cyclogenesis - University of Utah · 2019. 11. 21. · Cyclogenesis: The formation of a cyclone....

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Cyclogenesis

JimSteenburgh

UniversityofUtah

jim.steenburgh@utah.edu

801-581-8727

Suite480/Office488INSCC

Suggestedreading:Lackmann(2011),Section5.3

Motivation

Weneedadynamicallybasedconceptualframeworktounderstandanddiagnose

thedevelopmentanddecayofsurfacecyclonesandanticyclones.

Definitions

Cyclogenesis:Theformationofacyclone.

Cyclolysis:Thedecayofacyclone.

Relationshipbetweenpressure,height,andvorticitytendencies

Conceptualframeworksforcyclogenesisareoftenbasedonvorticityratherthan

pressure.Pressuretendencies,heighttendencies,andgeostrophicvorticity

tendenciesarecoupledthroughthegeostrophicwindrelationshipsothat:

!𝜕𝑝𝜕𝑡%&

∝ !𝜕𝑍𝜕𝑡%)

∝ *−𝜕𝜁-𝜕𝑡 .

&∝ *−

𝜕𝜁-𝜕𝑡 .

)

Thus,pressure,height,andgeostrophicvorticitytendenciesareintrinsicallylinked

• Anincreaseinsurfacevorticityisassociatedwithheightandpressurefalls

• Adecreaseinsurfacevorticityisassociatedwithheightandpressurerises

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Vorticityperspective

Diagnosescyclogenesisusingthevorticityequation,whichignoringfriction,tilting,

andsolenoidaleffectscanbewritten

𝐷(𝜁 + 𝑓)𝐷𝑡 = −(𝜁 + 𝑓) !

𝜕𝑢𝜕𝑥 +

𝜕𝑣𝜕𝑦%

or,basedoncontinuity

𝐷(𝜁 + 𝑓)𝐷𝑡 = (𝜁 + 𝑓) !

𝜕𝑤𝜕𝑧 %

Thus,lowertroposphericstretching(i.e.,ascent)isamechanismforvorticity

generationandcyclogenesis.

TheexamplebelowfromLackmann(2011)showsanupper-leveltroughand

associatedareaofCVAandascentovertakingalow-levelfrontalzone.Thelow-level

frontalzoneischaracterizedbyhighinitialabsolutevorticity.Stretchingassociated

withtheupper-leveltroughmosteffectivelygeneratesvorticityalongthefront(due

totheabsolutevorticitymultiplierinthevorticityequation),leadingtocyclogenesis.

Pressureperspective

Assuminghydrostaticbalance,pressureissimplytheweightoftheoverlying

atmosphere.Surfacepressurechangesarethusduetointegratedmassdivergence

(orconvergence).Cyclogenesisoccurswhendivergencealoftexceedsconvergence

atlowlevels,resultinginnetmassdivergenceandsurfacepressurefalls.

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Ultimately,thevorticityandpressureperspectivesareconsistent

FigureadaptedfromLackmann(2011)coursenotes

QGperspective(omegaequation)

QGascentresultsinstretchingandpressurefalls.Ignoringfriction,thefollowing

cancontributetoascent,pressurefalls,andcyclogenesis:

• Vorticityadvectionbecomingmorecyclonic(orlessanticyclonic)withheight

o TypicallylookforCVA@500mb

• Alocalmaximumintemperatureadvection

o Typicallylookforwarmadvection,suchasalongawarmfrontor

occludedfront

• Alocalmaximumindiabaticheating

• Theseeffectsaremostpronouncedwhenthestaticstabilityislow

o e.g.,anupper-leveltroughcaninducecyclogenesismoreeasilyifthe

staticstabilityislowthanhigh

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Synopticexperience(cyclogenesis):

1. CVAdownstreamofanupper-leveltroughcontributestopressurefallsand

cyclogenesis

2. Warmadvectionalongawarmoroccludedfrontcontributestopressurefalls

andcyclogenesis

3. Diabaticheatingcontributestopressurefallsandcyclogenesis

4. Infrontalcyclonedevelopment,allthreeprocessescontributetoamutual

amplificationofthesurfacecycloneandupperlevelwave

5. Yougetmorebangforthebuckifthestaticstabilityislow

6. Stretchingdownstreamoftopographycanalsocontribute

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Idealizedexample(fromLackmann2011coursenotes)

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Synopticexperience(anticyclogenesis):

1. AVAdownstreamofanupper-levelridgecontributestopressurerisesand

anticylogenesis

2. Alocalminimumincoldadvectioncontributestopressurerisesand

anticyclogenesis

3. Diabaticcooling(e.g.,radiationalcooling)contributestopressurerisesand

anticyclogenesis

4. Compressionupstreamoftopographycanalsocontribute

Classactivity:

UsingtheIDVGlobal-10daydiagnostic,examineanddescribeacyclogenesis

andananticyclogenesiseventsfromaQGperspective.

PVperspective

Cyclogenesisresultsfromthecouplingandmutualamplificationofsurface

temperatureandupper-levelcyclonicPVanomalies,withcontributionsfrom

diabaticallygeneratedPVcyclonicanomalies.

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PVAnomalies(fromThorpe1985viaHoskinsetal.1985)

AsurfacewarmanomalyactslikeacyclonicPVanomalyandasurfacecoldanomaly

actslikeananticyclonicPVanomaly(fromThorpe1985viaHoskinsetal.1985)

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CyclogenesisfromaPVperspective:

1. Upper-levelcyclonicPVanomalyovertakesalow-levelfrontalzone

2. Cycloniccirculationassociatedwithupper-levelPVanomalyinducesawarm

tonguealongthefrontalzone

3. WarmtongueactslikeacyclonicPVanomaly,inducingacycloniccirculation

thatextendsupwardandfurtheramplifiestheupper-levelcyclonicPV

anomaly

4. Theupper-levelandsurfacethermalanomaliesbecomephaselockedand

mutuallyamplify,resultingincyclogenesis

5. DiabaticallygeneratedPVcanbeathirdbuildingblockinthisprocess

FigurefromHoskinsetal.(1985)

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Anotherperspective:

FigurefromLackmann(2011)lecturenotes

TypicalPVdistributioninanextratropicalcyclone

FigurefromLackman(2011)lecturenotes

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Example:1979President’sDayStorm

DynamicTropopausePotentialTemperature(fromLackmann2011lecturenotes)

SLPandSfcPotentialTemperature(fromLackmann2011lecturenotes)

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Diabaticlower-troposphericPV(fromLackmann2011lecturenotes)

Crosssection(greenlineinabove,fromLackmann2011lecturenotes)

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Classactivity:

UsingtheIDVGlobal-10daydiagnosticand/ortheweather.utah.eduGFSDT

andSurfacediagnosticforaselectedregion,examineanddescribeacyclogenesis

andananticyclogenesiseventsfromaQGperspective.