cyclogenesis - University of Utah · 2019. 11. 21. · Cyclogenesis: The formation of a cyclone....
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1 Cyclogenesis Jim Steenburgh University of Utah [email protected] 801-581-8727 Suite 480/Office 488 INSCC Suggested reading: Lackmann (2011), Section 5.3 Motivation We need a dynamically based conceptual framework to understand and diagnose the development and decay of surface cyclones and anticyclones. Definitions Cyclogenesis: The formation of a cyclone. Cyclolysis: The decay of a cyclone. Relationship between pressure, height, and vorticity tendencies Conceptual frameworks for cyclogenesis are often based on vorticity rather than pressure. Pressure tendencies, height tendencies, and geostrophic vorticity tendencies are coupled through the geostrophic wind relationship so that: ! % & ∝ ! % ) ∝ *− - . & ∝ *− - . ) Thus, pressure, height, and geostrophic vorticity tendencies are intrinsically linked • An increase in surface vorticity is associated with height and pressure falls • A decrease in surface vorticity is associated with height and pressure rises
Transcript of cyclogenesis - University of Utah · 2019. 11. 21. · Cyclogenesis: The formation of a cyclone....
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Cyclogenesis
JimSteenburgh
UniversityofUtah
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.
