Download Surface Fronts

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)
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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)
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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!
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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)
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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)
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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
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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)
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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)
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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)
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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)
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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)
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