Forecas(ngMidla(tudeWeather

2:CycloneStructureandLife‐Cycle

HorizontalStructure

Thehorizontalstructureofthecycloneconsistsofapressurestructureandathermalstructure:

h?p://www.eoearth.org/upload/thumb/3/30/Midla(tude_cyclones.gif/260px‐Midla(tude_cyclones.gif

HorizontalStructure

Thehorizontalstructureofthecycloneconsistsofapressurestructureandathermalstructure:

ThePressureStructureconsistsofapressureminimumatthecenterofthecyclone,withwindsthatblowcounter‐clockwise. h?p://www.eoearth.org/upload/thumb/3/30/Midla(tude_cyclones.gif/260px‐Midla(tude_cyclones.gif

HorizontalStructure

Thehorizontalstructureofthecycloneconsistsofapressurestructureandathermalstructure:

ThePressureStructureconsistsofapressureminimumatthecenterofthecyclone,withwindsthatblowcounter‐clockwise.

TheThermalStructureconsistsofathinbandofquicklychangingtemperaturesthatseparatesthecoldairfromthewarmair.Thisbandiscalledabarocliniczone,andformsthecyclone’sfronts.

h?p://www.eoearth.org/upload/thumb/3/30/Midla(tude_cyclones.gif/260px‐Midla(tude_cyclones.gif

HorizontalStructure

HorizontalStructure

HorizontalStructure

HorizontalStructure

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ColdFronts

Becausecoldairisdenserthanwarmair,itpushesitswayintotheregioninhabitedbywarmair(infrontofthecoldfront)

Thewarmairrisesupoverthedomeofcoldairpushinginatasteepangle

WarmFronts

Intheregionofthewarmfront,warmairisslowlypushingcoldnorthward

Thiscreatesashallowrampofcoldairthatwarmairrisesontopofasitpushesthecoldairoutoftheway

OccludedFronts

•  Coldfrontscanmovemorequicklythanwarmfronts

•  Whenacoldfrontcatchesupwithawarmfront,thewarmairissqueezedoutoftheway,leadingtoanoccludedfront

•  Thisisthe“lastgasp”forthefront,andocclusionoccursasthestormreachespeakintensity,thenweakensanddies

OccludedFronts

Ver(calStructure

h?p://www.atmos.uiuc.edu/~snodgrss/Midla(tude_cyclone_files/image018.jpg

Thever?calstructureofthecycloneismorecomplicated:

Thever(calstructureofthecycloneistypifiedbya?ltofthelowpressureatupperlevels.Thelowpressureatupperlevelsisnotjuststackedontopofthelowpressureatthesurface.

Thelowpressureatupperlevelsisusuallyfoundslightlywestofthelowatthesurface.

Ver(calStructure

L

Ver(calStructure

LL

Ver(calStructure

•  Thiswestward(ltwithheightisactuallyadvantageousforthecyclone:

Atupperlevels,theregionbetweenalowpressure“trough”andahighpressure“ridge”toitseastistheperfectloca(onforthesurfacecyclone. L H

L H

Ver(calStructure

•  Thiswestward(ltwithheightisactuallyadvantageousforthecyclone:

Inthisregion,theairisdiverging–moreairismovingawayfromacentralpointthaniscomingin.

Ver(calStructure

h?p://www.atmos.uiuc.edu/~snodgrss/Midla(tude_cyclone_files/image018.jpg

Thedivergenceatupperlevelshelpstoevacuateairfromtheaircolumn.

Thiscreatesazoneoflowpressureatlowerlevels.Aslongastheuppertroughstayswestofthesurfacecyclone,thedivergencewillpumpairoutofthecolumnandmakethecyclonestronger.

CycloneLifecycle

Next,we’regoingtolookatthecyclonelifecycle,andseehowthehorizontalandver(calstructureofthecyclonechangesasitgrows,reachesmaximumstrength,andfinallydecays.

CycloneLifecycle

•  Cyclonestypicallylastfrom5‐7days,duringwhichthecyclonepassesthroughdis(nctphasesofdevelopment:–  IncipientStage– MatureStage– OccludedStage– DecayingStage

IncipientStage

Westartwitha“sta?onaryfront”,whichisjustaregionofstrongtemperaturecontrastbetweencoldpolarairandwarmtropicalair

IncipientStage

Westartwitha“sta?onaryfront”,whichisjustaregionofstrongtemperaturecontrastbetweencoldpolarairandwarmtropicalair

Asmalllow‐pressuredisturbancestartstocirculateair,crea(ngwarmfrontsandcoldfronts

MatureStage

Cyclonestrengthenstomaturestage:• Sealevelpressuredrops• PressuregradientandWindsincrease• Morecloudsandprecipita(on• Be?erdefinedfronts• “Comma”appearance

h?p://www.ems.psu.edu/~nese/ap3014vi.gif

OccludedStage

Frontalocclusionbeginsasthecoldfrontcatchesuptothewarmfront

Stormisatitsstrongest–lowestsea‐levelpressure,strongestwinds,strongprecipita(on

“Deathknell”–cyclonecenterisdetachedfromwarmair,frontsandcyclonewillstarttoweaken

h?p://improve.atmos.washington.edu/improve2/IOP/wx_images/011222‐011223images/sat2001122212.gif

DecayingStageCyclonecenternowcompletelyseparatedfromwarmair

Sea‐levelpressurerises

Pressure‐gradientandwindsdiedown

Precipita(onandcloudscease

Thisstormhasusedallofitsavailableenergy

h?p://improve.atmos.washington.edu/improve2/IOP/wx_images/011222‐011223images/sat2001122212.gif

CycloneLifecycle

Thelifecycleofthecycloneisapparentinitshorizontalstructure:– Strengthandloca(onofwarmandcoldfronts– Developmentofoccludedfront– Minimumsea‐levelpressure

Howcanweseethisprocessinthever?calstructure?

CycloneLifecycle

•  Recallthatthepoten?alforacyclonetogrowisdependentupontheupperlevellowpressuresystem(the“lowpressuretrough”)beingslightlywestofthesurfacecyclone.

•  Asthecyclonematures,thenoccludesanddecays,thisver?calstructurechangesfrombeingfavorabletothecyclonetobeingunfavorable.

CycloneLifecycle

•  Whilethecycloneisgainingstrength(incipientmaturestages),theuppertroughiswestofthesurfacecyclone(favorablever(calstructure)

•  Oncethecyclonereachesmaximumstrength(occlusion),theuppertroughisrightontopofthesurfacecyclone(unfavorablever(calstructure)“ver?callystacked”

CycloneLifecycle

Incipient Mature OccludedL L L

L L L

CycloneLifecycle

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CycloneLifecycle