DynamicalconstraintsonITCZmigra3oninplanetaryatmospheresusingamoistGCM

SeanFaulk,JonathanMitchell(UCLA)SimonaBordoni(Caltech)

AMSConferenceonHurricanesandTropicalMeteorologyApril20th,2016

ITCZonEarth

Observedannual-meanprecipita3onin2013

(Friersonetal.,2013)

“ITCZ”onTitanreachespole

(BouchezandBrown,2005)

NorthPolarLakes SouthPolarClouds

(Imagesource:NASA/JPL/ASI/USGS)

ITCZonEarth

Observedannual-meanprecipita3onin2013

(Friersonetal.,2013)

Ourques(on:Whydoesn’tEarth’sITCZreachsummerpole?

ITCZisassociatedwithascendingbranchofHadleycell

Duringnorthernsummersols3ce HadleycelledgeITCZ

Hadleycelledge=la3tudewheremeridionalflowvanishes;wellcorrelatedwithenergyfluxequator(Kangetal.,2008)andmaximumzonalMSEinmostofouraquaplanetsimula3onsITCZ=la3tudeofmaximumzonalprecipita3on

Observa3onsofAsianmonsoonsector

(BordoniandSchneider,2008)

Zonalmeanprecipita3on Zonaland3memeanstreamfunc3on

Moiststa3cenergy(MSE)vs.precipita3onEquinox Monsoononset

ThermodynamicargumentsforITCZposi3ononEarth

1)  Convec3onanduplicgenerallyoccuroverwarmestsurfacewaters,justequatorwardofla3tudeofmaximumMSE(PrivéandPlumb,2007)oratenergyfluxequator(Kangetal.,2008,2009)•  Con3nentaldistribu3on(Philanderetal.,1996)andoceanheat

transport(Friersonetal.,2013)helpexplainITCZ’snorthernannual-meanposi3onasaresultofwarmerwatersnorthofequator

2)  SurfaceheatcapacityaffectsmovementofITCZonEarth(Bordoniand

Schneider,2008;Donohoeetal.,2013)•  Oceninvokedaspartofexplana3onforfar-reachingHadleycellson

Titan

HowdodynamicsaffectITCZposi(on?

ITCZonTitan

(Mitchelletal.,2006) (Mitchelletal.,2009)

TitanITCZreachespoleduringsols3ce,aspartofaglobalHadleycell

Zonalmeanprecipita3on Zonaland3memeanstreamfunc3on

(HeldandHou,1980)

DeterminedbyTOAenergybalance

Useamoistconvec3onaquaplanetGCMwithaseasonalcycle

•  ModeldevelopedbyFrierson(2006)andFrierson(2007)usingFMSspectralcore–  zonallysymmetric,slabmixedlayerocean–  nocloudsorwatervaporfeedbacks–  moistconvec3on

•  3-D;64x128horizontalresolu3on;25ver3callevels1)  Simula3onsrunfordecreasedheatcapacity–keepingeverythingelseEarth-like

•  10-yearseasonalrunswheretop-of-atmosphereinsola3onisempiricalfittoEarth’scurrentinsola3on(Hartmann,1994)

2)  Simula3onsrunundereternalsols(ceforcingfor10years

3)  Simula3onsrunwithΩ=αΩEforα=4,2,1,1/2,1/4,1/6,1/8,1/16,1/32

DecreasedheatcapacityhasminimaleffectonITCZmigra3on

Zonalmeanprecipita3on(mm/day)

MaximumMSEcondi3onfailsineternalsols3cecase

Zonaland3memeanstreamfunc3onZonal/3memeanprecipita3onandMSE

Zonalmeanprecipita3on

Decreasingrota3onhasmajoreffect–ITCZreachespolebeyondΩE/8

ITCZvs.otherdiagnos3cs

Boundarylayerforces

HadleycelledgeITCZ

Bernoulligradient Coriolisζu

drag

Boundarylayerforces

Boundarylayerforces

Summary

•  ControlsonEarth’sITCZares3llunclearandmostoftheliteraturehasfocusedonthermodynamicmechanismsaswellasontheplanetEarth,sowehaveapproachedtheproblemfromaplanetaryperspec3veandwithanemphasisondynamics

•  Wefindthatsurfacetemperature/MSEandheatcapacitydonotalwayscontrol

ITCZposi3on,andthatrota3onrateand/orradiusarethedominantcontrolsinanaquaplanetcontext

•  WeexamineboundarylayerdynamicsandfindtheITCZmigra3ontobewell

correlatedwithfric3onandotherforces.

ITCZandenergyfluxequatorscaleaccordingtopowerlaw

(Caballeroetal.,2008)