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Simulation of the Global Hydr ologicalCycle in
the CCSM Community Atmosphere Model
(CAM3): Mean Features
JamesJ. Hack�, Julie M. Caron, StephenG. Yeager, Keith W. Oleson,
Marika M. Holland, John E. Truesdale,and Philip J. Rasch
NationalCenterfor AtmosphericResearch,Boulder, Colorado80307
�NCAR, P.O.Box 3000,Boulder, CO.80307,email: [email protected]
1
Abstract
The seasonalandannualclimatologicalbehavior of selectedcomponents
of thehydrologicalcyclearepresentedfrom coupledanduncoupledconfigura-
tionsof theatmosphericcomponentof theCommunityClimateSystemModel
(CCSM),theCommunityAtmosphereModel Version3 (CAM3). Theformu-
lationsof processesthatplay a role in thehydrologicalcycle aresignificantly
morecomplex whencomparedwith earlierversionsof theatmosphericmodel.
Major featuresof the simulatedhydrologicalcycle will be comparedagainst
available observational data,and the strengthsand weaknesseswill be dis-
cussedin thecontext of thefully-coupledmodelsimulation.
1. Intr oduction
Theaccuratetreatmentof Earth’shydrologicalcycle,thecirculationof waterin theclimate
system,is centralto scientificexplorationsof climatedynamicsandclimatechange.The
globalwatercycle is an integral partof theglobalenergy cycle andhenceplaysa funda-
mentalrole in determininglarge-scalecirculationandprecipitationpatterns.Thecomplex
webof feedbackslinking hydrologicalprocesseswith theenergy cycleoperateoveracon-
tinuumof timeandspacescales.
Watercanbefoundin all threephasesin theclimatesystem,andisastronglyradiatively-
activeatmosphericconstituentin all forms.Theliquid andfrozenphasesaremostcommon
in theform of clouds.Cloudsplay a dominantrole in regulatingtheenergy budgetof the
planet,andtheirbehavior remainsamajorsourceof uncertaintyin ourability to projectthe
effectsof climatechange(e.g.,StephensandWebster1981;Cesset al. 1990;IPCC2001).
They coolEarthby reflectingsolarradiationbackto space,while at thesametimewarming
theplanetby absorbingthermalradiationemittedfrom thesurfaceandlowerregionsof the
2
atmosphere.Theprocessesresponsiblefor phasetransitionsof wateralsocontributeto the
diabaticforcingof Earth’sdynamicalcirculations,andarekey to theoverallenergy budget.
This is particularlytrue for the thermallydrivencirculationsin the tropicsandsubtropics
(Chahine1992).
Thehydrologicalcyclebeginswith theevaporationorsublimationof waterfromEarth’s
surfacewhereit is transportedby theambientmotionfield. Whentheair is lifted, it cools
andallows waterin thevaporphaseto condensein clouds,whereit canexist in both the
liquid or frozenphase.Microphysicalprocessesdeterminewhetherthecloudcondensate
is re-evaporated,changesphase,or growsparticleslargeenoughin sizeto precipitateback
to thesurface.Oncetheprecipitationreachesthesurfaceit canbere-evaporated,produce
runoff thatfindsits way into lakes,rivers,andoceans,or infiltrate thesurfaceandbestored
in thewatertable.All of theseprocessesoperateonawide rangeof timeandspacescales,
andareverydifficult to quantifyobservationally. Themostreliableobservationsof hydro-
logic processesarelimited to relatively long time andlargespacescales.As such,current
observationaldataprovide relatively weakcontraintson the formulationof hydrological
processesin global models.A major challengeto the designof global climatemodelsis
to realisticallyincorporatethemany physicalprocessesinvolvedin thehydrologicalcycle
that operateon scalesof motion distinctly separatefrom thoseof the larger-scaleresolv-
ablecirculation,but stronglyinfluencethebehavior of theatmosphereonall timeandspace
scales.
Version3 of theCommunityAtmosphereModel (CAM3) is the latestin a succession
of generalcirculationmodelsthathave beenmadewidely availableto thescientificcom-
munity, originatingwith theNCAR CommunityClimateModel (CCM). This modelis the
atmosphericcomponentof theCommunityClimateSystemModel 3.0 (CCSM3),a fully-
coupledmodelingframework thatcanbeusedfor abroadclassof scientificproblems.The
3
CCSM3representsthe latestgenerationof this modelingframework, andis discussedin
moredetailby Collins et al. (2005a).CAM3 incorporatesa significantnumberof changes
to thedynamicalformulation,thetreatmentof cloudandprecipitationprocesses,radiation
processes,andatmosphericaerosols,andis discussedmorefully in Collins et al. (2005b).
Therepresentationof cloudandprecipitationprocesseshasbeensignificantlyrevised,in-
cluding separatetreatmentsof liquid and ice condensate,large-scaleadvection,detrain-
ment,andsedimentationof cloudcondensate;andseparatetreatmentsof frozenandliquid
precipitation(Boville et al. 2005).Theparameterizationof radiative transferhasbeenup-
datedto include a generalizedtreatmentof cloud overlap(Collins et al. 2001) andnew
treatmentsof longwave andshortwave interactionswith watervapor(Collins et al. 2002a,
2004). Finally, a prescribedclimatologicaldistribution of sulfate,soil dust,carbonaceous
species,and seasalt basedupon a three-dimensionalassimilation(Collins 2001; Rasch
et al. 2001) is usedto calculatethe direct effectsof troposphericaerosolson the heating
rates(Collins etal. 2002b).This latterchangeis noteworthy in thecontext of whatfollows
giventhattheradiativeeffectsof atmosphericaerosolhasbeenshown to stronglyinfluence
thebehavior of hydrologicalprocesses(Ramanathanet al. 2001;Menonet al. 2002).
The CAM3 hasbeendesignedto provide simulationswith comparablelarge-scalefi-
delityoverarangeof horizontalresolutionsfor severaldifferentdynamicalapproximations.
Thesemodificationsrequireadjustmentsto parametersin the physicspackageassociated
with cloud andprecipitationprocesses.Consequently, the detailedhydrologicalprocess
behavior hassomedependenceon horizontal resolutionand the formulation of the dy-
namicalcore. Someof theseissues,alongwith thesensitivity of thesimulatedclimateto
modelresolutionarediscussedin moredetail in Hack et al. (2005),Yeageret al. (2005),
andDeWeaver andBitz (2005). Thestandardconfigurationof theCAM3 is basedon an
Eulerianspectraldynamicalcore,wheretheverticaldiscretizationmakesuseof 26 levels
4
(L26) treatedusingsecond-orderfinite differences(Williamson 1988). The vertical do-
main is essentiallythesameasin earliermodels,but employs 8 additionallevelsto better
refinethe uppertroposphereandlower stratosphere.The discussionthat follows will fo-
cuson the standardCAM3 configurationthat usesa 26-level 85-wave triangularspectral
truncation(T85L26). This truncationtranslatesto a 1.4�transformgrid ( � 150 km near
theequator)on which non-linearandparameterizedphysicstermsareevaluated.This re-
flectsa four-fold increasein thenumberof horizontaldegreesof freedomwhencompared
to earliermodels(Hack et al. 1994;Kiehl et al. 1998;Kiehl andGent2003). A 22-year
5-memberensembleusingobservedseasurfacetemperaturesandobservedseaice is used
to characterizethemeanfeaturesof thesimulatedhydrologicalcycle in theuncoupledcon-
figuration.Thesesimulationcharacteristicsarethencontrastedwith simulationproperties
obtainedfrom thefully-coupledCCSM3.
Therearea largenumberof observationaldatasetsrelatedto Earth’s hydrologicalcy-
cle. The datasetswe will use include the 40-yearreanalysisproductERA40 from the
EuropeanCenterfor MediumRangeWeatherForecasting(ECMWF).Thearchiveconsists
of monthly meandataon 23 pressurelevels in the vertical at 2.5 degreeresolution. The
dataareregriddedto T42spectralresolutionfor our analysis.TheNimbus-7CloudMatrix
total clouddataarederived from the TemperatureHumidity InfraredRadiometer(THIR)
andTotalOzoneMappingSpectrometer(TOMS)measurementsfor theperiodApril 1979-
March1985.Retrieval algorithmsaredescribedin detail in Stowe et al. (1988)andStowe
et al. (1989). The Global PrecipitationClimatologyProject(GPCP)Version-2Monthly
PreciptationAnalysisis a globalprecipitationdataseton a 2.5degreegrid extendingfrom
1979-2003(Adler et al. 2003). It is a merged datasetconsistingof satellitemicrowave
andinfrareddataandsurfacerain gaugedata.TheCPCMergedAnalysisof Precipitation
(CMAP) is aglobalmonthlyprecipitationdatasetona2.5degreegrid, coveringtheperiod
5
1979-1998(Xie andArkin 1997). The NationalAeronauticsandSpaceAdministration
WaterVaporProjectglobal watervapordataset(NVAP) is a watervaporand liquid wa-
ter patharchive at 1 degreeresolutionthat extendsfrom 1988-1999(Randelet al. 1996).
The blendedanalysisincludessatelliteretrievals of watervaporfrom the Television and
InfraredOperationalSatellite(TIROS) OperationalVerticalSounder(TOVS), theSpecial
SensorMicrowave/Imager(SSMI), aswell asradiosondemeasurements.The SSMI col-
umnwatervaporandcloudliquid waterarederivedfrom satellitemicrowave radiometers
asdescribedin WentzandSpencer(1998). The MODIS total columnwatervaporprod-
uct wasobtainedfrom near-IR andIR algorithms,whilecloudliquid waterpathis derived,
alongwith a numberof physicalandradiativecloudproperties,usingIR andvisible algo-
rithms(King etal. 2003).WeusetheISCCPD2 griddedcloudproductdatafor totalcloud.
This is a monthlymean,globaldatasetonanequalareagrid (Rossow andDuenas2004).
2. Mean-StateSimulation Properties: UncoupledConfigu-
ration
a. GlobalMeanProperties:
We begin our discussionof thesimulatedhydrologicalcycle by examiningtheglobalan-
nualbudgetof waterin theCAM3 usingaa22-year5-memberensembledescribedearlier.
The land surfacein thesesimulationsis fully interactive. The simulatedglobal annual
precipitationrate of 2.86 mm/day is approximately7% larger than the CMAP satellite
estimate. We also note that the magnitudeof the hydrologicalcycle, as definedby the
global annualprecipitationrate, is slightly more than8% weaker than in the previously
documentedatmosphericmodelin this series(Hacket al. 1998).Thereductionin thehy-
drologicalcycle is largely attributableto significantchangesin thesurfaceenergy budget
6
associatedwith the introductionof specifiedatmosphericaerosols,andwill be discussed
elsewherein this specialissue. Figure 1 shows the breakdown of the precipitationand
evaporationexchangesof waterin absolutetermsbetweentheatmosphereandland,ocean,
and seaice surfaces,along with runoff from the land and ice to the oceans(definedas
thedifferencebetweenprecipitationandevaporation).Therelative distribution of surface
exchangeis remarkablysimilar to renormalizedobservationalestimates(e.g.,Peixotoand
Oort 1992).Theobservationalestimatesof precipitation,evaporation,andrunoff, arenor-
malizedby theCAM3 simulatedannualglobalprecipitationrate,andshow thatall terms
in thesurfaceexchangesgenerallyagreeto within a few percentof therelativepartitioning
containedin theobservationalestimate.
Figure1 alsoshows the time averagedstorageof waterin theatmospherein all three
phasesassimulatedby theCAM3, alongwith estimatesof watervapor(i.e., precipitable
water)andcloud liquid waterretrievedfrom MODIS. We have usedMODIS estimatesin
this comparisonbecausetheseretrievalsprovide themostcomprehensive globalcoverage
of cloud liquid water, but not necessarilythemostdefinitive retrievalsof precipitableand
cloud liquid water. The total reservoir of water in its frozen, liquid, and vapor stateis
brokendown by its distributionover land,oceanandice. Fromthisperspective,theCAM3
doesa reasonablejob of simulatingthe distribution of waterwith regardto surfacetype.
Sincethesenumbersarestronglyweightedby the fractionalareasof ocean,landandsea
ice, it’ s no surprisethat thelargestfractionfor eachphaseresidesover theoceans.Figure
1 suggeststhattheCAM3 generallyoverestimatesprecipitablewater, overestimatescloud
liquid waterover theoceans,andunderestimatescloudliquid waterover landandseaice.
Anotherway to look at the distribution of water, andits exchangewith the surface,is to
quantifytheaveragepropertiesfor eachof thethreeunderlyingsurfaces,asshown in Tables
1 and2.
7
Table1 shows precipitationandevaporationdatafor the CAM3, evaporationdatafor
the ERA40 reanalysis,andprecipitationdatafrom the CMAP climatology. We have not
includedERA40 precipitationestimatesbecauseof known spin-upproblemsin the anal-
ysis of precipitation(SakariUppala,personalcommunication).The evaporationsideof
thewaterbudget,however, doesnot suffer asmuchfrom hydrologicalspin-upproblems,
andcompareswell with the da Silva et al. (1994)climatology(Anton Beljaars,personal
communication).CMAP providesonly onesideof thewaterbudget,but providesa useful
quasi-independentmeasureof themagnitudeof thehydrologicalcycle,andthedistribution
of precipitationacrosssurfacetypes.To someextent,thedisagreementsin theseestimates
helpillustratecontinuinguncertaintyin quantifyingthemagnitudeof Earth’s hydrological
cycle,evenon long time scales.Althoughthemagnitudeof theCAM3 hydrologicalcycle
is largerthantheCMAP estimateusingglobalannualprecipitationasthemeasure,it bears
amuchcloserrelationshipto thereanalysisif weuseglobalannualevaporationasthemea-
sure. In a relative senseprecipitationratesaregreaterover landandseaice in theCAM3
whencomparedto oceanicratesusingCMAP estimatesasthereferenceobservation.
Thecomponentsof waterstoragein theatmospherearegenerallydifficult quantitiesto
observe on globalscales.Much of this datacomesfrom satelliteretrievals,oftenblended
with in-situ and/oranalysisdata,and is most often limited to vertical integrals of pre-
cipitable water and cloud water. Table 2 shows the simulatedprecipitablewater, cloud
liquid water, andcloud ice waterby surfacetype for the CAM3, alongwith comparable
estimatesfrom NVAP, MODIS, andERA40. Onecanseethat thereareconsiderabledif-
ferencesbetweenthe variousobservationalestimates,which areof similar magnitudeto
differenceswith the CAM3. Generallyspeaking,theCAM3 appearsto agreereasonably
well with NVAP andERA40estimatesof precipitablewater, whichareslightly higherthan
theMODIS retrievals. Cloud liquid waterover theoceansis higherin CAM3 thanany of
8
theestimates,but falls betweentheERA40andMODIS valuesoverbothlandandseaice.
b. ZonalMeanProperties:
The zonally averagedseasonalandannualdistribution of precipitationfor the CAM3 is
shown in Figure2 in comparisonwith precipitationestimatesfrom CMAP. In mostrespects
the CAM3 exhibits similar biasesto thoseseenin the CCM3 simulation. The amplitude
of the tropical precipitationin the Inter-Tropical ConvergenceZone(ITCZ) is generally
well captured,althoughthereis a slightly moreexaggerateddouble-ITCZthanin CCM3,
mostnotablyduring DJF. The simulatedlocationof the DJF ITCZ maximum,morethan
10�northof theanalyzedCMAP maximumillustratesa majorproblemwith therepresen-
tation of tropical precipitation,which is the persistenceof ITCZ-like precipitationin the
NorthernHemisphereyearround.This is in contrastto theobservationalestimates,which
show a clearseasonalmigrationof ITCZ precipitationacrosstheequator. Subtropicalpre-
cipitationminimaaregenerallydisplacedtoo far polewardseasonallyandannually, asare
thesecondaryprecipitationmaximain theextratropicalstormtracks.Thepolewardshift of
theCAM3 SouthernHemispherestormtrackresultsin amodestpositiveprecipitationbias
whencomparedto thesatelliteretrievals.
Thezonallyaveragedseasonalandannualevaporationrateis shown in Figure3 for the
CAM3 andCCM3. CCM3is usedin thiscomparisonbecauseof problemswith identifying
a comparableglobalobservationaldataset,andto illustratethereductionin themagnitude
of the hydrologicalcycle betweenCCM3 andCAM3. Thesefiguresclearly show a sig-
nificant andsystematicreductionin the CAM3 evaporationrateswhencomparedto the
CCM3. This reductionin themagnitudeof thehydrologicalcycle is primarily associated
with the introductionof a climatologically-specifieddistribution of atmosphericaerosol
that producea significantreductionof absorbedsolar radiationat the surface. As was
9
thecasefor the CCM3 atmosphericmodel,themostvigoroustransferof waterto theat-
mosphereoccursin the subtropicswith evaporationratesreachingmaximumvaluesnear
15�N and15
�S. Consistentwith observationalanalyses,theSouthernHemisphereoceans
aretheprincipalsourceof waterpoweringtheatmospherichydrologiccycle in theCAM3.
The suppressionof evaporationin the vicinity of ITCZ convection is a realistic feature
of theCAM3 simulation,andis in goodagreementwith correspondingoceanicestimates
(e.g.,seeOberhuber1988;Kiehl andTrenberth1997;Doney etal. 1998;LargeandYeager
2004). We alsonotea substantialreductionin thesurfaceflux of waterto theatmosphere
poleward of 80�
N which introducesa large changein the waterbudgetover the Arctic
whencomparedto theCCM3.
The zonally averagedseasonalandannualnet surfaceexchangeof water,�����
, is
shown for CAM3 andCCM3 in Figure4, and is quantifiedin units of energy (where1
mm day��� 29.055Wm� ). TheCAM3 simulatesa strongseasonalmeridionaloscilla-
tion in thesourceregions,but a relatively weakseasonalmovementof theequatorialwater
sink. The weakmeridionalexcursionof the net watersink in the deeptropics is largely
attributableto theunrealisticallyweakseasonalmigrationof ITCZ precipitation.The re-
gions10�N – 40
�N and10
�S – 40
�S arewell definedsourceregionsof total water, where
the deeptropicsandhigh-latitudeextratropicsrepresentthe principal sinks. In most re-
spects,the CAM3 net waterbudgetbearsremarkablesimilarity to the CCM3, especially
consideringtherelatively largelocalchangesto theindividualcomponentsof thewaterex-
change.Thelargestchangesin�����
occuralongtheequator, andpolewardof 60�N. The
equatorialdifferencesarelargely a consequenceof reducedprecipitationrates,manifested
in the form of a strongerdoubleITCZ, particularlyapparentin the Indian Ocean. Over
the Arctic the evaporationdeficit is nearlytwice aslarge asin the CCM3, largely dueto
systematicreductionsin thesurfaceevaporationrate.
10
Thezonallyaveragedprecipitablewater, or verticalintegralof thespecifichumidity, is
shown in Figure5, alongwith theNVAP climatology. TheCAM3 is systematicallymoister
than the CCM3, and in betteragreementwith zonally-averagedobservational estimates
suchasNVAP. Thelargestbiasis presentyearroundnear30�N, exceeding4 kg m� (or 4
mm)overmostof theregionbetween10�and40
�N. As wewill show, theagreementin the
zonalmeandistributionof precipitablewateris theconsequenceof afortuitouscancellation
of errorsin thelongitudinaldistribution.
As mentionedearlier, cloudsprovide importantforcingson theclimatesystemthrough
their modulationof the radiative heatingfield. The climatologicaldistribution of cloud
and cloud condensateis thereforeworthy of somediscussion. The radiative effects of
the simulatedcloud field are discussedin Collins et al. (2005b),wherewe confinethis
discussionto the extent of cloudcover andthe simulatedliquid waterandice waterpath
lengths.
Theannuallyandzonallyaveragedmeridionaldistributionsof total cloudamountare
shown in Figure6 for CAM3, ISCCP, andNimbus7. TheCAM3 cloudfield is markedly
differentfrom theCCM3. Totalcloudcover in thetropicsandpolewardof theextratropical
stormtracksis significantlyreducedin theCAM3. This is dominatedby a sharpreduction
in highcloudovermostof theglobe,andreductionsin mid andlow-level cloudathigh lat-
itudes.Thereductionsin tropicalhighcloudarecompensatedby increasesin middle-level
cloud,wherelow level cloudhassystematicallyincreasedequatorwardof 60�N and60
�S.
The reductionin high cloud is moreconsistentwith ISCCPestimates,while the increase
in low level cloud amountis more consistentwith Warrenet al. (1988). Despitesome
improvementin thedistributionof simulatedcloudamount,importantbiasesin themerid-
ional distribution remainin CAM3. Oneof the moreobvious longstandingdeficiencies
is the locationof the minima in subtropicalcloud cover, near20�latitudein the observa-
11
tional record,but closerto 30�in the CAM3 simulation. This differencehasimportant
consequencesfor the radiative budgetof the subtropicswhere,for example,the tropical
shortwave cloud forcing is too broadandthereforetoo strongfor muchof thesubtropics.
Theradiativeissuesrelatedto theCAM3 simulationof cloudandcloudopticalpropertiesis
not thefocusof thismanuscript,but will bediscussedelsewhereusingtheISCCPsimulator
developedby Klein andJakob (1999)andWebbet al. (2001).
As notedin Table2, condensedwaterin theatmosphereis severalordersof magnitude
smallerthan storagein the vaporstate,and yet is of comparableclimate importancein
termsof modulatingtheglobal radiationbalance(e.g.,Wielicki et al. 1995;Kiehl 1994).
Zonallyandannually-averageddistributionsof liquid waterpathareshown in Figure7 for
theCAM3, andfor severalsatellite-derivedproducts.TheCAM3 exhibits sharplydefined
maximafor cloudliquid waterin thedeeptropicsandat 60�N and60
�S. Simulatedcloud
waterin thetropicsandsubtropicsagreesmostcloselywith theSSMI retrievalsof Wentz
andSpencer(1998),andrepresentsa � 30%overestimateof cloudwaterin theITCZ for
boththeMODIS andNVAP retrievals.Cloudwaterin theextratropicalstormtracksis ap-
proximatelytwice aslargeasin theITCZ, andapproximatelytwice aslargeasdiagnosed
by any of theavailableretrievals.Theoneexceptionis anew MODIS retrieval underdevel-
opmentby membersof theNASA CERESScienceTeam,whichshowshigh latitudecloud
liquid waterpathsof comparablemagnitudeto theCAM3 simulation(personalcommuni-
cation,P. Minnis). Unlike theCCM3,theseasonalbehavior of thesimulatedzonalaverage
of cloudliquid waterdoesnotshow astrongseasonaloscillationathigh latitudes.TheJJA
simulationshows thestrongestdeparturefrom theannualmeandistribution, with slightly
enhancedliquid waterpathsin theITCZ andNorthernHemispherehigh latitudes.Similar
enhancementsareseenin thevariousobservationalestimates,althoughtherelative biases
discussedearlierremain.
12
A quantityfor whichlittle in thewayof globallyobserveddataareavailableis icewater
path.Zonally, annually, andseasonallyaverageddistributionsof icewaterpathassimulated
by the CAM3 are shown in Figure 8. As is the casefor cloud liquid water, cloud ice
waterexhibits largedifferencesbetweenthetropicsandextratropics.SouthernHemisphere
extratropical ice water pathsare nearly three times as large as in the ITCZ, exceeding
40 gm m� . Unlike the liquid water distribution, ice water hasa very strongseasonal
cycleathighlatitudes,with maximumextratropicalvaluesoccuringin therespectivewinter
hemispheres.Thereis alsoamuchstrongerseasonalshift in cloudiceat low latitudeswith
greatertropical icewaterloadingduringBorealsummer.
c. Vertical Structure:
Temperatureandwatervaporarethetwo statevariablesthat jointly definethemoiststatic
stabilityof theatmosphere.Theability to properlysimulatetheverticaldistributionof wa-
ter vaporis stronglyconstrainedby biasesin thesimulatedtemperaturestructure.Figure
9 shows theCAM3 annualzonalaveragedifferencesof temperatureandspecifichumidity
from the ERA40 reanalysisclimatology. Overall, the CAM3 doesa relatively good job
of reproducingtheanalyzedthermalstructure.Simulatedtemperaturesarewithin 1�K to
2�K of theanalyzedfield for mostof thedomainboundedby 50
�N and50
�S.Overall, the
CAM3 temperaturesimulationrepresentsa modestimprovementover theCCM3. Tropi-
cal tropopauseerrorsarenearlyhalvedwhencomparedto CCM3,andhigh-latitudelower
tropospherictemperatureshave beensignificantlywarmed.A sizablepartof thewarming
changeis associatedwith theincreasedhorizontalresolution,mostnotablyin high-latitude
mid-to-uppertropospherictemperatures(seeHacket al. 2005). Improvementsto the for-
mulationof CAM3 cloudprocessesexplainstheremainingimprovementsto thetempera-
turesimulation,particularlyfor thelowertroposphereathigh-latitudes(Boville etal.2005).
13
Despitetheseimprovements,thedifficulty in properlysimulatingpolartropopausetemper-
aturesremains,a longstandingdocumentedproblemfor atmosphericgeneralcirculation
models(Boeret al. 1992).
Global observationaldataon the vertical distribution of water in the atmosphereare
notoriouslydifficult to find, whereanalysisproductsprovide thebestavailableestimates.
Atmosphericanalysescontinueto containuncertaintiesin the moisturefield (e.g.,Tren-
berthandGuillemot 1995),sincethe watervapordistribution continuesto dependupon
theparameterizedtreatmentof processesinvolvedin thehydrologicalcycle. Nevertheless,
comparisonof thereanalysisproductagainstlocally availableradiosondeobservationssug-
gestthat the vertical structureof the biasesshown in Figure9 arerobust. Thesezonally
averagedbiasesgenerallyshow a wetterthananalyzedatmospherethroughoutmostof the
domain. The major exceptionis the meridionally-broadlow-level dry biasbetween600
and900 mb in the tropics,exceeding1 gm kg� in the zonalannualmean. The overall
structureof thewatervaporbiasis similar to CCM3,but slightly exaggeratedin amplitude.
Preliminaryanalysesof this errorsuggestthat theverticalstructurein thetropics,suchas
thepositivewatervaporanomalyat500mb,is stronglydeterminedby theform of parame-
terizedmoistconvection.Evidencethattheselarge-scalebiasesarerealis shown in Figure
10,whichillustratesverticalprofilesof ��� andspecifichumidityoverYapIslandin thetrop-
ical westPacific duringthemonthof July. Thesefiguresshow how theERA40reanalysis
comparesto radiosondedata,andthat the lower troposphericdry biasandmid-to-upper
troposphericmoistbiasarerobustfeaturesof thesimulation.Thedry biasmaximizesnear
850mbreaching3 gm kg� . Watervaporbiasesof this magnitudeandstructurehave a
significantimpacton themoiststaticstability of thetropicalatmosphere,asis seenin the
��� profiles,andarelikely to play animportantrole in thelow latitudedynamicalresponse
to diabaticheating.
14
Thezonalaverageof thesimulatedverticaldistributionof condensateis shown in Fig-
ure 11. Thecolor shadedregionsshow liquid waterconcentration,andthecontouredre-
gionsshow ice waterconcentration.The locationof the freezinglevel is alsoshown for
reference.Most of the liquid watershown in Figure7 residesbelow 900 mb with con-
centrationsrangingfrom 0.05– 0.15 gm m�� in the zonalannualmean. The mid- and
high-latitudeextratropicsexhibit very strongvertical gradients,while the vertical distri-
bution in the deeptropicsis considerablymorediffuse. The cloud ice waterdistribution
generallyreachesit’ smaximumconcentrationseveralkm abovethefreezinglevel,between
500mb and600mb in theextratropicalstormtracksandnear300mb in thedeeptropics.
Maximumice waterconcentrationsreach0.007gm m�� and0.003gm m�� in therespec-
tivezonalannualextratropicalanddeeptropicalmeans.Thehighlatitudeseasonalswingin
ice waterpathis primarily determinedby changesin ice waterconcentrationin thelowest
kilometerof theatmospherein theNorthernHemisphere.TheSouthernHemispheresea-
sonalcycle is largelydeterminedby icewaterloadingchangesthroughoutthetroposphere.
Also, muchof thehigh latitudecloudcondensateconsistsof mixedphaseclouds,whereas
iceandliquid waterregimesaremoreclearlyseparatedin thetropics.
Finally, we illustratethe vertical structureof the meridionaltransportof watervapor
in Figure12. Meanmeridionalwater transportis shown in the left panels,andtransient
meridionalwater transportis shown in the right set of panels. As shown in Figure 4,
thedeeptropicsareasinkof moisture,thesubtropicsareasourceof moisture,andregions
polewardof 40�aresinksof moisture.As mightbeexpected,thetransportof waterfrom the
subtropicsinto thedeeptropicsis generallyconfinedto thelower1500� of theatmosphere
andlargely handledby themeanmeridionalcirculation.This transportexhibits thestrong
seasonalasymmetriesassociatedwith theHadley Circulation(Trenberthet al. 2000).The
majority of watervaportransportto higherlatitudesoccursover a slightly deeperportion
15
of the atmosphere,occurringlargely in the form of transienteddy transport. Although
muchweaker thantheequatorwardtransportby theHadley Circulation,a third of thetotal
poleward transportoccursin the indirect or FerrelCirculation. At higher latitudeseddy
transportstowardthepoledominatetransportby themeancirculation(PolarCell), which
actsto movewatervaporfrom thepolarregionsto lower latitudes.
d. HorizontalStructure:
In this section,we will examinethe horizontaldistribution of vertically integratedmea-
suresof water and surfacewater exchangein the CAM3. We begin with the annually
averagedprecipitationfield shown in Figure13. AlthoughtheCAM3 simulationcaptures
many of theobservedfeaturesin theglobalprecipitationdistribution, it continuesto share
many of thesamebiasesexhibitedby theCCM3. Mostof theavailableretrieval dataagree
that the mostserioussimulationerrorsoccur in the form of excessive precipitationover
the westernIndian Ocean,the centralsubtropicalPacific, and in the vicinity of Central
America. The CAM3 alsocontinuesto underestimatethe strengthof the Atlantic ITCZ.
Another longstandingsimulationdeficiency is a tendency for the simulatedtropical pre-
cipitationmaximato remainin theNorthernHemisphereyearround,andaslightly greater
tendency for reducedprecipitationalongtheequator, particularlyin theIndianOcean.This
is in sharpcontrastwith mostsatelliteestimates,which show aclearseasonalmigrationof
ITCZ precipitationacrosstheequator, includingtheIndianOcean.
The precipitationanomaliesin the westernIndian Oceanare relatedin part to defi-
cienciesin theZhang-McFarlaneclosureassumptions,a topicwhich hasbeenexploredby
several investigators(Xie andZhang2000;Zhang2003). Themostseriousmanifestation
of theseproblemsappearsin the form of excessive precipitationratesover the Arabian
Peninsuladuring the NorthernHemispheresummermonths. Other factorscontribute to
16
this biasthroughouttheyear, including the tendency to inaccuratelyshift precipitationto
the northernIndian Oceanduring the borealwinter, coupledwith an overactive anddis-
placedprecipitationregime to the westandnorthwestof Madagascarextendingfrom the
MozambiqueChannelinto theIndianOceaneastof Tanzania.Borealsummeralsoexhibits
anextremelyoveractive precipitationregime just northof theequatorand1000km to the
southwestof the Indian subcontinent.The excessive precipitationin the centralPacific
subtropicsis associatedwith two simulationchallenges.TheCAM3 continuesto havedif-
ficulty in properlypositioningtheSouthPacific ConvergenceZone(SPCZ),which is too
strongin amplitudeandtoo zonalin structure,not extendingfar enoughinto thesouthern
extratropics.TheSPCZalsohasa tendency to extendtoo far east,anothersymptomof the
tendency for themodelto producea doubleITCZ. ThenorthernPacific biasis associated
with a poorsimulationof thevery well definedprecipitationpatternthatextendsfrom the
SouthChinaSeathroughthePhilippineSeaandinto thetropicalequatorialPacific during
themonthsof July throughAugust.Thisprecipitationpatternis representedasa relatively
diffuseextensionof the southeastAsian Monsoonwell into the centralPacific subtrop-
ics, andis a longstandingprecipitationbiasin theCCM andCAM models.Othernotable
featuresof the precipitationdistribution is the inability to capturethe seasonalcycle of
precipitationoff thewestcoastof CentralAmerica,andweaker thananalyzedprecipitation
ratesalongtheextratropicalwesternboundarycurrents.Precipitationoverthelandsurfaces
generallytendsto beexcessive,especiallyover theCongo.Exceptionsincludelargeareas
over theAmazonBasin,andmuchof UnitedStateseastof thecontinentaldivide. Finally,
simulatedprecipitationratesin thehigh-latitudeextratropicalstormtrackregionscontinues
to beslightly higherthancurrentsatelliteretrievalssuggest.
The simulatedevaporationfield (not shown) illustratesthe importantrole playedby
the oceansurfaceasa sourceof watervaporto the atmosphericgeneralcirculation. As
17
suggestedby the zonal means,both the northernand southernoceanscontribute to the
evaporationof water vaporyear round,but with importantseasonallongitudinal migra-
tionsof evaporationcenters.Thesimulationexhibits a clearevaporationminimumin the
ITCZ yearround,with extensiveregionsof highevaporationin therespectivewinterhemi-
spheres.Borealwinter includesevaporationmaximaalongthewesternboundarycurrents
(theKuroshioandGulf Stream),theRedSea,theeasternBay of Bengalandeasternequa-
torial Pacific, all of which exceed10 mm day� ( � 290W m� ) in the3-monthseasonal
mean.Otherfeaturesincludemaximain thewesternsubtropicalPacific, thewesternequa-
torial Atlantic, andSouthernIndian Oceanwith evaporationrates � 6 mm day� ( � 175
W m� ). As was the casewith CCM3, broadregionsof evaporationarealsoseenover
muchof SouthAmericaandSouthernAfrica exceeding4 mmday� ( � 120W m� ) in the
seasonalaverage.During theBorealsummer, thewell definedevaporationcenterstransi-
tion to anextensiveregionof highevaporationacrossthesouthernoceans,with maximain
the SouthernIndianOceanandTropicalWestPacific. Evaporationmaximain the north-
ernPacific oceanmigrateeastwardto thevicinity of theHawaiianIslandswith maximum
evaporationratesof 6 mmday� . Theprincipalevaporationregimein theAtlantic migrates
into theSouthernHemisphere,andcontinentalevaporationmovesinto theNorthernHemi-
sphere,mostnotablyeasternNorth America, India, large portionsof eastandsoutheast
Asia,andsub-SaharanAfrica.
Together, the evaporationandprecipitationfields definethe propertiesof freshwater
exchangebetweentheatmosphereandEarth’s surface. Theannuallyaveragedhorizontal
distribution of�����
is shown in Figure14 for the CAM3. We notethat a comparable
globalobservationaldatasetdoesnotexist. Theprincipaltropicalprecipitationfeaturesare
clearlyvisible. Local waterdeficitsin the ITCZ generallyexceed4 mm day� in thean-
nualmean.TheeasternPacific subtropics,centralAtlantic subtropics,andsouthernIndian
18
Oceansubtropics,aretheprincipalsourcesof waterfor theatmosphere.TheCAM3 simu-
latesa largeseasonalcycle in�����
over muchof SouthAmerica,CentralandSouthern
Africa, India, andSoutheastAsia, mostly a reflectionof the seasonalmigrationof deep
convectionin responseto solar insolation. Similar seasonalvariability is seenover most
of Europeextendinginto CentralAsia,andover muchof North America.Most of Europe
anda largeportionof North Americacanbeclearlycharacterizedaswatersourceregions
duringJJA, andwatersink regionsduringDJF.
Thehorizontaldistribution of theannually-averagedprecipitablewater, andits differ-
encefrom the NVAP climatology, is shown in Figure15. To a large extent, the CAM3
doesaverygoodjob of capturingthestructureandcorrectmagnitudeof precipitablewater
in the atmosphere.Thereare,however, importantlarge-scalesystematicbiases,despite
exceptionallygoodagreementin thezonalmeanstructure.Thelongitudinallycompensat-
ing arrangementof thesebiasesis responsiblefor thegoodagreementin thezonalmean,
wheresomeof theseregional biasesare well correlatedwith biasesin the precipitation
distribution. Precipitablewateris generallyoverestimatedthroughoutmostof thePacific
basin,in the westernIndian Ocean,ArabianSea,andcentralAfrica. In sharpcontrast,
thesimulationexhibits a largespatially-coherentdry region stretchingfrom theAmericas,
acrosstheequatorialAtlantic, NorthernAfrica, andinto SouthernandSoutheastAsia. In
generalterms,thesimulationis systematicallydry over continentalregions,mostnotably
duringthewarmseason.Thesewatervaporbiasesarelocally significant,particularlyover
SaharanAfrica wherethey canexceed10mm,or oftenhalf of of theobservedprecipitable
water.
Figure16 shows theannualglobaldistribution of cloud liquid waterandcloud ice for
theCAM3 simulation.Theextratropicalstormtracks,featuresof thelow latitudetropical
circulation, suchas the subtropicalsubsidenceregimes,and continentaldeserts,are all
19
clearlyvisible in thecloudliquid waterfield. Theliquid waterpathfrequentlyexceeds200
gm m� in the stormtracks,in contrastwith many of the satellite-retrieved cloud liquid
waterclimatologies.Liquid waterloadingat low latitudesis generallyin betteragreement
with satellite-derivedvalues,althoughpathlengthsin thesubtropicalsubsidenceregimes
are considerablysmaller, particularly in the SouthernHemisphere.This is a surprising
featureof thesimulation,giventhegreaterthanobservedcloudradiative forcing of these
regionsin the simulation. Cloud ice sharesmany of the sameregional characteristicsas
cloud water. The tropical distribution is highly correlatedwith areasof deepconvective
activity, suchasovertheCongo,westernIndianOcean,TropicalWestPacific,andAmazon.
As suggestedby the zonal meansin Figure 8, significantly greaterice water loading is
foundathigh latitudesin thestormtrackregions,whereicewaterpathsarefrequentlywell
in excessof two timesthemaximumicewaterpathsseenin thetropics.
3. Low FrequencyForcedVariability: UncoupledConfigu-
ration
Theseasonalcycleandchangesto theequatorialSSTdistributionassociatedwith El Nino-
SouthernOscillation(ENSO)aretwo examplesof majormodesof low frequency variabil-
ity in the climatesystem.Theseareessentiallyforcedmodesof variability in uncoupled
integrationsof theCAM3, andprovideausefulbasisfor evaluatingthesimulatedlocaland
far-field responsesascomparedto observations.
The CMAP andGPCPanalysesof global precipitationprovide an observationalop-
portunity to quantitatively examinetheCAM3 simulatedprecipitationresponseto ENSO.
Figure17 is a Hovmoller plot of precipitationanomaliesasestimatedby CMAP averaged
over the deeptropics (averagedbetween10�N and 10
�S) and the CAM3 simulationof
20
precipitationfor the periodJanuary1979 throughDecember2000. The CMAP product
shows theevolution of strongpositiveandnegativeprecipitationanomaliesin responseto
the warm andcold phasesof the observed ENSOcycle. Generally, the CAM3 doesex-
tremelywell at capturingboth the structureandamplitudeof the anomalypatternin the
centralandeasternPacific. Theeastwardextensionof thewarmphaseanomaliesarewell
reproduced.Themostseriousdeficiency is in thesimulationof theanomalypatternin the
westernPacificandIndianOcean,which is muchmoreweaklyrepresentedthanobserved.
A secondway of examiningtheresponseof thehydrologicalcycle to ENSOis to ex-
plore the spatialpatternof the anomalyresponseassociatedwith the time averagedpre-
cipitationdifferencebetweena specificwarmandcold event. This approachalsohasthe
advantageof amplifying the responseto the ENSOcycle. Figure18 shows the monthly
averagedprecipitationdifferencebetweenJuly 1994 (warm phase)andJune1999 (cold
phase)asanalyzedby GPCPandassimulatedby CAM3. Both panelsshow a very large
positive precipitationanomalystretchingacrossthe centralequatorialPacific flanked by
negative anomaliesto the north, west, andsouth(in the SPCZ).The CAM3 simulation
doesa very goodjob of representingthestructureandamplitudeof thepositive anomaly.
Thestructureandmagnitudeof thenegative anomalyresponseis not aswell represented,
particularly in the westernequatorialPacific andeasternequatorialPacific. The western
Pacific anomalyis too strongimmediatelynorth of the equatorand too weak along the
equator. This responsepatternis consistentwith the time-dependentresponseshown in
theFigure17 Hovmoller diagram.Nevertheless,theCAM3 simulationdoesa remarkably
goodjob of capturingtheoverall patternandamplitudeof theresponse,includingthefar-
field responseseenin theAtlantic andtheWesternIndianOcean.An importantexception
is therainfall anomalyover theAmazonbasin,which is veryweaklyrepresented.
Finally, weexaminetheability of theCAM to simulatetheseasonalmigrationof water
21
vaporbetweentheNorthernandSouthernHemispheres,which representsa regularmajor
meridionalredistribution of massin theatmosphere,andhasanimpacton Earth’s angular
momentumbudget(e.g.,Lejenaset al. 1997).As seenin thezonalmeansof Figure5, the
CAM3 correctlysimulatesa strongseasonalmeridionalmigrationof precipitablewater.
Figure19showsthisseasonalredistributionof watervaporby subtractingtheJJA distribu-
tion of precipitablewaterfrom theDJFdistribution. Despitethebiasesdiscussedearlier,
theseasonalredistribution of watervaporis well representedin theCAM3. Thestructure
of the seasonalresponseis very similar to the observed climatology, even on relatively
small spatialscales.Thereare large-scalesystematicbiases,suchasthe slightly weaker
seasonalcycle in the NorthernHemisphere,and slightly strongerseasonalcycle in the
SouthernHemisphere,thatleadto localdifferencesin amplitude.But thepropertiesof this
modeof variability aregenerallywell representedin theCAM3 simulation.Theseresults
demonstratethe ability of the CAM3’s hydrologicalcycle to respondto lower-frequency
externallyimposedforcing.
4. Mean-StateSimulation Properties: Coupled Configura-
tion
In this sectionweprovideanoverview of thehydrologicalcycleasrepresentedin CCSM3
coupledsimulations,whichemploy theCAM3 astheatmosphericcomponent.Wewill ex-
aminetheprincipaldifferencesin thehydrologicalcycle assimulatedby theatmosphere,
along with major featuresof the hydrologicalcycle asseenfrom the perspective of the
land,ocean,andseaicecomponentmodels.Thisdiscussionwill employ astandardCCSM
controlsimulationin which theatmosphereandlandmodelsarerepresentedon a T85L26
transformgrid, andtheoceanandsea-icemodelsmake useof a nominal1�horizontaldis-
22
cretization.TheT85x1configurationof thecoupledmodelis whathasbeenusedto doc-
umentthe CCSM3simulationsfor internationalclimate-changeassessments(seeCollins
et al. 2005a).
a. Atmosphere:
In anoverallsense,theCCSM3atmosphericglobalwaterandenergy cyclebudgetremains
remarkablysimilar to the uncoupledCAM3 simulation. The top of atmosphereenergy
budgetremainswithin 0.2 Wm� , while the individual componentsof thesurfaceenergy
balanceremainwell within 1 Wm� in the global annualmean. The global cycling of
waterin theCCSM3atmosphereis nearlyidenticalto thecharacterizationshown in Figure
1 for theuncoupledmodel. Themagnitudeof theglobalhydrologicalcycle is reducedby
approximately1%, primarily dueto a reductionin the magnitudeof the waterexchange
over landandseaice,but with comparablelevelsof runoff. Globalannualstorageof water
vaporandcondensatein the atmospherealso remainswell within 1% of the uncoupled
controlsimulation.Seasonally, thesedifferencesin measuresof theglobalwatercyclevary
only slightly morethanin their annualmeans.
Althoughglobalannualmeasuresof thehydrologicalcycle arevirtually identical,the
detailedregional behavior of the simulatedhydrologicalcycle in coupledmodeincludes
somenotabledifferences.Theseanomaliescanbe seenin the zonalmeanquantitiesre-
lated to the exchangeandstorageof water in the atmosphere(seeFigs. 2, 3, 4, and5).
Thereis a remarkableshift in thesurfaceexchangeof waterfrom theNorthernto South-
ernHemispheretropicsin thecoupledmodel.NorthernHemispheretropicalprecipitation
ratesarereducedby 1 mm/dayin thezonalannualmean,andareenhancedby morethan
twicethisratenear10�S.Thismeridionalshift producesasignificantandunrealisticchange
to the freshwaterbudgetover the tropical oceans,mostnotablyduring the Borealwinter
23
(seeSection4b). Although precipitationanomaliesappearin both the Atlantic andPa-
cific basins,thezonalmeananomalyis dominatedby changesover thePacific. This takes
the form of an unrealisticenhancementof a southernandmorevigorousbranchof ITCZ
convectionextendingacrossthe Pacific basinfrom the warm pool to the Equadorcoast
(seeFig. 20). Thechangeto theprecipitationdistribution is symptomaticof theso-called
double-ITZCproblemthatplaguesmany coupledmodels(e.g.,seeDavey etal.2002).De-
spitetheoverestimatedprecipitationratesin thesoutherntropicalPacificandsoutheastern
tropicalAtlantic, severalotherfeaturesin theprecipitationdistributionaresignificantlyim-
provedin thecoupledconfiguration.TheseincludeprecipitationoverCentralAmericaand
theCaribbean,alongthewesternmid-latitudeboundarycurrents,over theArabianPenin-
sula, and over the NorthernIndian Ocean. Precipitationreductionsin the north central
subtropicalPacific alsorepresentmodestimprovementswhencomparedto theuncoupled
simulation.
Changesin theprecipitationdistribution areassociatedwith similar shifts in thestor-
ageof waterin theatmosphere.Precipitablewatermovesfrom theNorthernto Southern
Hemisphereshowing a double-peaked tropical distribution in the zonal mean,which is
dominatedby anomaliesthatmaximizein DJF. Largepositiveanomaliesexceeding10mm
appearin the southcentraltropical Pacific andsoutheasterntropical Atlantic. Negative
anomaliesof similarmagnitudearelocatedovermuchof thetropicalandsubtropicalNorth-
ern Hemispherewith maximacenteredover the ArabianPeninsulaandCentralAmerica.
Generallyspeaking,changesto the precipitablewaterfield representadditionaldegrada-
tionsof thesimulationwhencomparedto observationalestimates.Thecloudcondensate
distribution reflectsthechangesto thedistribution of precipitationandprecipitablewater.
Cloudwaterandcloudicefollow theconvectivesourceregions,whichhavemigratedto the
southernoceans.Thesechangesto thehorizontaldistribution of waterstronglyimpactthe
24
energy budgetatboththetopof theatmosphereandsurface.Largelocalanomaliesareseen
in both clear-sky andall-sky radiative fluxesat the surfaceandat the top of atmosphere,
exceeding40Wm� for all-sky fluxes.Sinceprecipitationandradiativeprocessesareinte-
grally involvedin driving thetropicalcirculation,significantchangesto thelow level wind
field arealsoseenin the centralPacific andeasternAtlantic. Thesechangesgive rise to
differencesin themeridionalsurfacelatentandsensibleheattransfers,furtheraffectingthe
freshwaterandheatbudgetsover theoceans.
An exampleof CCSM3tropicalvariabilityof precipitationis shownin therightHovmoller
panelin Figure17. This shows muchweaker tropical variability than in the uncoupled
model. The responseis relatedto the strengthof CCSM simulatedENSOevents(asop-
posedto specifiedENSO eventsin the uncoupledsimulation),coupledwith the altered
dynamicalstructureof the deeptropics and a tendency for most of the precipitationto
occuraway theequator.
b. Ocean:
Oceantransportof freshwaterpreventsthedevelopmentof significantlocal trendsin ocean
salinitywheremeannetfreshwaterflux is stronglypositiveor negative. Netfreshwaterflux
is mainly a functionof precipitationandevaporationwhich have a geographicdistribution
determinedby large-scaleatmosphericcirculationslike the Hadley cells. The freshwater
forcingof theoceanis thuspredominatelyafunctionof latitude.Theocean’sprinciplerole
in the hydrologicalcycle of the climatesystemis to transportthe net positive freshwater
flux resultingfrom precipitationin the tropicsandhigh latitudestowardsthe midlatitude
evaporationzones.The oceanalsomovesfreshwaterpoleward from ice melting regions
to ice forming regionswheremeannet freshwaterflux is negative. Lastly, theoceanmust
redistributethelarge,highly-localizedinflux of freshwaterarisingfrom river runoff.
25
Netfreshwaterflux into theoceanasafunctionof latitudeis shown in Figure21,which
comparesthefully-coupledCCSM3control(T85x1)to astand-aloneoceanmodelsolution
(x1ocn)aswell asto anestimateof climatologicalfreshwaterflux (LY2004)derivedfrom
observedatmosphericandoceandatasetsspanning1984-2000,asdescribedin Largeand
Yeager(2004).Thex1ocnandLY2004curvestrackeachotherclosely, sincethey usethe
sameblendedprecipitationdataset.Themajordifferencebetweenthetwo is thattheformer
couplesobserved atmosphericstatefields spanning1958-2000to a fully evolving ocean
modelwhereasthelattercouplesthesamefieldsto anobservedSSTdataset.River runoff
fluxes from both computationsare identical and are basedon the climatologicalgauge
estimateschemeoutlinedin Large andYeager(2004). Thesecurvesarecharacterizedby
significantuncertainty, but neverthelessprovide somemeasureof real freshwaterfluxes,
with andwithoutpolarprocessesincluded,whichcanbecontrastedwith thecoupledmodel
solutionwheredeviationsarelarge.
In additionto differencesin SST, thex1ocnandLY2004curvesdeviateathighlatitudes
becausetheoceanmodelincorporatesice formationandicemeltflux algorithmsfor which
thereareno correspondingobservationaldatasets.Thus,at extremepolar latitudeswhere
theLY2004curve is missing,thex1ocnshows largenegative freshwaterfluxeswhereice
formationresultsin brine rejection. The x1ocnindicatesmorepositive freshwater input
thanLY2004nearice edgelatitudes( 65�S, 70
�N) wheremelting takesplace. As in the
uncoupledocean,T85x1 freshwaterflux is negative at high polar latitudes,andshows a
jumpto positiveneartheiceedgeto valueswhichexceedtheobservedflux estimate.Com-
paredto bothobservationally-basedbenchmarks,thereis excessivecoupledfreshwaterflux
to theoceanbetween����� � ��� � � andlessflux between�� �! � � ��� � , in bothhemispheres.
The SouthernHemisphereexcess(A) resultsprimarily from excessive precipitation(see
Fig. 22) togetherwith a moreequatorward peakin ice melt flux, which relatesto overly
26
extensiveicecoveragein theAtlantic andIndianoceansectorsof theSouthernOcean(Hol-
landet al. 2005).
Themidlatitudefreshwaterflux deficitnear30�S(B) arisesfrom a lackof coupledpre-
cipitationcomparedto observations(seeFig. 22)aswell asameanincreasein evaporative
flux over theselatitudes. The NorthernHemispherehigh latitudesarealsocharacterized
by someexcessive precipitationandmoreequatorward ice melt, but muchof the excess
freshwaterflux in thevicinity of 60�N (C) is dueto muchhigherthanobservedriver runoff
fluxesin the Arctic (seeSection4c). As in the South,thereis lesscoupledprecipitation
andmoreevaporationbetween�� �! � � ��� � N, resultingin a freshwaterflux deficit (D).
The freshwaterflux to the coupledoceanis mostunrealisticin the tropics,wherethe
doubleITCZ createsa spuriouspeakin zonalmeanprecipitationat � "#! � S. A peakin
T85x1 freshwater flux at the Equator(E) is relatedto colder SSTswhich generateless
evaporationin thePacific alongwith excessive precipitationin thewesternequatorialPa-
cific andIndianOcean.Thepositive flux biassouthof theequatorin Figure21 is further
exacerbatedby excessive runoff from the Congo. River runoff anomaliesare relatedto
precipitationanomaliesovercontinents,asdiscussedin section4c.
Thebiasesin T85x1zonalmeansurfacefreshwaterflux giveriseto biasesin themean
meridionaltransportof freshwaterby the ocean. In Figure22, the northward freshwater
transportsof thecoupledanduncoupledoceanmodels,computedfrom Eulerianmeanad-
vection,arecompared.Eddytransportsaremissingfrom thesecurves.Both modelsshow
polewardfreshwatertransportat high latitudesassociatedwith ice growth/meltprocesses.
While x1ocnshowssignificantfreshwatertransportsouthacrosstheEquator(about1/3 of
which occursin theAtlantic basin),theglobalzonalmeanfreshwatertransportacrossthe
Equatorin T85x1is verynearzero.In coupledCCSM3,thenegativefreshwaterflux zones
atsouthernmidlatitudes( �$"#! � � ��! � S)receive freshwatervia oceantransportfrom south-
27
ern high latitudesaswell as from the southerntropics,wherefreshwater input is higher
thanobservationalestimates.Sincetropicalprecipitationin natureis muchmoreasymmet-
ric aboutthe equatorthanit is in CCSM3(seeFig. 22), the x1ocntransportsfreshwater
southwardacrosstheequatorin eachoceanbasinin orderto compensatethesouthernmid-
latitudeevaporationzones.ThecoupledCCSM3oceantransportsaboutasmuchfreshwa-
ter northwardacrosstheequatorin theAtlantic asx1ocntransportssouthwardacrossthe
equator, andthereis muchlesstransportof freshwatersouthwardacrosstheequatorin the
Pacific.
Therewould appearto beanoverly robusthydrologicalcycle in theCCSM3in which
excessive midlatitudeevaporationin theSouthernHemisphereis relatedto excessive pre-
cipitation in thesoutherndeeptropicsaswell asin theSouthernOcean.Theoceanmust
thereforetransportmorefreshwaterpoleward from the tropicsthanis estimatedfrom ob-
servations,muchof it northward.Excessivehighnorthernlatituderiver runoff (section4c)
resultsin too muchfreshwatertransportsouthward to theextratropics.Theoriginsof the
biasedfreshwatercycle aredifficult to pinpoint, but areprobablyrelatedto the lack of a
dampingmechanismwhich would inhibit air-seafreshwaterexchangein wayscompara-
ble to heatexchange,whentheoceanbecomestoo freshor salty. SubtropicalSeaSurface
Salinity(SSS)andSeaSurfaceTemperature(SST)in thesoutharefresherandwarmerthan
observedin CCSM3LargeandDanabasoglu(2005).Thissuggeststhatexcesstropicalpre-
cipitation transportedto the subtropicsby the oceanrendersthe midlatitudeupperocean
toofreshandstable,thusinhibiting deepmixing whichwould lowertheSST. Anomalously
highSSTresultsin theenhancedevaporationrateswhich,afteratmospherictransportback
to the tropics, recursasexcessive precipitation. Processstudiesindicatethat, at leastin
the Atlantic, correctingthe westcoastoceanSSTbiasgreatlyreducesexcessive tropical
precipitationLargeandDanabasoglu(2005).
28
c. Land:
The hydrologicalbudgetover land in CCSM3is a balancebetweenprecipitation,evapo-
transpiration,runoff, andthechangein storagein soilsor snow. As seenin Table3, there
is no appreciablechangein storageduring the time period analyzedhere. Both annual
meanprecipitationandrunoff comparefavorablywith observations,with precipitationbe-
ing about3% high andrunoff about4% low if glaciersareincludedin themodelestimate,
and aboutright with glaciersexcluded. We note that observationsof runoff do not in-
cludemostof Greenlandandall of Antarctica.Estimatesof globalevapotranspirationare
not available. However, Brutsaert(1984),basedon a numberof estimates,proposesthat
evapotranspirationis about60-65%of precipitation. Simulatedevapotranspirationin the
CCSM3is 63%of precipitation.
Evaporationfrom thegroundis thelargestcomponentof evapotranspiration(59%)fol-
lowedby canopy evaporation(28%)andtranspiration(13%). Otherestimatesof thepar-
titioning of global evapotranspirationsuggestthat transpirationshouldbe the dominant
componentfollowedby groundevaporationandcanopy evaporation.In particular, Choud-
huryetal. (1998),usingaprocess-basedbiophysicalmodelof evaporationvalidatedagainst
field observations,foundthatthepartitioningwas52%(transpiration),28%(groundevap-
oration),and20%(canopy evaporation).Furthermore,sincephotosynthesisis coupledto
transpirationthroughstomatalconductance,theunderestimateof transpirationhasimplica-
tionsfor carbonassimilationin themodel.Globalphotosynthesisis about57 PgC, which
appearsto beabout50%low (Table3). Thedominantform of runoff in CCSM3is surface
runoff (52%of total runoff), followedby drainagefrom thesoil column(41%),andrunoff
from glaciers,lakes, and wetlands(7%). This latter runoff term is calculatedfrom the
residualof thewaterbalancefor thesesurfaces.This termmayalsobenon-zerofor other
surfacesaswell becausethesnow packis limited to a maximumsnow waterequivalentof
29
1000kg m� .Zonal annualaveragevaluesof the hydrologic cycle are shown in Figure 23. The
CCSM3 simulationoverestimatesprecipitationnorth of 45�N, generallyunderestimates
it in thenortherntropics,andoverestimatesit in southernSouthAmerica. The latitudinal
distribution of evaporationgenerallyfollows thatof precipitationwith a maximumin the
tropics.Generally, therunoff biasescoincidewith thoseof precipitationsuggestingthatim-
provementsin thesimulatedprecipitationmayleadto improvementsin therunoff. At high
latitudes,theprimaryactive hydrologicalcomponentis runoff. At otherlatitudes,ground
evaporationgenerallydominates.An exceptionto this is in the deeptropics (10S-10N)
wherecanopy evaporationis equallyimportant.Transpirationis thesmallestcomponentof
evaporationat all latitudes.
Total runoff from the landmodel is routedto the oceanusinga river transportmodel
(Olesonet al. 2004).Thus,biasesin runoff have thepotentialto affect seasurfacesalinity
andregional oceancirculation. The annualdischarge into the global oceanis shown in
Figure24. Total discharge is 1.33Sv. Discharge excluding Antarcticais about1.25Sv,
which is about6% higherthantheestimateof Dai andTrenberth(2002). Theriver trans-
port schemedoesnot accountfor lossof waterdueto humanwithdrawal or impoundment
of water, seepageinto groundwater, or evaporationfrom the river channel. In particular,
consumptionof water for irrigation may accountfor someof the discrepancy. Doll and
Siebert(2002)estimatenetandgrossglobalirrigation requirementsas0.035Svand0.078
Sv, respectively. The loss of freshwater from Antarcticais estimatedto be 0.07 Sv by
Vaughanet al. (1999),which is thesameasthat from CCSM3.However, this comparison
is fortuitousbecausethemajorityof Antarcticrunoff from CCSM3comesfrom thecapping
of snow over glaciers.More detailedglaciermodelsneedto beincorporatedinto CCSM3
to properlydescribeglacialprocessesincludingiceberg calvingandbasalmelting.
30
Clearly, therearenotabledeficienciesin themodeleddischargeat certainlatitudesthat
arisefrom thelandrunoff fields. In particular, dischargefrom theAmazonandtheCongo
is 42%low and109%high, respectively. Thedeficienciesin partitioningof evapotranspi-
ration describedpreviously areevident in the hydrologicalbudgetof the AmazonBasin
(not shown). The partitioningof annualevapotranspirationin the model is 49% canopy
evaporation,30%groundevaporation,and21%transpiration.As discussedin Dickinson
etal. (2005),toomuchwateris interceptedby thecanopy andre-evaporatedin thewetsea-
son,thusresultingin limited wateravailability for plantrootsparticularlyin thedry season.
Photosynthesisexhibits a significantdeclinein thedry season,which affectstheability of
the dynamicglobal vegetationmodelto correctlysimulatethe compositionof vegetation
in this region (Levis andBonan2005). The year-roundwarm bias in this region that is
pronouncedin thedry seasonis furtherconfirmationthatthesimulationis deficient.
While improvementsin theprecipitationfield suppliedby theatmospheremodelwould
likely improvethelandhydrologicsimulationin theAmazonBasinandglobally, thereare
clearly aspectsof the land hydrology that requireattention. Currentresearchis focused
on improving the sunlit/shadedtreatmentof photosynthesis,stomatalconductance,and
transpiration,andtheparameterizationof canopy interception.
d. SeaIce:
As icegrows from seawater, it rejectssaltbackto theocean,resultingin a relatively fresh
ice cover with approximately4ppt salinity. If ice dynamicsis excludedandequilibrium
climateconditionsareconsidered,thelocal icegrowth is balancedby local icemeltandthe
netlong-termmeanseaicefreshwaterflux to theoceanis zero.Evenunderthermodynamic
only conditionshowever, theconsiderableseasonalcycle of the ice/oceanfreshwaterflux
canmodify theoceanbuoyancy forcingandinfluenceoceanmixing. Whenseaicedynam-
31
icsareconsidered,thetransportof relatively freshseaiceredistributeswaterin thesystem,
influencingtheglobalhydrologicalcycle. This hasthepotentialto modify the largescale
oceancirculationin both the southern(e.g.,GoosseandFichefet1999)andthe northern
(e.g.,Hollandet al. 2001)hemispheres.
In the SouthernHemispherethereis net seaice growth alongthe Antarctic continent
which is thentransportedequatorward. Figure25 shows the annualmeanmeridionalice
transportsimulatedby theCCSM3control integration.As theseaice hasonly 4pptsalin-
ity, this ice volumetransportis nearlyequivalentto a freshwatertransport.The transport
reachesa maximumof approximately0.25Sv at 65S.Estimatesderivedfrom satelliteice
motionobservationsandsparseice thicknessobservations(Weatherlyet al. 1998)suggest
a maximumvaluebetween0.05and0.1Sv. Comparedto theseestimates,theCCSM3has
excessive meridionalice transportin the SouthernHemisphere.The CCSM3 simulated
southernhemisphereice motion comparesquite well to observed estimates(not shown).
However, the ice thicknessis excessive, particularly in the Weddell Sea(Holland et al.
2005),resultingin thehigh meridionaltransport.This excessive ice transportandmelting
alongthe ice edge,modifiestheoceanseasurfacesalinity conditions,resultingin a fresh
biasalongtheAntarcticseaiceedgein thesouth-westernAtlantic.
In CCSM3,thelong-termaveragefreshwaterstoragein Antarcticseaiceequals15,630
km� . This correspondsto anannualaverageareaof 12 million km with anaveragethick-
nessof approximately1.4m anda salinity of 4ppt. As discussedin Hollandet al. (2005),
the simulatedareaof Antarctic seaice is large comparedto observations,which have an
annualaverageof approximately9-10million km .In theNorthernHemisphere,thereis netseaicegrowth in theArctic basin,resultingin
anetlossof waterfrom theArctic ocean.TheArctic ice is transportedby windsandocean
currentsand entersthe north Atlantic throughFram Strait. This provides an important
32
sourceof freshwater to the Greenland-Iceland-Norwegian seasand hasthe potential to
influenceoceanicdeepwater formation in this region (e.g., Holland et al. 2001). The
annualmeanflux of seaice throughFramStraitin theCCSM3T85-gx1controlintegration
is 0.08Sv. This agreeswell with theobservedestimateof 0.09Sv givenby Vinje (2001).
The flux hasa considerableannualcycle (Fig. 26) reachinga maximumvalueof almost
0.12Sv in late winter whenthe ice thicknessis at a maximumandthe windsareat their
strongest.As theicevolumeflux dependsonboththethicknessandvelocityof theseaice,
thegoodagreementwith observationssuggeststhatbothof thesepropertiesarereasonably
simulated.Thisdoesappearto bethecase,asdiscussedfurtherin Hollandetal. (2005)and
DeWeaver andBitz (2005). On the long-termaverage,the northernhemisphereCCSM3
seaicecovers10million km with ameanthicknessof approximately2 m. Accountingfor
theicesalinity, this representsa freshwaterstorageof 18,450km� .
5. Summary
We have presentedselectedfeaturesof the simulatedhydrologicalcycle for the CCSM3
CAM3 for bothcoupledanduncoupledapplicationsof themodel. TheCAM3 exhibits a
weaker hydrologicalcycle whencomparedwith predecessormodels,andcloserin mag-
nitudeto observationalestimates.Therelative distribution of surfacewaterexchangeand
atmosphericwaterstorageby surfacetype is in goodagreementwith observationalesti-
mates.Major precipitationandevaporationfeaturesaregenerallywell captured.
Although many featuresof water in the climate systemare well reproducedby the
CAM3, the modelcontinuesto exhibit several importantlongstandingsystematicbiases.
Thelongitudinaldistributionof precipitablewater, andits verticaldistribution,remaintwo
significantexamplesof thesedeficiencies.Biasesin theverticaldistribution of waterap-
pearsto bestronglylinkedto theparameterizedtreatmentof moistconvection.Thelarge-
33
scalelongitudinalanomaliesin precipitablewateraremuchmoredifficult to understand
andwill requireamorecomprehensiveanalysisof thewatervaporbudget.
Perhapsthe mostimportantweaknessin the simulationis the tendency for CAM3 to
form double-ITCZstructuresin thedeeptropics,andto inadequatelysimulatetheseasonal
meridionalmigrationof tropical precipitation.This simulationchallengeaffectsmostat-
mosphericgeneralcirculationmodelsatsomelevel,andis of particularimportancebecause
theerroris amplifiedwhentheatmosphereis coupledto afully interactiveocean.Givenits
importanceto thecoupledsimulation,identifying thereasonsfor thedouble-ITCZbiasis
likely to bethehighestpriority simulationchallengethatneedsto beaddressedin thenext
generationof theCCSMCAM.
Acknowledgement We would like to acknowledgethe substantialcontributions to the
CCSMprojectfrom theNationalScienceFoundation(NSF),Departmentof Energy (DOE),
theNationalOceanicandAtmosphericAdministration,andtheNationalAeronauticsand
SpaceAdministration.In particular, Hack,Truesdale,andCaronwishto acknowledgesup-
port from theDOE ClimateChangePredictionProgram,andNCAR WaterCycleAcross
ScalesInitiative.
This studyis basedon modelintegrationsperformedby NCAR andCRIEPIwith sup-
portandfacilitiesprovidedby NSF, DOE,MEXT, andESC/JAMSTEC.
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List of Tables
1 AnnualAveragePrecipitationandEvaporationRatesbySurfaceType(mm/day)
45
2 AnnualAverageStorageof Vapor, CloudWater, andCloudIce by Surface
Type(mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3 Annualaveragesof global land precipitation(�
), evapotranspiration(�
),
andrunoff ( ' ) (mm day-1). Thecomponentsof�
aretranspiration(�)(
),
evaporationof canopy interceptedwater(��*
), andgroundevaporation(�)+
).
Thecomponentsof runoff aresurfacerunoff ( '-, ), drainagefrom thesoil
column ( '-. ), and runoff from glaciers,wetlands,and lakes and snow-
cappedsurfaces( '-/�021 ) (mm day-1). Photosynthesis(�43
) hasunits of
PgC. Observationsfor�
arefrom Willmott andMatsuura(2001), ' from
Feketeet al. (1999),and�43
from Schlesinger(1991). Glaciersin Green-
land andAntarcticaare includedin the model runoff. The observations
have no dataover theseregions. ExcludingGreenlandandAntarcticain
themodel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Table1: AnnualAveragePrecipitationandEvaporationRatesby SurfaceType(mm/day)
Ocean Land SeaIce Global
P E P E P E P E
CAM3 3.22 3.61 2.26 1.48 1.37 0.43 2.87 2.87
CMAP 3.11 —- 1.93 —- 1.10 —- 2.68 —-
ERA40 —- 3.58 —- 1.49 —- 0.41 —- 2.84
Table2: AnnualAverageStorageof Vapor, CloudWater, andCloud Ice by SurfaceType
(mm)
Ocean Land SeaIce
Vapor Liq. Ice Vapor Liq. Ice Vapor Liq. Ice
CAM3 27.92 .1270 .0191 18.29 .1055 .0199 6.40 .1530 .0334
MODIS 26.59 .0982 —– 15.64 .1311 —– 4.45 .2017 —–
NVAP 25.65 .1127 —– 20.15 —– —– 5.96 —– —–
ERA40 28.27 .1170 .0358 19.77 .0718 .0339 5.84 .0274 .0470
Table 3: Annual averagesof global land precipitation(�
), evapotranspiration(�
), and
runoff ( ' ) (mmday-1).Thecomponentsof�
aretranspiration(�)(
), evaporationof canopy
interceptedwater(�)*
), andgroundevaporation(��+
). Thecomponentsof runoff aresurface
runoff ( '-, ), drainagefrom thesoil column( '5. ), andrunoff from glaciers,wetlands,and
lakesandsnow-cappedsurfaces( '-/�021 ) (mmday-1).Photosynthesis(�23
) hasunitsof Pg
C.Observationsfor�
arefrom Willmott andMatsuura(2001), ' from Feketeetal. (1999),
and�43
from Schlesinger(1991). Glaciersin GreenlandandAntarcticaareincludedin
themodelrunoff. Theobservationshavenodataovertheseregions. ExcludingGreenland
andAntarcticain themodel.
� � �)( ��* ��+ '6 '-, '5. '-/�021 '- �23CCSM3 2.12 1.33 0.18 0.37 0.78 0.79 0.41 0.32 0.06 0.82 57.2
Observed 2.06 —- —- —- —- 0.82 —- —- —- 0.82 120
List of Figures
1 SimulatedGlobal WaterBudget. Storagetermsare in mm andexchange
ratesare in mm/day. Quantitiesin [ ] arederived from MODIS, while
quantitiesin ( ) area renormalizedversionof theglobalwatercycle de-
scribedin PeixotoandOort (1992). . . . . . . . . . . . . . . . . . . . . . 51
2 Zonally-averagedAnnual, DJF, andJJA precipitationrate in mm/dayfor
CAM3, CCSM3,andCMAP. . . . . . . . . . . . . . . . . . . . . . . . . . 52
3 Zonally-averagedAnnual, DJF, and JJA evaporationrate in mm/dayfor
CAM3, CCSM3,andCCM3. . . . . . . . . . . . . . . . . . . . . . . . . . 53
4 Zonally-averagedAnnual,DJF, andJJA netsurfacewaterflux 7 �8���69 in
:<; � for CAM3 andCCSM3.. . . . . . . . . . . . . . . . . . . . . . . . 54
5 Zonally-averagedAnnual, DJF, andJJA PrecipitableWater in =?> ; �@ for
CAM3, CCSM3,andNVAP. . . . . . . . . . . . . . . . . . . . . . . . . . 55
6 Zonally-averagedtotalcloudfractionfor CAM3 comparedwith ISCPPand
Nimbus7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
7 Zonally-averaged,ocean-onlycloud liquid waterpath(LWP) in > ; � for
CAM3, CCSM3,MODIS, SSMI,andNVAP. . . . . . . . . . . . . . . . . 57
8 Zonally-averagedAnnual, DJF, and JJA cloud ice water path (IWP) in
> ; �@ for CAM3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
9 Zonally-averagedannualmeantemperatureandspecifichumidity differ-
encesbetweenCAM3 andERA40. . . . . . . . . . . . . . . . . . . . . . . 59
10 Profilesof equivalentpotentialtemperatureandspecifichumidityfor CAM3,
ERA40,andRAOBSatYapIsland( ACBD�FEHGJI�"#K�LCBM"NE � ). . . . . . . . . . . . . 60
11 CAM3 zonally-averagedannualmeancloudliquid waterconcentration(color
countours),ice waterconcentration(black contours),with 273K freezing
level contour(red)for reference. . . . . . . . . . . . . . . . . . . . . . . . 61
12 Zonally-averagedAnnual,DJF, andJJA mean(left column)andtransient
(right column)meridionalmoisturetransportin >O� ; =P>�Q for CAM3. . . . 62
13 Annualmeanprecipitationin mm/dayfor CAM3 comparedwith CMAP. . . 63
14 Annual,DJF, andJJA netsurfacewaterflux 7 �R�R�69 in:<; �@ for CAM3. 64
15 Annualmeanprecipitablewaterin =?> ; �@ for CAM3 comparedwith NVAP. 65
16 Annualmeancloudicewaterpath(IWP)andcloudliquid waterpath(LWP)
in > ; � for CAM3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
17 Equatorial( "#! E 3S� "#! E G ) precipitationanomaliesfor CAM3, CMAP, and
CCSM3.TheCAM3 andCMAP periodincludes1979-2000,andtheCCSM3
is anarbitraryrepresentative22-yearperiodfrom thecontrolsimulation . . 67
18 Warm (July 1994)minuscold (June1999)event precipitationanomalies
for CAM3 andGPCP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
19 Amplitude of seasonalprecipitablewaterchangesin mm for CAM3 and
NVAP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
20 Annualmeanprecipitationin mm/dayfor CCSM3comparedwith CAM3. . 70
21 Net freshwaterflux for theCCSM3(T85x1),uncoupledoceanmodel(x1
ocn),andobservations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
22 GlobalandAtlantic Basinnorthwardfreshwatertransport. . . . . . . . . . 72
23 Zonally averagedannualmeanland valuesfrom CCSM3comparedwith
observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
24 Totalaccumulateddischargefrom A�! E G from landinto theoceans.. . . . . 74
25 Theannualmeanmeridionalicetransportin theSouthernHemispherefrom
CCSM3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
26 Thesimulatedannualcycleof FramStraitseaice transportin Sv. . . . . . . 76
Figure1: SimulatedGlobal WaterBudget. Storagetermsarein mm andexchangerates
arein mm/day. Quantitiesin [ ] arederivedfrom MODIS, while quantitiesin ( ) area
renormalizedversionof theglobalwatercycledescribedin PeixotoandOort (1992).
Figure2: Zonally-averagedAnnual,DJF, andJJA precipitationratein mm/dayfor CAM3,
CCSM3,andCMAP.
Figure3: Zonally-averagedAnnual,DJF, andJJA evaporationratein mm/dayfor CAM3,
CCSM3,andCCM3.
Figure4: Zonally-averagedAnnual,DJF, andJJA netsurfacewaterflux 7 �T�T�69 in:<; �
for CAM3 andCCSM3.
Figure5: Zonally-averagedAnnual,DJF, andJJA PrecipitableWaterin =?> ; �@ for CAM3,
CCSM3,andNVAP.
Figure6: Zonally-averagedtotalcloudfractionfor CAM3 comparedwith ISCPPandNim-
bus7.
Figure7: Zonally-averaged,ocean-onlycloudliquid waterpath(LWP)in > ; �@ for CAM3,
CCSM3,MODIS, SSMI,andNVAP.
Figure8: Zonally-averagedAnnual,DJF, andJJA cloudice waterpath(IWP) in > ; �@ for
CAM3.
Figure 9: Zonally-averagedannualmeantemperatureand specifichumidity differences
betweenCAM3 andERA40.
Figure10: Profilesof equivalentpotentialtemperatureandspecifichumidity for CAM3,
ERA40,andRAOBSat YapIsland( A�BD� E GJIC"UK�LCBM" E � ).
Figure11: CAM3 zonally-averagedannualmeancloud liquid waterconcentration(color
countours),ice water concentration(black contours),with 273K freezing level contour
(red)for reference.
Figure12: Zonally-averagedAnnual,DJF, andJJA mean(left column)andtransient(right
column)meridionalmoisturetransportin >O� ; =?>�Q for CAM3.
Figure13: Annualmeanprecipitationin mm/dayfor CAM3 comparedwith CMAP.
Figure14: Annual,DJF, andJJA netsurfacewaterflux 7 �R�V�29 in:<; �@ for CAM3.
Figure15: Annualmeanprecipitablewaterin =?> ; �@ for CAM3 comparedwith NVAP.
Figure16: Annualmeancloudice waterpath(IWP) andcloudliquid waterpath(LWP) in
> ; �@ for CAM3.
Figure 17: Equatorial( "#!WE 3R� "U!WE#G ) precipitationanomaliesfor CAM3, CMAP, and
CCSM3. The CAM3 andCMAP period includes1979-2000,and the CCSM3 is an ar-
bitrary representative22-yearperiodfrom thecontrolsimulation
Figure18: Warm (July 1994)minuscold (June1999)event precipitationanomaliesfor
CAM3 andGPCP.
Figure19: Amplitudeof seasonalprecipitablewaterchangesin mmfor CAM3 andNVAP.
Figure20: Annualmeanprecipitationin mm/dayfor CCSM3comparedwith CAM3.
Figure21: Net freshwaterflux for theCCSM3(T85x1),uncoupledoceanmodel(x1 ocn),
andobservations.
Figure22: GlobalandAtlantic Basinnorthwardfreshwatertransport.
Figure23: Zonallyaveragedannualmeanlandvaluesfrom CCSM3comparedwith obser-
vations.
Figure24: Totalaccumulateddischargefrom A�!WEHG from landinto theoceans.
Figure25: The annualmeanmeridionalice transportin the SouthernHemispherefrom
CCSM3.
Figure26: Thesimulatedannualcycleof FramStraitseaice transportin Sv.