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)