Describing the Components of the Water Transport in the Martian Atmosphere

2 Pages·2003·0.04 MB·English

by  Montmessin, F.

Third Mars Polar Science Conference (2003) 8089.pdf
DESCRIBINGTHECOMPONENTSOFTHEWATERTRANSPORTINTHEMARTIANATMOSPHERE..F.Montmessin,
R.M.Haberle,NASAAmesResearchCenter-MoffettField,F.Forget,LaboratoiredeMeteorologieDynamique-France,P.Rannou,
M.Cabane,Serviced’Aeronomie-France.
Introduction: Inthispaper,weexaminethemeteorolog- andmid-latitudesprecedestheincorporationofwaterwithin
icalcomponentsdrivingwatertransportintheMartianatmo- theascendingbranchoftheHadleycellinthenortherntropics.
sphere. Aparticularemphasisisgiventotheroleofresidual Likewise,themechanismbywhichwaterreturnstothenorth
meancirculationandwatericecloudsindeterminingthegeo- residualcapincludeswatertrappingintheseasonalCO cap,
2
graphicalpartitioningofwatervaporandfrost. thankstoanintensemixingofairmassesacrossthecapedge.
Modeldescription: Foroursimulations,weusetheGen- Duringtherecessionofthecap,seasonalwaterfrostiscarried
eralCirculationModel(GCM)developpedatLaboratoirede polewardbyasuccessionofsublimation/recondensationpro-
MeteorologieDynamique(Paris/France)[1]tosimulateMar- cesses occuring within poleward warm fronts and equatorial
tianhydrologicalcycleforvariousorbitalconfigurations. The coldfronts.
model includes usual representations of processes like sur- With this in mind, describing water transport in terms of its
facesublmation,cloudformationandsedimentation(withpre- meteorological components becomes a necessary task in or-
dictedcloudparticlesizes)[2]. Mostofoursimulations dertofullyappreciatethemechanismscontrollingtheMartian
Watercycleandcirculation: Usingthesamedecompo- watercyclestability.
sition as that found in [3], it is possible to recast the water Results of our analysis on an annual average and at specific
transport in terms of circulation components. Indeed, it is seasonswillthusbepresented.
staightforwardtodemonstratethatthetotalmeridionaltrans-
Watercycleandclouds:Followingtheirfirstobservation
portofatracerspeciesincludesthecontributionofthemean
ofwhathasbeensincecalledthe"Aphelioncloudbelt",i.e. a
meridionalcirculation,thatoftransienteddiesandthatofsta-
cloudystructureencirclingtheequatorduringnorthernspring
tionarywaves.
andsummer,[7]proposedamechanism,involvingclouds,to
Followingthisapproach,weexpressthetotaltransportofwa-
predict preferred storage location of water with changes of
tervaporatagivenlatitudeandatagivenheightas[q¯v],where
periheliondate. Theso-calledClancyeffectcomesfromthe
theq¯vsymboldenotesthetimeaverageand[]symbolsdenotes
potential ability of clouds to sequester water below the re-
thezonalmeanoftheproductqv,qbeingthemassmixingra-
turning branch of the equinoctial Hadley cell if the latter is
tioofwaterandvthemeridionalwind. Accordingto[3],[q¯v]
synchronized with the aphelion season. In practice, aphe-
canbewrittenas:
lion season implies decreasing atmospheric temperatures in
[q¯v]=[q¯][v¯]+[q0¯v0]+[q¯∗v¯∗] (1) thetropicsandthuslowerlevelsofcondensation/precipitation.
Perihelionseason,onthecontrary,comeswithenhancedsolar
with the prime symbol 0 expressing the departure from the forcingandwarmertemperatureswhichallowwatertobecar-
timeaverage(q0 = q−q¯andv0 = v−v¯)whereasthestar riedbytheHadleycelltowardsthewinterhemispherewithout
experiencingtheeffectsofcloudsedimentation.
symbolbeingrelatedtothedeparturefromthezonalaverage
(q∗ =q−[q]andv∗ =v−[v]). Onanannualaverage,[7]suggestsanetfluxofwatertowards
the hemisphere for which summer is timing with aphelion.
In short, total water transport [q¯v] is the sum of the mean
Whereasthecurrentorbitalconfigurationshouldthereforefa-
meridionalcirculationcomponent[q¯][v¯],thatoftransiented-
dies [q0¯v0] and that of non-travelling waves [q¯∗v¯∗]. Eq. (1) vorthenorthernhemisphere,italsoimpliesthatthissituation
isreversedwhentheperihelionisshiftedof180o.
canbeintegratedoverheighttoyield
[7]evensuggestedthatthecurrentpositionofthepermanent
Z 0 dp Z 0 dp Z 0 dp Z 0 dp watericecapisaconsequenceofthiseffect. Accordingly,the
[q¯v] = [q¯][v¯] + [q0¯v0] + [q¯∗v¯∗] permanentwatericecapshouldmovetothesouthpolewhen
g g g g
ps ps ps ps aphelionoccursduringsouthernsummer.
(2)
Ontheotherhand,[8]showedthat,regardlessofexcentricity
Thelatterequation,akintoazonal-andvertical-meanexpres-
andapheliondate,thesouthtonorthtopographicslopeapplies
sionofthewatermeridionaltransport,synthetizesthegeneral
a major component to the general circulation biasing cross-
behaviorofthemeridionalfluxfields. Giventhatqisthesum
equatorialmassflowstowardsthenorthhemisphere.
ofbothatmosphericwatervaporandwaterice(clouds),eqs.
The purpose of our study is to confront the Clancy effect to
(1)and(2)canbefurtherrecastedtoyieldtherespectivecon-
thatinducedbytopographicforcingwithinthecontextofGCM
tributionsofvaporandclouds. Mostofthewatercyclestudies
simulations. Ultimately,wewilldiscussthestabilityofaper-
todate[4,5]havefocusedonthesoleroleoftheHadleycellin
manent cap in the south pole that recent observations may
cross-equtorialflowsofmoisture. However,[4]and[5]have
confirm[9].
shownthatbaroclinicactivityplaysakeyroleincyclingwater
inandoutthenorthresidualcap. Indeed,[6]concludedthat Reference
theobservedextractionofwaterfromthenorthpolarcapcan [1]F.Forgetetal.,JGR104,24155(1999)
notbereproducedwithatwo-dimensionnalcirculationmodel. [2]F.Montmessinetal., proceedingsofthemarsVIconfer-
Accordingto[5],horizontalmixingofmoisturebetweenhigh ence,Pasadena(2003)
Third Mars Polar Science Conference (2003) 8089.pdf
Watertransportcomponents: F.Montmessinetal.
[3]J.P.PeixotoandA.Oort,PhysicsofClimate,Amer. Inst. [6]R.HaberleetB.Jakosky,JGR95,1423(1992)
ofPhysics(1992) [7]T.Clancyetal.,Icarus122,36(1996)
[4]H.Houbenetal.,JGR102,9069(1997) [8]M.RichardsonandJ.Wilson,Nature416,298(2002)
[5]M.RichardsonandJ.Wilson,JGR107,10.129/2001JE001536 [9]T.Titusetal.,Science299,1048(2003)
(2002)