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1
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
5
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.
7
PVAnomalies(fromThorpe1985viaHoskinsetal.1985)
AsurfacewarmanomalyactslikeacyclonicPVanomalyandasurfacecoldanomaly
actslikeananticyclonicPVanomaly(fromThorpe1985viaHoskinsetal.1985)
8
CyclogenesisfromaPVperspective:
1. Upper-levelcyclonicPVanomalyovertakesalow-levelfrontalzone
2. Cycloniccirculationassociatedwithupper-levelPVanomalyinducesawarm
tonguealongthefrontalzone
3. WarmtongueactslikeacyclonicPVanomaly,inducingacycloniccirculation
thatextendsupwardandfurtheramplifiestheupper-levelcyclonicPV
anomaly
4. Theupper-levelandsurfacethermalanomaliesbecomephaselockedand
mutuallyamplify,resultingincyclogenesis
5. DiabaticallygeneratedPVcanbeathirdbuildingblockinthisprocess
FigurefromHoskinsetal.(1985)
9
Anotherperspective:
FigurefromLackmann(2011)lecturenotes
TypicalPVdistributioninanextratropicalcyclone
FigurefromLackman(2011)lecturenotes
10
Example:1979President’sDayStorm
DynamicTropopausePotentialTemperature(fromLackmann2011lecturenotes)
SLPandSfcPotentialTemperature(fromLackmann2011lecturenotes)
11
Diabaticlower-troposphericPV(fromLackmann2011lecturenotes)
Crosssection(greenlineinabove,fromLackmann2011lecturenotes)
12
Classactivity:
UsingtheIDVGlobal-10daydiagnosticand/ortheweather.utah.eduGFSDT
andSurfacediagnosticforaselectedregion,examineanddescribeacyclogenesis
andananticyclogenesiseventsfromaQGperspective.