis • Usealtimetersettingandreduce/extrapolate pressuretothemeanelevationofwestern U.S.surfacestations – Surfacebasedinversions/coldpoolsinvalleysandbasins – Terraininducedflows(thermallyordynamicallydriven) Sample1500-mpressureanalysis • Contrastsinfrontalintensityandpositionbetweenlowand highelevationstations Source:SteenburghandBlazek(2001) 10 2/21/17 ImprovingFrontalAnalysis • UseMesoWest:Highstationdensity ImprovingFrontalAnalysis • UseMesoWest:Highstationdensity Sitewithnocturnalinversion (frontaltemperaturechangemasked) Sitewithoutnocturnalinversion (frontaltemperaturechangeevident) Conventionalvs.MesoWest frontalanalysis Source:SteenburghandBlazek(2001) Source:SteenburghandBlazek(2001) ImprovingFrontalAnalysis • UseMesoWest:Richclimatology ImprovingFrontalAnalysis • UseMesoWest:Multielevation stations Highelevation WeakFropa LowElevation StrongFropa Useclimatologies tounderstandwhenterrain-inducedflowsmaskcyclonicwind shiftsassociatedwithfronts AtKettleButte,terrainchannelinginSnakeRiverPlainresultsinSWpost-frontalwind Source:SteenburghandBlazek(2001) Source:SteenburghandBlazek(2001) Summary • Topographycanaffectthestructureofalow-levelcoldfrontin severalways – Frontscanberetardedbypre-frontaldownslope(e.g., Foehn)and blockingofthepost-frontalairmass windwardofthetopography – Frontsmayrotateanticyclonically (poleward portionofmountain)or cyclonically(equatorward portionofmountain)duetodevelopmentof mountainanticyclone – Along-valleyorgapwindsmayacceleratefrontsthroughlowland regions – Low-levelandupper-levelportionsofafrontmaybecomedecoupled • Frontalanalysisincomplexterrainisdifficult,butcanbeaidedover westernU.S.byMesoWest • Let’slearnmorebybeginningthislab (turninlaterforgrade) 11