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Atmosphericsensitivitytomarginal-ice-zonedrag:localandglobalresponses

IanA.Renfrew1*,AndrewD.Elvidge1,2andJohnM.Edwards2

1SchoolofEnvironmentalSciences,UniversityofEastAnglia,Norwich,UK2MetOffice,Exeter,UK*Correspondingauthor:Prof.IanRenfrew,CentreforOceanandAtmosphericSciences,SchoolofEnvironmentalSciences,UniversityofEastAnglia,NorwichResearchPark,Norwich,NR47TJ,UK.Email:i.renfrew@uea.ac.ukFirstsubmission:29thOctober2018Revisedsubmission:19thJanuary2019Forsubmissionto:QuarterlyJournaloftheRoyalMeteorologicalSociety

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Abstract[247/250words]Theimpactofaphysically-basedparameterizationofatmosphericdragoverthemarginal-ice-zone(MIZ)isevaluatedthroughaseriesofregionalandglobalatmosphericmodelsimulations.Thesea-icedragparameterizationhasrecentlybeenvalidatedandtunedbasedonalargesetofobservationsofsurfacemomentumfluxfromtheBarentsSeaandFramStrait.TheregionalsimulationsarefromMarch2013andmakeuseofacollectionofcold-airoutbreakobservationsinthevicinityoftheMIZforvalidation.Theglobalmodelanalysisusesmultiple48-hourforecaststakenfromastandardtestsuiteofsimulations.OurfocusisontheresponseofthemodelledatmospheretochangesinthedragcoefficientovertheMIZ.Wefindthattheparameterizationofdraghasasignificantimpactonthesimulatedatmosphericboundarylayer:forexample,changingthesurfacemomentumfluxbytypically0.1-0.2Nm-2(comparabletothemean)andlow-leveltemperaturesby2-3KinthevicinityoftheMIZ.ComparisonsagainstaircraftobservationsoveranddownwindoftheMIZshowthatsimulationswiththenewseaicedragschemegenerallyhavethelowestbiasandlowestroot-mean-squareerrors.Thewindspeedandtemperaturebiasesarereducedbyupto0.5ms-1and2Krespectively,comparedtosimulationswithtwosettingsofthepreviousdragscheme.Intheglobalsimulationstheatmosphericresponseiswidespread–impactingmostoftheArcticandAntarcticsea-iceareas–withthelargestchangesinthevicinityoftheMIZandaffectingtheentireatmosphericboundarylayer.Keywords:surfacemomentumflux;surfacedrag;seaice;atmosphericboundarylayer;MetUM;

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1.IntroductionThemarginal-ice-zone(MIZ)isthebandofpartiallyice-coveredwaterthatseparatestheice-freeoceanandthemainArcticorAntarcticsea-icepack.Morphologicallyitisheterogeneous;consistingofavarietyofsea-icetypeswithbrokenorraftedfloesonarangeofspatialscales,interspersedwithleadsandlargerpatchesofopenwater.TheMIZisrelativelydynamic:respondingtothewind,oceancurrentsandinternaldynamics(Notz2012),aswellasoceanwaves(Kohoutetal.2014).InrecentyearsthesummertimeArcticMIZhaswidened,from~100kmtoalmost150km(StrongandRigor2013).Furthermore,thistrendisprojectedtocontinueovercomingdecadeswithmostclimatemodelssuggestingatransitiontoa‘newArctic’isunderway;where,insummer,sea-iceconditionsforthewholeArcticregionwillbeanalogoustoaMIZ(e.g.Sigmondetal.2018).Forthisreason,determiningthesensitivityoftheatmospheretodragimpartedbytheMIZisofincreasingimportanceforimprovingourunderstandingandpredictabilityofweatherandclimateintheArctic.Theatmosphericboundarylayer(BL)usuallychangesrapidlyovertheMIZ.Duringoff-iceflowthereistypicallyarapidincreaseinBLtemperatureandhumidityconcomitantwithincreasesinsurfaceturbulentheatfluxes,andatransitionfromstabletosaturatedneutralorunstablestratification(e.g.Brümmer1996,1997;RenfrewandMoore1999).BLclouddevelopmentdownstreamiscommon(e.g.Youngetal.2017),oftenintheformofcloudstreetsorotherformsofshallowconvection.NumericalsimulationsshowthisBLdevelopmentissensitivetothedistributionofsea-ice,withsignificantdifferencesintemperature,humidity,wind,cloudandsurfacefluxesovertheMIZanddownstreamdependingontheicefractionanddistribution(e.g.Liuetal.2006;Gryschkaetal.2008;Chechinetal.2013).Duringon-iceflowaninternalstableboundarylayerwilltypicallydevelopastheBLiscooledthroughdownwardsurfaceheatfluxes(e.g.Overland1985).Cloudyboundary-layersarecommonasthemoistureladenon-iceflowcools.Inshort,theMIZmarksazoneofchangefortheBL,aswellastheoceansurface;forabroadercontextseeVihmaetal.(2014).Atmosphericboundary-layerchangesacrosstheMIZarearesultofchangesinthesurfaceexchangeofmomentum,heatandmoisture.Thesearechallengingquantitiestomeasuredirectly,butobservationsarevitaltounderstandBLprocessesandrepresentthesefluxesinnumericalmodels.OverwatermomentumexchangecanberepresentedbytheCharnockformula,whichprovidesanaerodynamicroughnesslength(z0)thatisdependentonthefrictionvelocity(𝑢∗)–Charnock(1955).Oversolidseaicethemomentumfluxhasoftenbeenrepresentedbyaconstantz0orconstantneutraldragcoefficient(CDN);observationshavesuggestedCDNbetween1.2-3.7x10-3dependingonicemorphology(e.g.Overland1985;Castellanietal.2014;Pettyetal.2016).OvertheMIZ,momentumfluxobservationshavebeenscarce,butlimitedobservationshavefoundrelativelylargeCDNvaluesandapeakinCDNovertheMIZ–asreviewedinOverland(1985),LüpkesandBirnbaum(2005),andElvidgeetal.(2016).TherecentstudyofElvidgeetal.(2016,hereafterE2016)usedlow-levelaircraftobservationstogenerateover200estimatesofaerodynamicdragovertheMIZ–doublingthenumberofobservationsavailableintheliteratureatthattime.E2016confirmedtherewasapeakindragcoefficientforicefractionsof0.6-0.8.Theyalsoillustratedthatthemorphology

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oftheicewascriticalindictatingitsroughness:e.g.,fortherelativelysmallunconsolidatedfloesoftheBarentsSeathemedianCDNwas2.5x10-3,whileforthelargersmootherfloesofFramStraitthemedianCDNwas1.2x10-3.TheaerodynamicdragovertheMIZiscomposedoftwocomponents:askindragandaformdrag(seeFig.1).Theskindragbeingduetofrictionandtheformdragduetopressureforcesonverticalfacessuchasfloeedgesorridges(Arya1973).Innumericalweatherandclimatepredictionmodelsthissurfacedragmustbeparameterizedviaasurfaceexchangescheme.Overseaicethishastraditionallybeentreatedrathercrudely,oftenwithonevalueofCDNfor100%seaiceandsonoaccountingforicemorphology(i.e.noaccountingfordifferencesinformdrag).Overpartiallyice-coveredgridboxesa“mosaicmethod”iscommonlyemployed,whichtakesaweightedaverageofthefluxesoverwaterandovericeinproportiontotheirsurfaceareas(e.g.Vihma1995).However,thisapproachisnotappropriateovertheMIZ.ItresultsinalinearfunctionofCDNwithicearea(A),incontrasttothepeakinCDNforA=0.6-0.8seeninobservations(E2016).InameticulousseriesofarticlesLüpkesandcolleagueshavedevelopedahierarchyofphysically-basedparameterizationsformomentumexchangeoverseaicewhichaimtocapturetheeffectsofformdrag.Herethetotalsurfaceexchangeisparameterizedby

𝐶$% = (1 − 𝐴)𝐶$%_-./01 + 𝐴𝐶$%_340 + 𝐶$%5 (1)whereCDN_waterandCDN_ice,aretheneutraldragcoefficientsoverwaterandiceandCDNfistheneutralformdragcoefficientrepresentingthedragdueicefloeedges.ThisapproachfollowsfromAyra(1973)andisdevelopedanddiscussedmostrecentlyinBirnbaumandLüpkes(2002),LüpkesandBirnbaum(2005),Lüpkesetal.(2012),andLüpkesandGryanik(2015).Lüpkesetal.(2012)providedetailsofahierarchyofschemes,prescribingCDN10asafunctionofvarioussea-iceproperties,suchasicefractionand(optionally)freeboardheightandfloesize.NoteCDN10istheneutraldragcoefficientreferencedtoaheightof10metres.OneLüpkesetal.(2012)schemeisbecomingwidelyadopted(Table1;Fig.2);forexample,itisavailableasanoptionintheCICEsea-icemodel(Tsamadosetal.2014;Hunkeetal.2015)andhasrecentlybeenimplementedbyusintheMetOfficeUnifiedModel(MetUM).ThisschemeissummarisedbyE2016,whoshowthatthefunctionalformofthisparameterizationisagoodfittotheirlargesetofaircraft-basedobservations.E2016providerecommendedvaluesforkeyparametersforthisschemeintheirTable1anditistheimpactsofthisnewsea-icedragscheme(hereafter‘L12E16’)thatweinvestigatehere.ThesurfaceexchangesofheatandmoistureovertheMIZarelesswellobservedthanmomentumexchange,asoftensimilaroceanandatmospherictemperaturesleadtopoorlyconstrainedfluxestimates(e.g.E2016).Intheabsenceofmoreobservationsandimprovedunderstanding,mostnumericalmodels(includingtheMetUM)useconstantscalarroughnesslengths.Inthisstudyweleavethescalarroughnesslengthsattheirdefaultvalue(z0t=z0q=0.2×z0_skin,wherez0_skinisaprescribedinterfacialroughnesslength,i.e.itdoesnotincludeanyformdragcontribution;seeTable1forvalues).Thisprescriptionhassomejustificationfromobservations(Andreas2002;Andreasetal.2010),butlargerMIZdatasetsarerequiredfora

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moresophisticatedapproach.Consequently,thedifferencesintheheatandmoistureexchangecomefromanydifferencesinz0_skinand–notingthatthescalarexchangecoefficientshaveadependencyonz0–theinclusionofformdragfortheL12E16scheme.NoteLüpkesandGryanik(2015)makesometheoreticalsuggestionsforhowz0tandz0qcouldvaryovertheMIZ,butwithoutcomprehensiveobservationstoverifytheirsuggestions,herewehavechosenthesimplerapproachofusingdefaultvalues.HereweinvestigatetheimpactoftheL12E16schemeontheatmospherethroughaseriesofnumericalmodelexperiments,i.e.weevaluateatmosphericmodelsensitivitytoaphysically-basedsea-icedragscheme.Themodelisruninanatmosphere-onlyconfigurationforbothregionalandglobaldomains,soassessingbothlocalandglobalimpacts.WefindtheimpactontheBLissignificant,especiallyonnear-surfacewindsandtemperatures,andbeneficial.Insection2wepresentthedetailsofourapproach;sections3,4&5describethesurface-layer,BLandglobalresponsestothenewscheme–includingaquantitativeassessmentofaccuracy;section6providesadiscussionandconclusions.2.Methods2.1ThemodelTheMetUMisastate-of-the-artatmosphericmodelsolvingthedeepnon-hydrostaticcompressibleequationsforafluidandincorporatingnumerousparameterizationsforphysicalprocessessuchasradiation,microphysics,boundary-layerturbulenceandsurfaceexchange.ItisusedbytheMetOfficeforoperationalweatherforecastingandasacomponentinalltheirclimatemodels(e.g.Waltersetal.2017).Hereweusebothregional(limited-area)andglobalatmosphere-onlyconfigurationsofversion10.6oftheMetUM.Scientificconfigurations(whicharetypicallyavailableatseveralsuccessiveversions)arealsonamed,numberedanddocumentedaccordingtotheirfunction.Forexample,Waltersetal.(2017)describetheGlobalAtmosphere'GA6'configurationandtheGlobalLand'GL6'configurationthatdefinesthetreatmentofthelandsurfaceandalso,inuncoupledsimulationswithoutinteractiveoceanandseaicemodels,surfaceexchangeoveropenseaandsea-ice.Here,globalsimulationswillbebasedonGA6/GL6.TheregionaldomaincoversmostoftheGreenland,northernNorwegianandwesternBarentsSeasandiscentredatabout77oN,10oE,justsouthofSvalbard;itusesahorizontalgridspacingof4kmwith70verticallevels.Generallyphysicalparameterizationsfollowthoseusedoperationallyforthe‘UKV’configuration(seeKendonetal.,2012;Tangetal.2013).ThissetupoftheMetUMhasprovenreasonablyaccurateatsimulatingcasesofcold-airoutbreaksandpolarlowsinthisarea(e.g.Sergeevetal.2017),aswellascasesoforographicflowsintheAntarctic(Elvidgeetal.2015,2016;Orretal.2014;ElvidgeandRenfrew2016).Theglobalconfigurationusesahorizontalgridspacingof~40km(N320)with70verticallevelsandalsogenerallyfollowsoperationalsettings.2.2ObservationaldataTheregionalsimulationsareforaten-dayperiodfrom21-31March2013duringtheACCACIA(AerosolCloudCouplingandClimateInteractionsintheArctic)fieldcampaign.Thisperiodwasdominatedbyoff-iceflow,i.e.northerlycold-airoutbreakconditions.Low-levelresearch

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aircraftobservationsovertheMIZintheBarentsSeaandFramStraitweremadeon8separatedays(seeE2016)andtheseareusedasvalidationdataforournumericalexperiments.Thelow-level(typically30-40m)aircraftlegshavebeendividedinto209runsof9kminlength.ThisACCACIA‘surface-layerdataset’includesstandardmeteorologicalvariables(e.g.temperature,humidity,wind,pressure,etc),turbulent&radiativefluxes,andinfra-redderivedseasurfacetemperature.ThemeteorologicalvariableshavebeeninterpolatedtostandardheightsusingtheCOAREstability-dependentbulkfluxalgorithm.Theturbulentfluxesareestimatedfromcovariancesandhavebeencarefullyqualitycontrolled(followingPetersenandRenfrew2009;CookandRenfrew2015);195runshavegoodqualitymomentumfluxvalues.Figure3showswheretheseobservationswereobtained,colourcodedbysea-icefraction(asdeterminedfromtheaircraft’sseasurfacetemperature–seeE2016,AppendixB).Thesurface-layerdatasetcoversafullrangeofsea-icefractionsovertwoseparatelocations:intheBarentsSea,totheSEofSvalbard,wheresea-icewascharacterisedbyrelativelysmallunconsolidatedfloes;andinFramStrait,totheNorthofSvalbard,wheresea-icewascharacterisedbylargersmootherfloes.Inadditiontothesesurface-layerobservations,meteorologicaldatafromdropsondesprovidessomeobservationsofBLdevelopmentovertheMIZforonecase.2.3ExperimentalDesignTheregionalexperimentsareforthe21-31March2013ACCACIAperiod.Simulationsof24hourswererun,initialisedat00UTCeverydayfromtheMetOffice’soperationalglobalanalyses.Hourlymodeldatahasbeeninterpolatedinspaceandtimetomatchthatofthesurface-layerdataset.Theseobservationstypicallycorrespondtoaleadtimeof12-15hours.Theglobalmodelexperimentsincorporate24casestudyhindcastsof5-dayduration,with12casesacrosstheextended-wintermonths(NovembertoAprilinclusive)and12casesacrosstheextended-summermonths(May-October)between2011and2014.ThisexperimentaldesignisstandardpracticeforsensitivitytestingandtriallingparameterizationschemesintheMetUM(e.g.Elvidge,2016).ThreemodelexperimentshavebeencarriedoutwithdifferentspecificationsofsurfaceroughnessovertheMIZandseaice.TheexistingMetUMschemeusestwovaluesofz0overice:onefortheMIZ(z0_MIZ)setforA=0.7andoneforsolidseaice(z0_ice)setforA=1(seeTable1andFig.2).TheCDNforeachgridpointiscalculatedbyinterpolationbetweenthesevaluesandthatofopenwater,i.e.amosaicmethod–seeUMDocumentation(2016).Twosettingsoftheexistingschemeareused–SmoothandRough–whilethethirdexperimentusesthenewseaicedragscheme‘L12E16’.TheSmoothz0valueshaveoftenbeenusedinclimatemodelconfigurations,whiletheRoughz0valueshavebeenusedintheglobaloperationalforecastmodelformanyyearsandafewclimatemodelconfigurations(seeTable1).TheL12E16parameterizationbecamepartoftheglobaloperationalforecastingsystemon25thSeptember2018andwillalsobeusedinfutureconfigurationsoftheMetOffice’sclimatemodelsviatheGL8(GlobalLand8)configuration.Figure2showstheneutraldragcoefficient,CDN,asafunctionoficefractionforthethreeroughnesslengthsettings.CDNisverylargeforintermediateicefractionsintheRoughsetting,

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duetoaverylargevalueofz0_MIZ(originallymotivatedbysomeindirectAntarcticMIZestimatesofroughnessbyAndreasetal.1984).Ithasbeenknownforsometimethatthisverylargez0_MIZvalueisnotrepresentativeofmanyMIZenvironmentsandlowervalueshavebeensubstitutedwhereappropriate(e.g.Outtenetal.2009;Petersenetal.2009).However,ithasremainedintheoperationalconfigurationoftheMetUMpartlyasthealternative(theSmoothsetting)appearedtoosmoothandpartlybecauseitwasnotclearhowtoreplaceit.Figure2illustratesthatL12E16hasaCDNthatisupto35%higherthantheSmoothsetting,andmuchlowerthantheRoughsetting.Bydesign,L12E16fitstheobservationsofE2016verywell:approximatelyequaltothemedianvaluesofthe0.4,0.6and0.8icefractionbinsandwellwithintheinter-quartilerangeforallbins.TheSmoothCDNisalsowithintheobservedinter-quartileranges,whilsttheRoughCDNiswelloutsideofthem(theBarentsSeaandFramStraitdonothavesubstantialsea-iceridges).ItisworthnotingthatalthoughtheACCACIAaircraftobservationsareusedasvalidationdatafortheregionalmodelexperiments,theCDNvaluesinthemodelarederivedfromtheRough,SmoothandL12E16parameterizationswithicefractiontakenfromthemodel’soperationalsurfaceboundaryconditions,i.e.fromtheOSTIA(theOperationalSeasurfaceTemperatureandseaIceAnalysis)dataset.Inotherwords,theACCACIAperiodexperimentsareatestofthesethreeschemesabilitytoreproducetheobservedatmospherewithoutinsituobservationsofthesurface.Inboththeregionalandglobalmodelexperimentswearetestingthemodel’ssensitivitytoMIZdrag,bothnearthesurfaceandintheBL.WeprovideaquantificationofthissensitivitybothlocallyandgloballywhichwilllikelybecomparableinotheratmosphericmodelsthatemploycomparableBLparameterizations.Noteincomparingthesimulationsagainstmeteorologicalobservationsweareinvestigatingwhichdragschemeperformsthebest,withallotherfactorsheldthesame,i.e.allotherparameterizationsandtheinitialisationdataareidenticalforeachsetofsimulations.3.SensitivityoftheatmosphericsurfacelayertoMIZdragAcomparisonofthethreeregionalMetUMsimulations(Rough,L12E16andSmooth)againsttheACCACIAsurfacelayer(SL)observationsissummarisedinTable2.GenerallytheMetUMsimulatestheatmosphericSLreasonablywellforthiscollectionofcold-airoutbreakcases.OveralltheL12E16simulationsarethemostaccurate:mostfrequentlyhavingthehighestcorrelationcoefficient,lowestbiasandlowestRMS(root-mean-square)errorofthethreeexperiments.ThecorrelationcoefficientsfortheL12E16andSmoothexperimentsaresimilar,withbothnoticeablyhigherthanfortheRoughexperiment.InadditiontheRMSerrorsfortheL12E16andSmoothexperimentsareoftensimilar.ThecleareststatisticaldifferenceisinasignificantlylowerbiasfortheL12E16simulationsforwindspeed,temperatureandpotentialtemperature,comparedtotheotherexperiments.Thebiasesinwindspeedandtemperaturereducetoonly-0.06ms-1and0.02K,whichareremarkablylowforthissortofobservationversusgrid-pointcomparisoninaremotepolarregion.Forexample,inRenfrewetal.(2009)windspeedandtemperaturebiasesof-0.7ms-1and-1.3KwerefoundfortheMetUM

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comparedtoasetofaircraft-basedSLobservationsduringcold-airoutbreakconditions.AlthoughtheL12E16biasinrelativehumidity(RH)isthelowest,intheRoughsimulationsanegativeRHbiascombinedwithapositivetemperature(T)biasresultinaverylowspecifichumiditybiasthroughacompensationoferrors.TheSLcomparisonsareillustratedasscatterplotsforwindspeedandTinFig.4.ForwindspeedtheL12E16experimentillustratesthelargescatterinsimulatedwindsthatiscommontoallthemodelexperiments(theRoughandSmoothcomparisonsappearsimilar–notshown).Evenforthebestperformingdragsetting(L12E16)thecorrelationcoefficientisonly0.53andtheRMSerrorsare>2ms-1forwindspeedsof4-10ms-1,whichcomparespoorlytosimilarcomparisonsformorehomogeneoussurfaceconditions,suchasovertheIrmingerorIcelandSeas(Renfrewetal.2009;Hardenetal.2015).Therelativelylargescatterinwindisalsoevidentfromthemeasuredsamplingvarianceinmomentumfluxwhichisapproximatelythreetimestheexpectedsamplingvariance(c.f.Drennanetal.2007).WesuggestthelargescatterinwindsandmomentumfluxisaresultoftheheterogeneityofthesurfaceroughnesselementsoftheMIZwhichaddsa‘stochasticelement’thatissimplynotaccountedforinthemodel.Thedifferencesintemperaturebetweenthethreeexperimentsareclearinthescatterplots(Fig.4).TherelativelylargeTbiasandscatteroftheRoughsimulationdataisevident.WhilethereareobviousimprovementsfortheL12E16andSmoothdata:thelowerbiasandRMSerrorsareclearandtheleastsquaresregressionlinesareclosertothe1:1line.BoththeL12E16andSmoothTcomparisonshavealowlinearregressionslope:lowtemperaturesarebiasedwarm,highertemperaturesarebiasedcold,byabout1K.AlsoincludedinTable2isacomparisonofmodeloutputagainsteddycovarianceturbulentfluxestimatesbasedontheaircraftobservations(seeE2016fordetails).Comparingmodeloutputdirectlytosuchfluxestimatesisproblematic,ascovariancefluxestimateshavearelativelylargerandomerrorassociatedwiththefinitesamplingofaturbulentprocess(seeDonelan1990;PetersenandRenfrew2009).Thissamplingerrorconfersanuncertainty(oftypically30%)toeveryfluxestimateandleadstorelativelylowcorrelationcoefficientsandrelativelyhighRMSerrors,comparedtothoseofthemeteorologicalvariables.Neverthelessacomparisonisinteresting.ItshowsthattheRoughmodeloutputcomparessignificantlylesswelltotheobservationsthantheL12E16andSmoothmodeloutputs.Themostrobuststatisticforthiscomparisonisthebias.Remarkablythebiasinmomentumflux(τ)is0.00Nm-2fortheL12E16experiment,comparedto0.11and0.03Nm-2respectivelyfortheRoughandSmoothexperiments(notetheobservedmeanτis0.13Nm-2).ThissuggeststhenewdragschemeisdoinganexcellentjobofparameterizingmomentumexchangeovertheMIZ.Recallalthoughthisdatasethasbeenusedtotunethenewdragscheme,thisisstillatestofitsperformance,becausethemodelusesadifferentsea-iceconcentrationfield(OSTIA),tothatobservedfromtheaircraftandusedintheparameterizationdevelopment.Thebiasesforthesensibleheatflux(SHF)andlatentheatflux(LHF)arelargeforallthreemodelexperiments.ExceptionallylargefortheRoughexperiment;forexample,theSHFbiasis132Wm-2,comparedtobiasesof54and46Wm-2fortheL12E16andSmoothexperiments

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(notetheobservedmeanSHFisonly19Wm-2).Back-of-the-envelopebulkfluxcalculationsshowtheselargebiasescannotbeexplainedbybiasesinthemodel’swindspeed,temperature,humidityorsurfacetemperature(Table2).Thereisasmallwarmbiasinsurfacetemperatureinalltheexperiments,butthisisnowherenearlargeenoughtoaccountfortheSHForLHFbiases.Insteadwebelievetheretobeaninadequacywiththeprescriptionofthemodel’sroughnesslengthsforheatandmoisture(sethereasz0t=z0q=0.2×z0_skin,wherez0_skinistheinterfacialroughnesslength).Ourcomparisonsuggestsz0tandz0qmaybetoohigh.Butthelackofreliableobservationsintheliteraturehasrestrictedfurtherinvestigationhere.Infutureworkweplantoanalysenewturbulentheatfluxdatasets,thendevelopandtestanimprovedparameterizationforz0tandz0q,andevaluatethenewsurfaceheatfluxes.Evensoitdoesnotseemplausiblethatimprovingthescalarroughnesslengthscouldfullyaccountfortheverylargeheatfluxbiasesseenhere.Itseemsthatthereareothermodelinadequaciesatplay,whichwealsoaimtoinvestigateinfuturework.Anadvantageofvalidatingagainstasetofaircraftobservationsiswecanevaluatethemodel’sabilitytocapturespatialdistributions.Figure5showsobservationsandmodeloutputfromthreedayswhenwehadmeasurementstothenorthofSvalbard–the25,26,and29March2013.Figure5(a)shows10-mwindspeedobservationsofbetween2-12ms-1fromaNtoENEdirectionandgenerallyslightlyhigherwindspeedsovertheMIZthanoverthesea-icetothenorth(i.e.anicebreeze;Chechinetal.2013).Figure5(b)showsflight-level(~35m)temperaturegenerallyincreasingtothesouthforeachflightandnoticeablyhigherforthemostsouth-easterlyflightwhenwindshadamoreeasterlycomponent.Observationsofspecifichumidityechothespatialchangesseeninthetemperature(notshown).Themodelsallgenerallycapturetheobservedspatialdistributionsinsurface-layermeteorology.Buttherearesomesystematicdifferences;forexample,thewindspeedisgenerallytoohighforthetwoflightstothenorthwestandtheupwindairtemperatureistoolowforthemostnortherlyflight(notshown).Thesesystematicdifferencesarecommontoallthemodelexperiments–andarelikelyduetoinadequatedataforinitialisationorproblemssimulatingthelarge-scaleflowevolution–soarenotdiscussedfurtherhere.InsteadwefocusonhoweachexperimentsimulatesthespatialchangesacrosstheMIZ.Figure5(c)-(f)showthechangeintemperature(ΔT)fromthemostnortherlypointineachflight(i.e.thecoldestpointsfortheseflights).TheobservedΔTisanincreaseofabout4-6KacrosstheMIZwithsomespatialdifferencesbetweeneachflight.AllthreemodelexperimentscapturethespatialdistributionofΔTbutoverestimatethemagnitude.IntheRoughsimulationsΔTreaches12Kinthetwomostnortherlyflights,anoverestimateofmorethan6Kformanypoints.IntheL12E16andSmoothsimulationsΔTisclosertothatobserved,butstillanoverestimatebytypically2-4K.ThemodeloverestimatesofΔTareprimarilyduetooverestimatesoftheSHFforthesecases(notshown),especiallyforthetwomostnortherlyflightswherethewindspeedsaresystematicallytoohigh.TheseflightsareillustrativeofthelargebiasesinSHFfoundoveralltheflights(Table2).Wehavealsoexaminedspatialdistributionsofthewindspeedforthesethreeflights,butnoclearspatialdifferencesareobviousandthereistoomuchscatterintheobservedwindstojudgewhichexperimentperformsbestatsimulatingthespatialdistribution.

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Thecomparisonofthethreesimulationsagainstsurface-layerobservationsovertheMIZisencouragingastheL12E16parameterizationrepresentsasignificantimprovementinaccuracyovertheexistingparameterization–especiallyfortheRoughsetting.TheRoughexperimenthastoomuchmomentumandheatbeingexchangedwiththesurface,leading(onaverage)tosignificantbiasesinwindspeedandtemperature.ThesensitivityoftheatmosphericSLtotheparameterizationchangesissurprisinglylarge–therearemeandifferencesof0.8ms-1and2.5K.4.SensitivityoftheatmosphericboundarylayertoMIZdragWenowinvestigatewhetherthesimulationdifferencesseenintheatmosphericsurfacelayerarecarriedintotheboundarylayer.Fortunately,wehaveanexcellentsetofobservationsofBLevolutionacrosstheMIZfromasetofdropsondesduringacold-airoutbreakon23March2013duringtheACCACIAfieldcampaign.Figure6showscross-sectionsofpotentialtemperature(θ),specifichumidity(q)andwindspeedfromthedropsondeobservations.Itillustratesanarchetypalinternalboundarylayer,whichisdeepeningwithdistancedownstreamwhereitisclosetoneutralstaticstability(across-sectionofθeillustratesthestabilityoftheBLwas,largely,saturatedneutral–notshown).ThisBLstructureistypicalforoff-iceflows(e.g.Brümmer1996,1997;RenfrewandMoore1999).TheBLtop(fromairbornelidar)risesfromaround1000to2000m;althoughnotethatovertheseaice(northof76oN)thereisacoldpoolofairwithasecondaryθinversionat500msituatedwithinthelarger-scaleBL.TheBLθandqincreasesteadilywithdistancesouth,whiletheBLwindspeedincreasesbetween74.5and72.5oNperhapsduetoanice-breezeeffectoradecreaseinthesurfaceroughnessoverthewater.Themodelcross-sections(notshown)arequalitativelysimilartoeachotherand,ingeneral,totheobservations.Therearesomedifferences:thesimulationsdon’thaveacoldpool(orsecondaryθinversion)overtheseaice;thesimulatedBLdepthislowerthanobserved,risingfromaround500to1500m,seeminglyduetoalowerBLovertheseaice;andtheBLtopθinversionsaretoodiffuse(acommonfeatureinsuchsimulations,e.g.Petersenetal.2009).Thedifferencesbetweenthesimulationsareonlyquantitative,forexampletheBLisslightlydeeper(~100m)intheRoughsimulationthanintheL12E16andSmoothsimulations.NotethatabovetheBLthereisalmostnodifferencebetweenthesimulations.TheBLiswell-mixed,solookingatdifferencesinmixed-layeraveragedvariablesisrepresentativeoftheBLresponse.Figure7showsobservedandsimulatedmixed-layeraveragepotentialtemperature(θml),specifichumidity(qml)andwindspeed(Uml),wherethemixed-layeraveragesarebetween50-400m.Allofthesimulationshavesignificantdiscrepanciesinθmlatthemostnortherlydropsondelocation.WhatisinterestingisthedownstreamchangeinθmlissimulatedreasonablywellintheL12E16andSmoothexperiments,butlesswellintheRoughexperimentwhichhastoogreatanincreaseinθmlovertheMIZ,andthennotenoughwarmingfurtherdownstream.ThisisconsistentwiththeoveralloverestimatedSHFintheRoughsimulations(Table2).Therearequalitativelysimilardownstreamchangesinqml:theL12E16simulationmatchestheobservationsreasonably

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well,whereastheRoughsimulationhastoohighagradientinqmlacrosstheMIZ.AllthesimulationsshowanincreaseinBLwindspeedinthecentreofthecross-section.TheRoughsimulationhasthismaximumaround74.5N,whiletheL12E16andSmoothsimulationshaveitaround74Ninbetteragreementwiththeobservations.Thelargestdifferencebetweenthethreesimulationsistothenorth.Thequantitiestendtoconvergewithdownstreamdistance,althoughthedifferencespersistforhundredsofkilometres.Inshort,theL12E16simulationreproducestheobservedspatialgradientsinBLtemperature,humidityandwindspeedmoreaccuratelythantheRoughsimulationforthiscross-section.ThiscaseisanarchetypalinternalBLdevelopmentandthereiseveryreasontosupposetheseresultswouldbegenericforsuchcold-airoutbreakcases.Indeed,examiningourotherACCACIAobservationsdoesprovidesomesupporthere(althoughthiscorroborationislimitedtothesurfacelayer).Itshouldbenotedthatwehaveonlyexaminedoff-iceflows(i.e.cold-airoutbreaks)wheretheBLrapidlytransitionsfromstaticallystabletoneutralconditionsacrosstheMIZ.Theneutralstabilityreflectstheconvectively-drivenmixingwhichlinkstheSLandBLandmeanstheirresponsesaresimilarinthemodelexperiments.Duringon-iceflows,wheretheBLislikelytobestablystratifiedwithshear-drivenmixing,itispossiblethelinkbetweentheSLandBLwillbeweaker.Atpresentwedonothaveaccesstoanappropriatedatasetforon-iceflow,sotestingthisdragparameterization’simpactontheBLduringsuchsituationsisreservedforfuturework.5.GlobalimpactsToexaminethebroaderscaleimpactsofsea-icedragontheatmospherewehaverunastandardtestsuiteofcasesinaglobalconfigurationoftheMetUMwithagridsizeof40km–soatypicalMIZwouldbe5-10gridpointswide.Figures8and9provideresultsfortheNorthernHemisphereandSouthernHemisphererespectively,showingmeansea-icefractionandthenthedifferencesbetweentheL12E16andRoughoutputfor10-mwindvelocities,near-surfaceairtemperatureandm.s.l.pressure;whereeachpanelrepresentsthemeandifferenceover12hindcasts,eachataleadtimeof48hours.TheimpactintheNorthernHemispherediffersgreatlydependingonthetimeofyear,essentiallybecausetheArcticMIZispolewardofthemajorlandmasses,sobothextendedwintertime(November–April)andsummertime(May–October)resultsareshown.FortheSouthernHemisphere,impactsarequalitativelysimilarbetweentheseasons,essentiallybecausetheAntarcticMIZisequatorwardofthelandmasssocold-airoutbreaksarestillcommoninsummer(Papritzetal.2015).Consequently,onlywintertimeimpacts,whicharegenerallyhigheramplitude,areshown.TheimpactoftheL12E16schemeiswidespread,affectingmuchoftheArcticandAntarcticregions.ThedifferencesinwindspeedandairtemperatureareconcentratedovertheMIZsofbothhemispheresandreflectreductionsinsurfacemomentumfluxandupwardheatfluxes,generallyleadingtostrongerwindsandlowertemperatures.TheexceptiontothisisduringsummerintheNorthernHemisphere,wherethemeanheatfluxispredominantlydownward(notshown)duetorelativelywarmairandweakeroff-iceflow;theL12E16schemereduces

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thisdownwardflux,resultinginaweakpositiveimpactonairtemperature(Fig.8f).TheareasthataremostsensitivetothenewschemeincludelargepartsoftheNorthernsubpolarseas–suchastheGreenlandSea,BarentsSea,SeaofOkhotsk,BeringSea,LabradorSeaandHudsonBay–wheresea-icecoveradvancesandretreatsseasonallyandiceareascanhavelargevariability(e.g.CavalieriandParkinson2012).IntheSouthernHemispheretheimpactisalmostcircumpolar,reflectingwhereiceconcentrationsareoftenbelow100%(ComisoandSteffen2001)orarehighlyvariable(ParkinsonandCavalieri2012).ThereisalmostnopartoftheSouthernOceanthatisunaffected.TheimpactintheSouthernHemispheresummerisspatiallysimilarforallvariables(notshown),butitislowerinmagnitudee.g.temperaturedifferencesof<1Kinsummer,comparedtomorethan2Kinwinter.TheimpactoftheL12E16schemeisalsosignificant.ComparedtotheRoughscheme,itreducesthewintertimeMIZsurfacedragbytypically0.1-0.3Nm-2(i.e.comparabletothemean–notshown);andresultant10-mwindspeedsandairtemperaturesby1-3ms-1and1-3K.Thesedifferencesareconsistentwiththeregionalexperiments(Table2)andaresubstantialcomparedtotypicalmodelerrors.Forexample,comparedtomodelbiasesinwindspeedandairtemperatureof0.5ms-1and2.1KforourRoughsimulationsagainstouraircraftobservationsovertheMIZ(seeTable2);0.12ms-1and0.43KforERAIreanalysesoverthecentralIcelandSea(Hardenetal.2015);and0.7ms-1and-1.3KforMetUManalysesoffSEGreenlandduringcoldairoutbreaks(Renfrewetal.2009).NotetheshadinginFigs.8and9hasbeenscaledtoemphasisesignificantdifferences–boldcoloursillustratedifferencesthataregreaterthantypicalmodelbiases.Theimpactonthem.s.l.pressurefieldisalsosignificantbutmorevariablespatiallyandseasonally(Figs.8g,h,9d).Thegreatestdifferencesareupto0.7hPainmagnitude.Thesearesubstantialdifferencescomparedtotypicalmodelbiases,forexample0.3hPaforourRoughsimulationsovertheMIZ(Table2);0.1hPaoverthecentralIcelandSea(Hardenetal.2015);and-1.0hPaoffSEGreenland(Renfrewetal.2009).IntheNorthernHemisphereduringwinter,m.s.l.pressuregenerallyincreasesovertheMIZanddecreaseselsewhere.ThegreaterpressureovertheMIZisprimarilyduetocoldertemperatures(Fig.8e),butthereareclearlysomecompensationsinthemassfieldmorewidely,leadingtodecreasesinpressure.Thesebroaderpressurechangesarecommensuratewiththepositiveimpactonthelow-levelwindfield,whichresultsinastrengtheningofcirculationpatterns.Theimpactisoneofwidespreadpolardivergence–oranincreaseinthemeannortherly,off-icewindsinparticularovertheGreenlandSeaandtheBeringStrait(Fig.8c)–andconsequentlyadecreaseinm.s.l.pressureoverthehighestlatitudesandoversomelandmasses,e.g.Alaska.DuringtheNorthernHemispheresummer,anticycloniccirculationisstrengthened(Fig.8d),leadingtoabroadamplificationofthenorthpolarhigh-pressurecentre(Fig.8h).TheimpactsonSouthernHemispherem.s.l.pressurearemorecomplicated,butalsocorrespondwiththechangesintemperatureandcirculation(Fig.9).TheimpactoftheL12E16schemeonsimulatedm.s.l.pressureandtemperatureisgenerallytoreduceknownbiasesintheoperationalMetUM(i.e.withtheRoughsettings)when

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comparedtoERAInterim(takenastruth).Inparticularalong-standingpositiveMetUMbiasinwintertimeNorthernHemispherepolarm.s.l.pressureissignificantlyimproved.6.ConclusionsandDiscussionTheimpactofMIZsurfacedragontheatmospherehasbeenevaluatedthroughthreesetsofnumericalweatherpredictionmodelsimulations.The‘L12E16’simulationsmakeuseofaphysically-baseddragscheme(Lüpkesetal.2012)thathasrecentlybeentunedwitharelativelylargesetofobservations(Elvidgeetal.2016);whiletheothertwosimulationsmakeuseofthedragschemeusedintheformeroperationalforecastconfigurationoftheMetOfficeUnifiedModel(Rough),andthatcommonlyusedinclimatemodelconfigurationsoftheMetUM(Smooth).Theimpactontheatmosphereissignificantandsurprisinglywidespread.Thedifferencesinsimulatedsurfacefluxesaretypically0.1Nm-2,70and40Wm-2formomentum,sensibleandlatentheatrespectively;leadingtodifferencesoftypically0.5ms-1,2Kand0.2gkg-1forsurface-layerwindspeed,temperatureandspecifichumidityrespectively.Comparingregionalsimulationsagainstacollectionofaircraft-basedobservationsoftheatmosphericsurfacelayeroveranddownstreamoftheMIZ,wefindthatthesimulationemployingtheL12E16schemeperformsthebestoverall.Ithasthelowestbiasinmomentumflux,windspeedandtemperatureandsignificantlyhighercorrelationcoefficientsandlowerRMSerrorsthantheRoughscheme.Theimprovementinthesimulatedmomentumfluxiscomparabletothemean.Basedontheabovewewouldrecommendaphysically-motivatedandvalidatedparameterizationschemeforatmosphericdragovermarginalseaiceisusedinweatherandclimatepredictionmodels.TheL12E16schemebecameoperationalintheMetOffice’sglobalforecastingsystemon25thSeptember20181;whilstasimilarschemewasimplementedon12May2015(cycle41)intheECMWFIntegratedForecastSystem.Thisstudyhaslimitations.Firstly,theprescriptionofroughnesslengthsforheatandmoisturehasnotbeenproperlyaddressed.HeretheirprescriptionisleftunchangedfromtheSmoothsettings.Butthisdecisioniscurrentlyapragmaticone:wedonotyethaveconfidenceinhowtoprescribethescalarroughnesslengths.Furtherresearchisrequiredtodefinescalar

1TheL12E16schemewaspartofalargerpackageofchangestotheglobalforecastingsystemthatweretestedforseveralmonthsaspartofPS41(ParallelSuite41)beforebecomingoperationalon25thSeptember2018.Overallthispackageofchangesledtoreductionsofaround2-5%inRMSerrorinm.s.l.pressureand500hPaheightforboththenorthernandsouthernextratropics,relativetotheoperationalsuiteovertheperiodofparallelrunning(May-September2018).Whilstitisnotpossibletounambiguouslyisolatethecontributionofthesea-icedragschemetothisimprovement,resultsfromotherpre-operationaltestssuggestthatthechangestosea-icedragwereamajorfactorinimprovingperformance,especiallyinthesouthernhemisphereextratropics.

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roughnesslengthparameterizations,validatetheseagainstlargeandreliableobservationdatasetsandthentesttheparameterizationsinmodels.Secondly,thevalidationdatasetusedhereisfromoneregionoftheArcticandforoff-iceflowsresultinginanatmospheretransitioningfrombeingstaticallystabletoneutralorunstable.AsubsequentstudywouldideallymakeuseofobservationsfromotherregionsoftheArcticorAntarcticandwouldincludesomeobservationsfromon-iceflows.Thirdly,thecomparisonofthethreesetsofsimulationsagainstobservationsmaybeaffectedbyfactorsthatarebeyondourcontrol,suchaschangesinthewayotherparameterizationschemesreacttothechangesinsurfaceforcingortheaccuracyofthesurfacerepresentation(e.g.theseaiceconcentration).Wehavedoneourbesttocontrolforthisbyusingthesameinitialisationfieldsandleavingallotherparameterizationsthesame.Theconsistencyofourfindingsovertheregionalandglobaldomainsdoesprovidesomeevidencethatourresultsarerobust,i.e.thatwehavemanagedtoisolatetheimpactsofsea-icedragontheatmosphere.Althoughtheresultspresentedhereareallforonespecificmodel(theMetUM)wewouldassertthatthesensitivitytosurfacedragthatisfoundislikelytobequalitativelyandquantitativelysimilarinothernumericalweatherandclimatepredictionmodels.Atleastforthefundamentalatmosphericpropertiesthatareexchangedwiththesurface–namelymomentum,heatandmoisture–andthesurface-layermeteorologicalvariablesthesedirectlyimpact–windspeed,temperatureandhumidity.Wewouldcontendthattherelationshipbetweenatmosphericsurfacedragandiceconcentrationisnowreasonablywellconstrained.However,thisrelationshipisanindirectone.Thesurfacedragisactuallyafunctionoficemorphology(ridgeheights,meltponddepths,floeheights,etc).Thisisdemonstratedbythefactthatthedragcoefficientfor100%seaice(CDN_ice,an‘endpoint’intheL12E16parameterization)variesdramaticallywithicemorphology;forexample,Elvidgeetal.(2016)foundmedianvaluesofCDN_ice=2.5x10-3fortheBarentsSea(smallerunconsolidatedfloes),comparedtoCDN_ice=1.2x10-3forFramStrait(largersmootherfloes)–seealsoCastellanietal.(2014).Thesearesubstantialdifferencesandinsomeregionssuchdifferencesmayoccuroverrelativelysmalldistances,evensub-gridscale.Theseaicepackcanbeasheterogeneousasthelandsurface,butatpresentisalltreatedasonesurfacetype.OneideatoaddressthiswouldbetoincorporateastochasticelementtosurfaceexchangeoverseaiceandtheMIZ.Stochasticparameterizationisnowwidely-usedinforecastmodelsand,inparticular,inensemblepredictionsystemswhereagreaterensemblespreadisoftensought(e.g.Palmeretal.2005).Itsefficacyinhigh-latitudelocationshasrarelybeenexamined(Jungetal.2016);oneexamplebeingtheimpactonsea-icepredictionofastochasticelementintheparameterizationofsea-icestrength(seeJurickeetal.2016).Astochasticsurfaceexchangeschemeforsea-iceareasmaybeworthinvestigatingasonewaytoincreaseensemblespreadofthehighlatitudeatmosphere.Surfaceexchangecanbedramaticallydifferentduetocontrastingsea-icemorphologiesanditisnotcurrentlyclearhowtoaccountforthisinmodels.Wheremodelsincludeasophisticatedsea-icecomponentthencharacteristicsofthesimulatedsea-icefieldcouldbeusedtoderiveCDN_icedirectly,andindeedCDNintheMIZdirectlytoo,sodispensingwiththeneedforthesortofMIZschemeinvestigatedhere.Developingsuchasurfaceexchangeschemethatisreliable

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andwell-constrainedwillrequireconsiderableeffort.Wheremodelsdon’tincludeasea-icecomponentthenassimilatingsea-icemorphologypropertiesdirectlymaybeappropriate,againwithconsiderableresearchrequired.ForthetimebeingwebelievethesurfaceexchangeschemefortheMIZinvestigatedherewillremainappropriateformanyyearsandparticularlyforuncoupledmodels.AcknowledgementsThisstudywasfundedbyNERC,theNaturalEnvironmentResearchCouncil,grantreferenceNE/I028297/1(ACCACIA)andNE/N009754/1(theIcelandGreenlandSeasProject).TheobservationsweregatheredaspartoftheACCACIAprojectwhichusedtheFacilityforAirborneAtmosphericMeasurement(FAAM)andBAS’sMASINaircraft.Wewouldliketothankallinvolvedinthefieldcampaign,particularlyIanBrooks,TomLachlanCopeandAlexandraWeiss.TheACCACIAsurfacelayerdatasetisavailableatCEDA(www.ceda.ac.uk).ThemodelsimulationswerecarriedoutonMONSooN,acollaborativefacilitysuppliedundertheJointWeatherandClimateResearchProgramme,astrategicpartnershipbetweentheMetOfficeandtheNERC.WewouldliketothankIanBrooksandEdBlockleyforusefuldiscussionsaboutthisresearch,andtwoanonymousreviewersfortheirconstructivecomments.ReferencesAndreas,E.L.2002:Parameterizingscalartransferoversnowandice:Areview.JournalofHydrometeorology,3(4),417-432.Andreas,E.L.,Tucker,W.B.,andAckley,S.F1984.:Atmosphericboundary-layermodification,dragcoefficient,andsurfaceheatfluxintheAntarcticmarginalicezone,J.Geophys.Res.-Oceans,89,649–661,doi:10.1029/JC089iC01p00649Andreas,E.L.,Horst,T.W.,Grachev,A.A.,Persson,P.O.G.,Fairall,C.W.,Guest,P.S.andJordan,R.E.,2010:Parametrizingturbulentexchangeoversummerseaiceandthemarginalicezone.Quart.J.RoyalMeteorol.Soc.,136,927-943.Arya,S.P.S.1973:ContributionofformdragonpressureridgestotheairstressonArcticice.J.Geophys.Res.,78,7092-7099,doi:10.1029/JC078i030p07092.Birnbaum,G.,andLüpkesC.2002:Anewparameterizationofsurfacedraginthemarginalseaicezone.Tellus,54A,107–123.doi:10.1034/j.1600-0870.2002.00243.xBrümmer,B.,1996:Boundary-layermodificationinwintertimecold-airoutbreaksfromtheArcticseaice.Boundary-LayerMeteorology,80,109-125.Brümmer,B.,1997:Boundarylayermass,water,andheatbudgetsinwintertimecold-airoutbreaksfromtheArcticseaice.MonthlyWeatherReview,125,1824-1837.

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Renfrew,I.A.,andMoore,G.W.K.,1999:Anextremecold-airoutbreakovertheLabradorSea:Rollvorticesandair–seainteraction.MonthlyWeatherReview,127,2379-2394.Sigmond,M.,Fyfe,J.C.andSwart,N.C.,2018.Ice-freeArcticprojectionsundertheParisAgreement.NatureClimateChange,8,404-408.Strong,C.,&Rigor,I.G.2013:Arcticmarginalicezonetrendingwiderinsummerandnarrowerinwinter.GeophysicalResearchLetters,40,4864-4868.Tang,Y.,Lean,H.W.,&Bornemann,J.2013:ThebenefitsoftheMetOfficevariableresolutionNWPmodelforforecastingconvection.MeteorologicalApplications,20(4),417-426.Tsamados,M.,Feltham,D.L.,Schroeder,D.,Flocco,D.,Farrell,S.L.,Kurtz,N.,Laxon,S.L.,andBacon,S.2014:ImpactofVariableAtmosphericandOceanicFormDragonSimulationsofArcticSeaIce.J.Phys.Oceanogr.,44,1329-1353,doi:10.1175/JPO-D-13-0215.1UMDocumentationPaper24,2016:TheParametrizationofBoundaryLayerProcesses,MetUMv10.5,MetOffice,UKVihma,T.1995:SubgridParameterizationofSurfaceHeatandMomentumFluxesoverPolarOceans,J.Geophys.Res.,100,22625–22646,doi:10.1029/95JC02498Vihma,T.,Pirazzini,R.,Fer,I.,Renfrew,I.A.,Sedlar,J.,Tjernström,M.,Lüpkes,C.,Nygard,T.,Notz,D.,Weiss,J.andMarsan,D.,2014.Advancesinunderstandingandparameterizationofsmall-scalephysicalprocessesinthemarineArcticclimatesystem:areview.AtmosphericChemistryandPhysics,14,9403-9450. doi:10.5194/acp-14-9403-2014Walters,D.N.,Best,M.J.,Bushell,A.C.,Copsey,D.,Edwards,J.M.,Falloon,P.D.,Harris,C.M.,Lock,A.P.,Manners,J.C.,Morcrette,C.J.andRoberts,M.J.,2011.TheMetOfficeUnifiedModelglobalatmosphere3.0/3.1andJULESgloballand3.0/3.1configurations.GeoscientificModelDevelopment,4(4),919-941.Walters,D.etal.2017:TheMetOfficeUnifiedModelGlobalAtmosphere6.0/6.1andJULESGlobalLand6.0/6.1configurations.GeoscientificModelDevelopment,10,1487–1520,doi:10.5194/gmd-10-1487-2017.Young,G.,Jones,H.M.,Choularton,T.W.,Crosier,J.,Bower,K.N.,Gallagher,M.W.,Davies,R.S.,Renfrew,I.A.,Elvidge,A.D.,Darbyshire,E.andMarenco,F.,2016:ObservedmicrophysicalchangesinArcticmixed-phasecloudswhentransitioningfromseaicetoopenocean.AtmosphericChemistryandPhysics,16,13945-13967,doi:10.5194/acp-16-13945-2016

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Setting Algorithm Met Office model configurations

L12E16 𝐶𝐷𝑁10 = (1−𝐴)𝐶𝐷𝑁10_𝑤𝑎𝑡𝑒𝑟 + 𝐴𝐶𝐷𝑁10_𝑖𝑐𝑒 + 𝐴

ℎ𝑓𝐷𝑖𝑆𝑐2𝑐𝑒2 Dln2Hℎ𝑓/𝑧0_𝑤𝑎𝑡𝑒𝑟Kln2H10/𝑧0_𝑤𝑎𝑡𝑒𝑟K

L

After Lupkes et al. (2012), where parameters are set as the E2016A values in Elvidge et al. (2016) and CDN10_ice = 1.6x10-3. Scalar roughness lengths: z0t_ice = z0q_ice = 0.1x10-3 m.

GL8 Operational Forecast system (from 25th September 2018, ‘PS41’)

Rough z0_MIZ = 100x10-3 m, z0_ice = 3x10-3 m; Equivalent to CDN10_MIZ = 7.5x10-3, CDN10_ice = 2.4x10-3

Scalar roughness lengths: z0t_ice = z0q_ice = 0.6x10-3 m here (also in GL6 and GSI6). z0t_ice = z0q_ice = 0.1x10-3 m in GL7.

GL7 and GL6 (Walters et al. 2017) GSI6 (Rae et al. 2015) Operational Forecast system (prior to 25th September 2018)

Smooth z0_MIZ = 0.5x10-3 m, z0_ice = 0.5x10-3 m; Equivalent to CDN10_MIZ = CDN10_ice = 1.6x10-3

Scalar roughness lengths: z0t_ice = z0q_ice = 0.1x10-3 m.

GL3 (Walters et al. 2011) GSI4 (Rae et al. 2015)

Table1ParameterizationsettingsfordragandscalartransferovertheMIZintheMetOfficeUnifiedModel.ThedragsettingsarethoseofthenewL12E16schemeandthepreviousdragschemewhichprescribesaroughnesslengthfortheMIZ(z0_MIZ)andanotherforsolidseaice(z0_ice),wheretwoprescriptionshaveoftenbeenused,referredtohereasRoughandSmooth.MetOfficeGlobalLand(GL)configurationsthatusethesesettingarenotedincolumn3.Inatmosphere-onlyconfigurationssurfaceexchangeovertheocean/seaiceisdefinedintheseGLcomponents.Incoupledatmosphere-ice-oceanconfigurationssurfaceexchangeovertheocean/seaiceisdefinedintheGlobalSeaIce(GSI)component–seeRaeetal.(2015).NotetheoperationalglobalforecastmodelhasusedtheRoughsettingsfrompriorto1993to2018(Smith1996,UMdocumentationpaper24,MetUMVersion3.1).TheL12E16schemebecameoperationalon25thSeptember2018.

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CorrelationCoefficient Bias RMSError

Experi-ment

Rough L12E16 Smooth Rough L12E16 Smooth Rough L12E16 Smooth

U 0.51 0.53 0.52 -0.51 -0.06 0.28 2.14 2.09 2.14T 0.86 0.93 0.93 2.12 0.02 -0.46 3.62 2.13 2.16θ 0.88 0.94 0.94 2.20 0.10 -0.39 3.66 2.11 2.13q 0.87 0.91 0.91 0.01 -0.14 -0.20 0.20 0.22 0.28RH 0.28 0.41 0.44 -13.9 -12.1 -13.3 17.1 15.1 15.6Tsfc 0.73 0.75 0.74 2.97 1.72 1.40 6.08 5.48 5.43p 0.99 0.99 0.99 -1.30 -1.02 -0.93 1.53 1.25 1.15τ 0.09 0.10 0.13 0.11 0.00 0.03 0.26 0.20 0.19SHF 0.15 0.36 0.40 132 54 46 188 80 73LHF 0.02 0.38 0.43 60 20 17 82 31 28Table2Comparisonstatisticsforthethreelimited-areamodelexperimentsagainsttheACCACIAsurface-layerobservationsandturbulentfluxestimates.Comparisonvariablesarewindspeed(U,unitsofms-1),temperature(T,K),potentialtemperature(θ,K),specifichumidity(q,gkg-1),relativehumidityw.r.t.water(RH,%),surfacetemperature(Tsfc,K)andpressure(p,hPa);aswellascovariancemomentumflux(τ,Nm-1),sensibleheatflux(SHF,Wm-2)andlatentheatflux(LHF,Wm-2).Thehighestcorrelationcoefficient,thelowestbiasandthelowestroot-mean-squareerrorsareinbold;thebiasbeingdefinedasmodel–observations.

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FigureCaptionsFigure1Asketchofthesurfacedragexertedontheatmosphereoverthemarginalicezone,showingthesurface(orskin)dragandtheformdragcausedbywindblowingagainstverticalfaces.Figure2Neutraldragcoefficient(CDN10)asafunctionoficeconcentrationforthethreeparameterizationstested:theL12E16scheme(followingLüpkesetal.2012andElvidgeetal.2016)andtheRoughandSmoothroughnesslengths(seeTable1).NoteCDN_iceissetto1.6x10-3.Aircraft-basedobservationsofCDN10fromFigure2aofElvidgeetal.(2016)areshownasmagentacircles(median)andwhiskers(inter-quartilerange).Figure3CompilationoficefractionobservationsfromtheACCACIAsurface-layerdataset(circles).Thisconsistsof209runsover8flightsbetween21&31March2013–seeTable2inElvidgeetal.(2016)fordetails.Thedataareshownhereintwopanels:tothesouthofSvalbardovertheBarentsSea,andtothenorthofSvalbardoverFramStrait.Typicalaircraftaltitudeswerearound35mabovesealevel.Alsoshownaredropsondelocations(blackdiamonds)fromflightB762on23March2013;andsea-icefractioncontours(from0.2-0.8)fromtheOSTIAanalysis.Figure4Scatterplotcomparisonsofflight-levelaircraftobservationsagainstMetUMoutputfor(a)windspeedfortheL12E16dragparameterizationsettingand(b)-(d)temperatureforthe(b)Rough,(c)L12E16and(d)Smoothdragsettings.Figure5Surface-layerobservationsandmodeloutputforthreeflightstothenorthofSvalbardon25,26and29March2013.Panelsshowobservationsof(a)windspeedat10m,(b)temperature,and(c)temperaturechange(DT);aswellasmodeloutputofDTfromthesurfacedragexperiments:(d)Rough,(e)L12E16,and(f)Smooth.Thevariableunitsare:ms-1andK.ContoursoficefractionfromtheOSTIAanalysisarealsoshown(from0.2to0.8).Figure6Cross-sectionsof(a)potentialtemperature(θ),(b)specifichumidity(q)and(c)windspeed,fromdropsondedataon23March2013.Cloudtopheightsderivedfromairbornelidarobservationsareplottedasblackdots.NotethewindsareapproximatelyfromlefttorightandtheMIZisbetween~76.5-75.5oN(thickblackline).Thecross-sectionis530kmlong.Theblacktrianglesmarkthedropsondereleaselocations–seeFig.3foralocationmapofthenorthernmost9dropsondes.Figure7Observedandsimulatedvariablesfromthedropsondecross-sectionof23March2013showninFig.6.Panel(a)showssea-icefractionfromOSTIAandfromtheMetUM;theremainingpanelsaremixed-layeraveragesof(b)potentialtemperature,(c)specifichumidityand(d)windspeedrespectively.Theblacktrianglesmarkdropsondereleaselocations(asinFig.6).Figure8CompositeglobalmodeloutputfortheNorthernHemisphereshowing(a,b)sea-icefraction;andthedifferencebetweentheMetUML12E16andRoughexperimentsfor(c,d)windspeed,(e,f)near-surfacetemperatureand(g,h)meansea-levelpressure.Theleft-handpanelsareaveragedoverallextendedwintercases(November-April);theright-handpanels

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areaveragedoverallextendedsummercases(May-October).Theunitsforwindspeed,temperatureandpressurearems-1,KandParespectively.Figure9CompositeglobalmodeloutputfortheSouthernHemisphereshowing(a)sea-icefraction,andthedifferencebetweentheMetUML12E16andRoughexperimentsfor(b)windspeed,(c)near-surfacetemperatureand(d)meansea-levelpressure,averagedoverallextendedwintercases(May-October).TheunitsarethesameasFig.8.