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Modeling Ice Sheets and Sea Level with the Community Earth System Model
William LipscombCESM Tutorial17 August 2017
Outline• Introduction
• IcesheetsinCESM1
• IcesheetsintheCESM2release
• IcesheetsbeyondCESM2
Sea-level rise over the past 25 years
Globalmeansea-levelrisefromsatellitealtimetrySLRrate=3.3± 0.4mm/yr,1993–2015
Nerem 2016
Growing ice sheet contribution to sea level
~1mm/yr fromGreenlandandAntarcticicesheets(~65%Greenland)~1mm/yr fromglaciersandicecaps~1mm/yr fromoceanthermalexpansion
Credit:ESA/NASA/PlanetaryVisions
AntarcticIceSheet• 60msea-levelequivalent
– 5minmarine-basedpartsofWestAntarctica
– 20minmarine-basedpartsofE.Antarctica
• Accumulationbalancedbyflowintofloatingiceshelves;littlesurfacemelting
• MasslossinrecentyearsfromWestAntarcticaandtheAntarcticPeninsula,triggeredbywarmoceanwaterreachingthebaseoficeshelves
Antarctic ice flow speed (Rignot et al. 2011)
GreenlandIceSheet
• 7msea-levelequivalent• Accumulationbalancedby
surfacerunoffandicebergcalving
• Growingmasslossinrecentyearsfromincreasedsurfacemeltingandrunoff,andfromthinningandaccelerationofoutletglaciers
Greenland surface ice speed (Greenland Ice Mapping Project)
IcesheetsinIPCCAR5• “UnderallRCPscenariostherateofsealevelrisewillvery
likelyexceedthatobservedduring1971–2010duetoincreasedoceanwarmingandincreasedlossofmassfromglaciersandicesheets.”
• Likelyrangeof21st centuryglobalmeansealevelrise:• 0.32to0.63m(RCP4.5,2081-2100)• 0.45to0.82m(RCP8.5,2081-2100)
• “Onlythecollapseofmarine-basedsectorsoftheAntarcticicesheet,ifinitiated,couldcauseglobalmeansealeveltorisesubstantiallyabovethelikelyrangeduringthe21stcentury….”
Marineicesheetinstability(MISI)
• IceinpartsofAntarctica,especiallytheAmundsen Searegion,isvulnerabletointrusionsofwarmCircumpolarDeepWater(possiblydrivenbychangesinwindforcing).
• Unbuttressed marineiceonareverse-slopingbedisunstable.• MISImayalreadybeactiveforPineIslandandThwaites glaciers.
Schematic of warm CDW reaching the grounding line (Jenkins et al. 2016)
Antarcticicesheetsensitivity
• LastInterglacial(125Kyearsago,CO2 =280ppm)– Globalmeansealevel6–9mhigherthantoday– Only~2mfromGreenland,0.4mfromoceanthermalexpansion,
henceanAntarcticcontributionof~5m• Pliocene (3Myearsago,CO2 =400ppm)
– Globalmeansealevel10–30mhigherthantoday– Maxof~7mfromGreenland,~5mfromW.Antarctica,so
probablyanE.Antarcticcontribution• EvenwithMISIincluded,itisdifficulttosimulatethismuch
retreatwithcurrentmodels.
– Arecriticalmechanismsmissing?
MechanismsforlargeAntarcticSLR?DeConto &Pollard(2016)suggestednewmechanisms:• Marineicecliffinstability(MICI)
– Icecliffsattheedgeofoutletglacierscanbe~1kmthick– Whenmorethan~90moficesitsoutsidethewater,atypicalicecliffwillcollapse(stress>1Mpa)
– Cliffretreatonareverse-slopingbedisdynamicallyunstable(similartoMISI).
• Hydrofracture– Increasedmeltingoniceshelvescouldgrowcrevassesandincreasecalving
– Loseofbuttressingshelvescouldtriggercliffinstability(e.g.,Jakobshavn inGreenland)
SimulatedAntarcticretreatDeConto &Pollard(2016)
• RCP8.5:77cmby2100,12mby2500– WAIScollapseby2250– MajorretreatinWilkes
andAuroraBasins• Atmosphericwarmingis
themaindriver,butoceanthermalmemoryinhibitsrecovery.
• Mechanismsareplausible,butratesarehighlyuncertain.o Coarse-resolutionicesheetmodel(10km)with
parameterizedfluxesatthegroundinglineo Prescribedoceanmeltrateswithnocoupling
DeConto &Pollard2016
Questions• Whatrangeofsea-levelriseshouldplannersand
policymakersassume?o Canwebeconfidentthat21st centurysea-levelrisewillnotexceed1m?
• CanCESMandotherEarthsystemmodelsprovideactionablesea-levelscience?
o SurfacemassbalanceprojectionsfromEarthsystemmodelsareincreasinglycredible.
o Butitremainsdifficulttoputanupperboundonthesea-levelcontributionfromAntarctica.
IcesheetsinCESM1• Formanyyears,globalclimatemodelsdidnotincludedynamic
icesheets.Icesheetsweretreatedasbigbrightrocks.o Thetraditionalviewwasthaticesheetsevolvedonmulti-
centuryandlongertimescales.Thisviewchangedwithobservationsoficesheetmasslossinthe1990sand2000s.
• In2009,theCESMLandIceWorkingGroupformedwiththegoalsof1) integratingawellvalidated,fullydynamicalicesheetmodel
inCESM2) determiningthelikelyrangeofdecade-to-century-scale
sea-levelriseassociatedwiththelossoflandice• CESMv1.0wasreleasedin2010withapreliminary
implementationofdynamicicesheets.
IcesheetsinCESM1CESM1includedtheGlimmerCommunityIceSheetModel(Glimmer-CISM).
• DynamicGreenlandicesheetonastructured5kmgrid
• Serialcode;Glide shallow-icedynamics(validforslowinteriorflows,butnotfastflowinicestreamsandiceshelves)
CESM1alsoaddedasurface-mass-balanceschemeforlandice.• Thesurfacemassbalance(SMB)forglaciatedregionsiscomputedbytheCommunityLandModelinmultipleelevationclasses,thensenttothecoupleranddownscaledtothelocalicesheetgrid.
• AdvantagestocomputingSMBinthelandmodel:o Coupleicealbedotoatmosphereonhourlytimescaleso Avoidduplicationofsnowphysicso Computationalsavings(landgridcoarserthanicesheetgrid)
IcesheetsinCESM1Land -> Ice sheet (10 classes)n Surface mass balancen Surface elevationn Surface temperature
Coupler
Atmosphere
Ocean
Sea Ice
Land surface(Ice sheet surface
mass balance)
Ice sheet(Dynamics)
Ice sheet -> Land
n No fields passed; placeholders only
GreenlandSMB:ComparisonwithRACMO2CESM RACMO2
SMB(Gt/yr) 359± 120 376 ± 117
• Goodmatchinablationzones• Accumulationisoverestimatedintheinteriorandunderestimatedin
thesoutheast(smootherorographyinCESM)
Courtesy of M. Vizcaíno!"#$%&'&()& *+!$,-%&.//&()&
Greenland ice sheet SMB (kg m-2 yr-1), 1960-2005
Red = net accumulationPurple = net melting
GreenlandSMB:20th v.21st century20th-century (1980-1999) RCP8.5 (2080-2099)
SMB (Gt/yr) 372± 100 -78± 143
Greenland ice sheet SMB (kg m-2 yr-1)
• ForRCP8.5,precipitationincreases,butmeltandrunoffincreasemore.• Warmingisgreatestinnorth(lessseaice),leastinsoutheast(weakerMOC).• AverageSMBisnegativeby2100,implyinglong-termdecayoficesheet.
Red = net accumulationPurple = net melting
Courtesy of M. Vizcaíno !"#$%""& '$#$%""&
Glimmer-CISMinCESM1• CapturesbroadpatternsofGreenlandiceflow,butoutlet
glacierstendtobeslowanddiffuse• Icesheettooextensiveinnorthandeast(positiveSMBbias)
(a) Greenland balance velocities based on observed thickness and the SMB of Ettema et al. (2009)
(b) Vertically averaged Greenland velocities simulated by Glimmer-CISM in CESM (top-ranking ensemble member, Lipscomb et al. 2013).
LandicegoalsforCESM2
• Parallelicesheetmodelwith“higher-order”flow;validinallpartsoftheicesheet
• Land-iceSMBcalculationssupportedbydefault;notjustforcustomLIWGsimulations
• Improvedtreatmentofsnowphysics(deepfirnlayers,insteadofsnowdepthcappedat1m)
• Supportfortwo-waycoupling:Changesinicesheetelevationandextentcanfeedbackontheclimate
HierarchyofStokesapproximations
• Previousgenerationoficesheetmodelsmostlyusedshallow-iceorshallow-shelfapproximations
• Newermodels(BISICLES,Elmer-Ice,ISSM,PISM,PSU,etc.)haveoneormorehigher-ordervelocitysolvers
• PrimarygoalforCISM2wasarobust,parallelhigher-ordersolver
Stokes3Dsolveforu,v,w,p
Higher-order(Blatter-Pattyn)3Dsolveforu,v
Depth-integratedhigher-order2Dsolveforu,v
Shallowiceapproximation
(Verticalshearstresses)
Shallowshelfapproximation(Membranestresses)
CISM2CISM2 wasreleasedinOct.2014:
• Codeathttp://oceans11.lanl.gov/cism/;git repoat https://github.com/cism
• Paralleldynamicalcore(Glissade)withsuiteofhigher-ordervelocitysolvers
• Incrementalremappingformassandtemperaturetransport
• Newtestcases(shallow-ice,higher-order)withPythontools
• Improvedcouplinginterface• PortedtoCESMbyBillSacks
SimulatedCISM2velocities.Top:GreenlandicesheetBottom:RossIceShelf
CISM2: ISMIP-HOM tests• Compared higher-order model results to community benchmarks
(Pattyn et al. 2008) for problems with small-scale variations in topography and basal traction
• Glissade’s higher-order solvers agree well with benchmarks
ISMIP-HOM Test A: Sinusoidal pattern in basal topography at 6 grid scales (Glissade output shown by black lines)
CISM2:Greenlandthickness
Observedicethickness(m)
CISMicethickness(m)after50ka spin-up
Modelminusobservedthickness(m)
• CISM2isrobustandefficientforlongGreenlandspin-ups(~1200yr/wallclockhouron4kmgrid).
• Aftera50kyr spin-upwithSMBforcingfromRACMO2,modelthicknessisclosetoobservations(abitthininnorthandwest,thickinsoutheast).
CISM2: Greenland velocities• Usingadepth-integratedHOsolver(DIVA)andpseudo-plasticsliding
law,CISMvelocitiesareingoodagreementwithobservations.
Observedsurfacespeed(m/yr,logscale)
ModeledsurfacespeedinCISM(m/yr,logscale)
CISM2: Greenland basal state• CISM’sdistributionoffrozenandthawedregionsissimilartoestimates
basedonobservationsandothermodels.
SynthesisofGreenland’sbasalthermalstatefromMacGregoretal.(2016)
Basalwaterdepth(m)inCISM;purple =frozen(nobasalwater),red =thawed(waterpresent).
CESM1.0 CESM2.0
Serial,shallowiceapproximation Parallel,higher-order approximation
One-waycoupling(CLMà CISM) Two-waycoupling(CLMßà CISM)with dynamic landunits
DownscalinginCISM/Glint,withSMBnotconserved
Downscaling inthecoupler,withSMBconserved
1-msnowpackinCLM Firn modelinCLM(10-msnowpack,improvedsnowdensity)
SMB computedonlyinrunsdonebyLIWG SMB computedinallruns
LandiceprogressinCESM
IcesheetsinCESM1Land -> Ice sheet (10 classes)n Surface mass balancen Surface elevationn Surface temperature
Coupler
Atmosphere
Ocean
Sea Ice
Land surface(Ice sheet surface
mass balance)
Ice sheet(Dynamics)
Ice sheet -> Land
n No fields passed; placeholders only
IcesheetsinCESM2Land -> Ice sheet (10 classes + bare land )n Surface mass balancen Surface elevationn Surface temperature
Coupler
Atmosphere
Ocean
Sea Ice
Land surface(Ice sheet surface
mass balance)
Ice sheet(Dynamics)
Ice sheet -> Landn Ice extentn Ice surface elevationn SMB mask
Ice sheet -> Oceann Solid and liquid fluxes
Ice sheet -> Atmosphere (offline)n Surface topography
Greenland surface mass balance in CESM2Overall Greenland climate is very good.
• SW and LW biases reduced compared to CESM1
• Recent tuning has improved winds and precipitation
• Model captures the western ablation zone
• Too little ablation in north; cold climate bias
GreenlandSMBinCESM2onCLMgrid(left)anddownscaledtoCISMgrid(right),comparedtoRACMO2(center).Red =accumulation,blue =ablation
Antarctic surface mass balance in CESM2
• Excellent Antarctic climate and SMB
• Improvements in snow physics have reduced spurious melting
• Good baseline for warmer climates with increased ablation AntarcticsurfacemassbalanceinrecentCESM2
tuningruncomparedtoRACMO2.Red =accumulation, blue =ablation
LandicegoalsforCESM2+• Supportmultipleicesheets(notjustGreenland)withappropriate
physicsandnumericso Antarcticao Paleo icesheets(e.g.,Laurentide)
• Worktowarddynamiccouplingofmarineicesheets
o Long-term:Interactivecouplingwithoceancirculationinsub-shelfcavitiesandadynamicice/oceaninterface
o Nearterm:UsesimplemodelsandparameterizationstolinkoceanGCMoutput(atvariousresolutions,withorwithoutcavities)tosub-shelfmelting
• ParticipateintheIceSheetModelIntercomparisonProjectforCMIP6
o ExperimentsforstandaloneISMsandcoupledAOGCM/ISMs
CurrentstateofCISM• User-friendly• Portable• Maintainableandextensible• Robust• Verified• Versatile
• SuiteofStokesparameterizations(BP,DIVA,SSA,SIA)• Butsomephysicsparameterizationsneedmoreresearch
• Efficient• NativeFortransolverisfastformoderateresolution
Greenland(~4km),adequateforfineresolution(~1–2km)• Solverimprovementswouldfacilitatewhole-icesheet
simulationsforAntarctica
NewCISMphysicsandnumerics• Newphysicsformarineicesheets
o Grounding-lineparameterization(complete)o Damage-basedcalving(inprogress)o Sub-shelfplumemodel(inprogress)o Marineicecliffinstabilityandhydrofracture
• Otherphysicsimprovementso Evolutionarybasalhydrologyo Fullyparallelisostasymodel
• Improvedefficiency(for1-2kmresolution)o Fastermatrixsolver(betterpreconditioningforiceshelves)o Betterloadbalancing(removeice-freeoceancells)
CISMAntarcticicesheetsimulations• CISM’s higher-order velocity solvers can handle Antarctica
out of the box at resolutions down to 1 km.• Challenge is to simulate realistic long-term evolution.
ObservedAntarcticsurfacevelocities(m/yr,logscale)
ModeledAntarcticsurfacevelocities(m/yr,logscale)
MarineIceSheetModelIntercomparisonProject
• MISMIP3d(Pattyn etal.2013)• Perturbedbasalslidingparametersgivelateralvariationandbuttressing,withcurvedgroundinglines
• CISMusesagroundinglineparameterization(GLP)toresolvesubgrid variationsinbasaltractions
• Thisallowsustomodelgroundinglinesaccuratelyatpracticalresolutions(~1–2km)
• MISMIPconsistsofidealizedexperimentsthattestamodel’sabilitytotrackgrounding-lineadvanceandretreat.DoestheGLreturntoitsstablestartingposition?
MISMIP3d: Applied basal perturbation
SSA withGLP(1km):GLreturnstostartposition(598km),closetoanalyticsolution(612km)
Black =startingposition; red =advance;lt.blue=return
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Channel length (km)
GL position throughout experiments
StndP75SP75R
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)
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StndP75SP75R
SSA withoutGLP(1km):GLtoofarretreatedatstart(504km)
andfailstoreturn
Startfromsteadystate;runfor100yearswithbasalperturbation;turnoffperturbationandreturntosteadystate.
MISMIP3d: Applied basal perturbation
SSA withGLP:GLreturnstostartposition(598km),closetoanalyticsolution(612km)
Black =startingposition; red =advance;lightblue=return
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StndP75SP75R
DIVAwithGLP:GLreturnstostartposition(558km);iceissofterwithverticalshearstresses
554 556 558 560 562 564 566 568 570 572 5740
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StndP75SP75R
• SSAgroundinglineisclosetoanalytic1Dsolution.• DIVAandBPresultsareclosetobenchmarkStokessolution.
MISMIP summary
• Veryhighresolution(~200m)doesnotseemnecessaryforaccurategrounding-lineresolution.WithaGLP,aresolutionof~1kmisadequate,evenwithasharptransitionzone.
• Depth-integratedviscosityapproximation(DIVA)appearstobeanaccurateandefficientcompromisebetweenBlatter-PattynandSSA.
• Thisisgoodnewsformodels!At1kmwithadepth-integratedsolver,itisfeasibletomodelentireicesheets,evenonCISM’sstructuredgrid.
Calving• CalvingaccountsforabouthalfofmasslossfromAntarctica
andGreenland.• CISMhasseveralsimplecalvingschemesbasedonice
geometryorlocation:o Calveallfloatingice.o Calveicewherethebedisdeeperthanacriticaldepth.o Calvefloatingicewhenthinnerthanacriticalthickness.
• Theseschemesdonotaccountforicemechanics.Icebergscalvewhentheiceissufficientlydamaged.
Schematicoffloatingicetonguewithcrevassesleadingtocalving.CourtesyofM.Whitcomb.
Damage-basedcalving• DOEcollaborationtodevelopadamage-basedcalvingmodel:U.
Michigan(J.Bassis,M.Whitcomb),LANL• Iceisdescribedbyadamagetracerrepresentingcrevassedepth.
CalvingoccurswhenD=1.• Tensilestressandbasalmeltingopencrevasses;gravitationalforces
closecrevasses.• Modelappliedtoobservedicestreamsandicetongues:Erebus,
Drygalski,PineIsland,Petermann
ModeleddamageofErebusIceTongue,comparedtoobservedthicknessprofile.CourtesyofM.Whitcomb.
Sub-shelfmelting
• Sub-shelfmeltratesaresometimesparameterizedasafunctionofdepth,failingtocapturethespatialstructureofmelting.
• ItisexpensivetorunoceanGCMsbeneathiceshelves(requiredgridresolution~2km),andnotalloceanmodelshavethiscapability.o POPgridhasaverticalwallattheshelfedge.o MOM6hasbeenrunwithoceancavities,butnotyet
operationalforglobalsimulations.
• Canweestimatesub-shelfmeltrateswithamodelofintermediatecomplexity?
Plumemodel• Holland,Jenkins&Holland(2008)modeledoceanflowinthecavity
beneathastaticiceshelf.TheysuggestedthatoceanGCMresultscanbeexplainedintermsofasteady-stateplumemodel.
• Theplumeisawell-mixed,buoyantlayerattheiceshelfbase,characterizedbythicknessD,temperature T,salinityS,andvelocityu=(u,v).
• Wearegiventhecavitygeometry(shelfbase,bedtopography) andtheambienttemperatureandsalinityTa andSa.
• TheplumemodelisbeingappliedtotheISOMIP+experimentsforsub-shelfoceanmodels(Asay-Davisetal.2016).
• T0 =-1.9oC,Tbot =1.0oC
• S0 =33.8psu,Sbot =34.7psu
Plumethicknessandvelocity
• Dominantflowisupslope(eastward),limitedbydraganddivertednorthwardbytheCoriolis force.
• Theplumethicknessis5to20minmostofthedomain.• Waterpilesuponthenorthcavityedge,drivinganeastwardjet.
Basalmeltrate
Plumemodel
• PreliminaryISOMIP+resultsfromPOP2X andMPAS-Ocean,courtesyofX.Asay-Davis
• LargestmeltratestendtobeinNWandSWcornersofthedeepcavity
• PlumemodelandMPAS-OceanarelessnoisythanPOP2x(noiseassociatedwithz-gridstair-stepping)
Plumesummary
• PlumevelocitiesandmeltratesarecomparabletotheresultsofoceanGCMs.o Thereissignificantvariabilityamongoceanmodels.ForISOMIP+
theplumemodelisdifferent,butnotclearlybetterorworse.
• AlthoughlessphysicallyrealisticthanoceanGCMs,theplumemodelissimpler,cheapertorun,andeasiertocouple.o Couldbeforcedbyanoceanmodelwithoutcavities;justrequires
verticalprofilesofTa andSa neartheshelfedge
• Nextsteps:o ApplytoMISOMIP1testcase(coupledoceanandicesheet).
o TestinmorerealisticGreenlandandAntarcticageometries.
IceSheetModelIntercomparisonProjectforCMIP6(ISMIP6)
• ISMIP6isanewtargetedactivityoftheClimateandCryosphere (CliC)projectoftheWorldClimateResearchProgram.
• Primarygoal:ToestimatepastandfuturesealevelcontributionsfromtheGreenlandandAntarcticicesheets,alongwithassociateduncertainty
• Secondarygoal:Toinvestigatefeedbacksduetodynamiccouplingbetweenicesheetandclimatemodels,andimpactsoficesheetsontheEarthsystem
• TheLIWGplanstorunISMIP6standaloneexperimentswithCISM(forbothicesheets)andcoupledexperimentswithCESM(forGreenland).
ExperimentaldesignforISMIP6
1. ExistingCMIP6experimentstobeanalyzed intermsoficesheetforcing
2.StandaloneicesheetexperimentsbasedonCMIP6modeloutputtoestimatepastandfuturesealevelrise,andexploreuncertaintyduetoicesheets
3.CoupledAOGCM-ISMexperimentstoexploreimpactsandfeedbacksduetoicesheets
CMIP6exptobeusedbyISMIP6(allAOGCM)
- Pre-industrial control- AMIP- 1%peryrCO2 to4xCO2- Abrupt4xCO2- CMIP6Historical Simulation- ScenarioMIP RCP8.5/SSP5x(uptoyear2300)- LastInterglacial PMIP
Standalone ISMIP6exp (ISMonly)
- ISMcontrol- ISMfor lastfewdecades(AMIP)- ISMforthehistoricalperiod- ISMforcedby1%peryrCO2 to4xCO2- ISMfor21st /23rd century(RCP8.5/SSP5x)- ISMforLastInterglacial- ISMspecificexperimentstoexploreuncertainty
Newproposed ISMIP6exp (coupledAOGCM-ISM)
- Pre-industrial control- 1%peryrCO2 to4xCO2- ScenarioRCP8.5/SSP5x(toyear2300)
Summary• Sea-levelriseisstillawide-openscientificproblem,largely
becauseofuncertaintiesinthedynamicsofmarineicesheets.
• TheCESMcommunity—withitsinterdisciplinaryworkinggroupstructure,linkstotheacademiccommunity,andexperienceindevelopingCESM1andCESM2—iswellequippedtotacklethescience.
• CISMisaccurate,efficientandversatileenoughtosupportsimulationsoftheGreenland,Antarcticandpaleo icesheetsoverthenextseveralyears.
• CISMcanreadilyincorporatenewdynamicsandphysicstosupportemergingscience.
LandIceWorkingGroupinfo
Webpage:http://www.cesm.ucar.edu/working_groups/Land+Ice/
Liaisons:GunterLeguy ([email protected]) andBillSacks([email protected])
Co-chairs:JanLenaerts([email protected]) andBillLipscomb([email protected])
Upcomingmeetings:• WinterLIWGmeeting,Boulder,10-11January2018• 23rdannualCESMworkshop,Colorado,18-21June2018