ICE, CLOUD, AND LAND ELEVATION SATELLITE-2 (ICESat-2) ATL12 Ocean Surface Height Release 001

Application Notes and Known Issues

James Morison 6/4/2019

Rev 1.0

Introduction ThisdocumentcontainsnotesfortheuseoftheICESat-2ATL12OceanSurfaceHeightproduct.Itincludesissuesthatareknowntothedevelopers,whichmaybefixedinfuturereleasesofthisproduct. Feedbackfromthecommunitywillbeaddedtofuturerevisionsofthisdocument.

ContentsNotes: The ATL12 Ocean Surface Height Product Philosophy and Brief Description......................3

Issues.............................................................................................................................................................................7 Issue1.MissingOceanSegments.....................................................................................................................7

Issue2.SurfaceFinding,SubsurfaceReturns,andHistogramTrimming....................................7

Issue3.UncertaintyinMeanSSH.....................................................................................................................7

Issue4.ErroneousDownlinkBands...............................................................................................................8 Issue5.SeaSurfaceHeightsAmongBeams................................................................................................9

Issue6.BathymetryTestandSurfaceType................................................................................................9

Issue7.SSBCalculation........................................................................................................................................9

Issue8.Highfreeboardsamplesduetoseastate....................................................................................9 Issue9.LowertransmittedenergyinBeam3(StrongBeam)........................................................10

Issue10.CloudFlagUsage...............................................................................................................................10 Issue11.OutputofbackgroundPhotonRate..........................................................................................10 Issue12.BinBoundaryDefinitions..............................................................................................................10

Notes: The ATL12 Ocean Surface Height Product Philosophy and Brief Description

TheIce,Cloud,andLandElevationSatellite-2(ICESat-2)providessatelliteoceanaltimetryunlikeanyother.ICESat-2hasbeendevelopedprimarilytomeasuretheheightoftheEarth’siceandlandathighspatialresolution.ToachievethisresolutionitcarriestheAdvancedTopographicLaserAltimeterSystem(ATLAS),aphoton-counting,multi-beamlidarpulsingat10kHz.Atthespeedofthespacecraft,eachbeamofATLASilluminates15mpatchofthesurfaceevery0.7mofalong-trackdistance.Giventhelowreflectanceoftheoceansurface,ofallthephotonsdetectedbyATLAS,ontheorderofonephotonperpulsereturnsfromtheoceansurface.ATLASdeterminestheapparentheightofthereflectingsurfaceforeachoneofthesephotonsalongwiththeapparentheightofalowerdensityofnoisephotons.Averagedoveralong-trackdistancetheseheightsformahistogramofheightsreminiscentofthewaveformsofotherradar(e.g.,CryoSat-2)andanaloglidar(e.g.,ICESat-1)satellitealtimeters,andthuswearetemptedtothinkintermsof“retracking”todecidewhatpartofthis“waveform”representstheaverageoceansurfaceovera“footprint”correspondingtotheaveragingdistance.

WehaveadoptedadifferentphilosophyinprocessingtheICESat-2.Wedonotthinkintermsof“footprint”and“retracking”intheusualway,buttreateveryphotonheightdocumentedintheATL03dataproductinputtoATL12asanindividualpointmeasurementofsurfaceheightaveraginglessthanameterapart,butwithax-ylocationuncertaintyontheorderof10m.Figure1showsATL03photonheightscollectedover7-kmofoceansurface.Thedensecloudofheightsrepresentingsurface-reflectedphotonsclearlystandsoutfromthelowerdensityofnoisephotonsaboveandbelowandrevealssurfacewaveswithabout2.5msignificantwaveheight(SWH)andanapparent470-mwavelength.Inprocessing,wefirstdistinguishbyahistogramtrimmingmethod,whichoftheseheightsoveranadaptivelychosenoceansegmentlength(typically7-km)arefromtruesurface

reflectedphotonsversusnoisephotons.Theresultant“receivedheighthistogram”isdeconvolvedwithaninstrumentimpulseresponse(IIR)histogramrepresentingtheheightuncertaintyassociatedwiththelidartransmitpulsewidthandotherinstrumentalfactors.Thisproducesasurfaceheighthistogramthat,withitsfirstfourmoments,istheprimaryATL12products.InadditionweanalyzethespatialseriesofsurfacephotonsheightstocharacterizesurfacewavesandcalculatethecorrelationofphotonreturnrateandsurfaceheightthatconstitutestheEMseastatebias(SSB)inthemeanseasurfaceheight(SSH).

Thus,theATL12Oceanproductincludeshistogramsandstatisticsofseasurfaceheightovervariablelength

Figure1.ATL03photonheights(magenta)fromOct.16,2018overthePacificOceanandtheEGM2008geoid(redline).Wavesinthedensephotoncloudareapparentaswellassubsurfaceandatmosphericnoisephotons.

along-trackoceansegments.ATL12photonheightsarederivedfromtheATL03oceanphotonheightdatawithsignalconfidencelevel1orhigherandwithin15moftheEGM2008geoid.Signalconfidence2,3,and4correspondtolow,medium,andhighconfidencethephotonissurfacereflectedandconfidencelevel1fillsa±15-mbufferaboutthehighconfidencelevelphotonheights.Thisconstraint,alongwiththegeoidbandcriteria,havebeenfoundtoeditoutheightsinerroneousdownlinkbandsassociatedwithhighbackgroundnoiserates.

Fromthispopulation,wefirstaccumulatephotonheightslikelyto

fallwithinthedistributionofoceansurfaceheightsuntil8,000candidatephotonsor7kmofalong-trackdistanceistraversed.Allthephotonsovertheresultingoceansegmentarethensubjectedtotwoiterationsselectingphotonheightsinthehistogramatlevelsabovethebackgroundrate.Afterthefirstiteration,alineartrendandaverageheightisremovedfromtheheightspriortotheseconditeration.Thetrendandaverageheightareretainedforoutputaspartoftheoceansegmentstatistics.Fromtheretheheightdatafollowstwopaths

Thehistogrampathconsidersthehistogramofthe

Figure2.ExampleofoneATL12file’sworthofoceansegmentlatitudesandlongitudesoverfourorbitscomprisingthefile.

Figure3ATL12oceansegmentstatistics.Upperleft:meanSSH,upperright:significantwaveheight=4xstandarddeviation,lowerleft:skewnessofseasurfaceheight,lowerright:kurtosisofseasurfaceheight.YellowDOTSareATL12ASAS5.1andbluedotsarefromourdevelopmentalMatlabcodeappliedtoATL03,whichgenerallydoesnotsegmentthedataexactlythesameasATL12.ASAS5.1andMatlabagreementisbetterwheresegmentsaremorecloselymatched.

receivedoceansegmentheightsanddeconvolvesthehistogramoftheinstrumentimpulseresponsetoproducethesurfaceheightdistribution.Thisisthenfitwitha2Gaussianmixturetoproducethefirstfourmomentsofthesurfaceheightdistribution.ThederivedheightsurfacehistogramandthefourmomentscomprisethemaintheATL12dataproducts

Thespaceseriestrackmaintainsthesurfaceheightsasanalong-trackseriesandbycorrelatingthephotonreturnratewithsurfaceheightat10-malong-trackscale,estimatessignificantwaveheightandtheEMseastatebias(SSB)ATL12dataproducts.Infuturereleases,analysesonthistrackwillincludemorewavepropertiesandestimatesofstatisticaldegreesoffreedomfortheaccumulationgriddedstatisticsintheATL17griddeddataproduct.

EachATL12datafilecoverstheworldoceanoverfourconsecutiveICESat-2orbits(Fig.2).ATL12filenamessuchasATL12_20181105031353_05730101_001_01.h5includethedate/timesequence(yyyymmddhhmmss=20181105031353)ofthestartofthefirstorbit,thereferencegroundtrack(_####=_0573),thecycle(##=01),theregion(##=01),andtherelease(###=001).

TheprocesseddataareforareasdesignatedasoceanaccordingtotheICESat-2oceansurfacemask.Theoceanmaskoverlapswithalltheothersurfacetypesinbufferzonesupto20-kmwide.Consequently,ATL12dataincludesbandsthatareinfactnotopenoceanbutwhicharecloseenoughtosealeveltofallwithin±15mofthegeoidtobeacceptedbyATL12’sprocessing.Examplesarethemarginalseaicezonesundertheseaicesurfacetypeandlow-lyingislandsunderthelandsurfacetype.FuturerevisionsaretoincludeabathymetrictesttoinsuretheATL12processingcoversonlyoceanwaters.

Exceptinspecialcasesorwhenoverlappingoneoftheothersurfacetypes,overtheoceanonlythethreestrongbeamswithgroundtracksseparatedby3kmaredownlinkedbyATLAS.TheATL12softwareprocesseseachstrongbeamindependently.AnexampleofoceanstatisticsmeasurebythemiddlestrongbeamoveroneATL03granuleintheNorthPacificisshowninFigure3foralloceansegmentsinthegranule.

Figure4.ICESat-2,CryoSat-2dynamicoceantopography(DOTinmeters)comparisonfortheGreenlandSeainNovember2018.Left:ICESat-2fromATL12Rel001,bin-averagedina25-kmgrid.Inthiscaseoceanandseaicemaskdataareincluded.Center:CryoSat-2LRMandpLRMfromRADSbin-averagedina25-kmgrid.Theseareopenwaterproducts.Right:ICESat-2DOTminusCryoSat-2DOTbin-averagedina25-kmgrid.DifferencesaretakenonlyforgridcellswithbothICESat-2andCryoSat-2data.

OurfirsteffortsatcalibrationandvalidationofICESat-2SSHshowgoodagreementwithCryoSat-2,especiallyintheSub-ArcticSeas.Figure4showscomparisonofICESat-2andCryoSat-2dynamicoceantopography,DOT=SSH-Geoid(EGM08)forthemonthofNovember2018bin-averagedina25-kmgrid.ICESat-2didnotgetasmanysurfacemeasurementsasCryoSat-2,almostsurelyduecloudcoverovertheNovemberGreenlandSea. However,thesameDOTpatternssuchasDOTsetuptowardsthecoastsassociatedwiththeEastGreenland,Norwegian,andWestSpitzbergencurrentsarealmostidenticallyshowninbothdatasets.ThedifferencebetweengriddedICESat-2andCryoSat-2DOTisontherightofFigure4.OverallthegridcellswithICESat-2andCryoSat-2,ICESat-2DOTis0.64cmhigherthanCryoSat-2DOT±16.6cmstandarddeviation.ThiscomparisonisremarkablegiventheearlyreleaseofICESat-2.

Figure5issimilartoFigure4exceptthecomparisonisdonefortheEasternEquatorialPacific(EEP)forJanuary2019.Inspiteofalowerdensityofgroundtracksatlowlatitude,ICESAt-2datacoverageintheEEPisarguablybetterthanintheGreenlandSea,likelybecauseofclearerskyconditions.TheICESat-2andCryoSat-2DOTpatternsintheEEPareverysimilar,showingamongotherthingsthetroughalong5°NlikelyassociatedwithNorthEquatorialCurrentandNorthEquatorialCounterCurrent.ThesimilaritybreaksdownalittleinthewesternpartoftheareawithICESat-2

DOTbeingalittlehigherthanCryoSat-2DOTinthenorthwestandlowerthanCryoSat-2DOTinthesouthwest.OnaverageICESat-2DOTis13.1cmlowerthanCryoSat-2DOT±12.2cmstandarddeviation.ThisisnotasgoodastheGreenlandSeacomparisonbutcomparableinmagnitudetootherICESat-2groundtruthcomparisonsforthisrelease.

Figure5.ICESat-2,CryoSat-2dynamicoceantopography(DOT)comparisonfortheEasternEquatorialPacificinJanuary2019.Top:ICESat-2DOT(m)fromATL12Rel001,bin-averagedina25-kmgrid,Center:CryoSat-2DOT(m)LRMandpLRMfromRADSbin-averagedina25-kmgrid,Bottom:ICESat-2DOTminusCryoSat-2DOT(m)bin-averagedina25-kmgrid.DifferencesaretakenonlyforgridcellswithbothICESat-2andCryoSat-2data.

Issues

TheissuesbelowarethosethatcouldaffectuseofATL12Rel.001presentlyorcouldaffectchangesinthewayATL12dataareusedinthefuture.

Issue1.MissingOceanSegments

DuetoissuessolelywiththeASAS5.1software,usersmayfindoccasionaldatagapswithmissingoceansegmentsinATL12Rel.001.Thesourcesofthesegapsareknownandhavebeencorrectedinthenewestgenerationofthecode

Issue2.SurfaceFinding,SubsurfaceReturns,andHistogramTrimming

Distinguishingsurfacereflectedphotonheightsinvolvesestablishingahistogramofallheightsinanoceansegmentandthensearchingoutwardfromthecenterofthehistogramtofindhighandlowlimitswherethehistogramlevelfallsbelowanestimateofthenoiselevel.Thephotonheightsbetweenthehighandlowlimitsareconsideredsurfacephotonheights.Todeterminethehighandlowlimits,thecurrentrelease(R001-ASAS5.1)comparesasmoothedversionofthehistogramtothemedianvalueofthesmoothedhistogram.Becausewiththesmallbinsize(1-cm)ofthecurrentprocessing,themedianofthesmoothedhistogramusuallyreflectsthenoisetails,andtheresultingtrimmingworksreasonablywell.However,ittreatsthesubsurfaceandabovesurfacetailsofthehistogramthesame.

Becausetheblue-greenlaserofATLASpenetrateswater,truesubsurfacereturnshavealwaysbeenaconcern,andthehighersubsurfacedensityofphotonsapparentinFigure1maybedueinparttosubsurfacescatteringintheocean.However,weseesimilarlyenhancedsubsurfacedensitiesover,cleardeepoceanwatersandevenoverlandwherepenetrationandbackscattershouldn’toccur.ConsequentlypresentthinkingexpressedintheATL03knownissuesisthatthesubsurfacenoiselevelisduetoforwardscatteringdelaysintheatmosphereofsurfacereflectedphotons.

Whatevertheircause,somesubsurfacephotonheightsarebeingincludedinthesurfaceheighthistogram,creatingwhatwethinkmaybeanorder1-3cmbiasinaverageSSH.Toreducethesensitivitytosubsurfacereturns,thepresentdevelopmentalcodefirstmakesasimpleestimateofthehighandlowhistogramlimitsandthenusesthesetodetermineseparateabovesurfaceandsubsurfacenoiselevels.Theultimatehighlimitisthenchosenwherethesmoothedhistogramfallsbelowafactor(e.g.,1.5)timestheabovesurfacenoiseandlowlimitischosenwherethesmoothedhistogramfallsbelowthesamefactortimesthesubsurfacenoise.Thismethodofreducingsubsurfacereturnswillbeinfuturereleases.Wealsoplantouseahistogramforsurfacefindingbasedonheightdeparturesfromasmoothedhighconfidencephotonsurfacetoeliminatesubsurfacereturnsfromunderthecrestsofwaves.

Issue3.UncertaintyinMeanSSH

Wehavefoundthewaveenvironment,ratherthaninstrumentalfactors,isprobablythebiggestcontributortouncertaintyinestimatesofthemeanSSH.Thisisillustratedbyouranalysisofmultipleoceansegments,ofwhichthedataofFigure1isone7-kmoceansegmentinthesouthernpartofthecentralNorthPacific.Thesesegmentsshowvariability

intheirmeanseasurfaceheightsofapproximately±0.12,muchgreaterthanwe’dexpectfrominstrumentalfactors.Thestandarderror,sL,intheestimatesofthemeanaregivenby

where sh is the standard deviation of the population and Ndf is the number of degrees of freedom. If every one of the ~ 8,000 photon heights were independent, even with a SWH = 2.5 m (sh = .625 m), the uncertainty in estimates of the mean would be less than 1 cm. The problem is that owing presence of long large waves, successive photon heights are far from independent. The fact that the underlying wave signal is periodic makes the estimate of the effective degrees of freedom in terms of correlation length scale more problematic. As a worst case if we set Ndf equal to the number of wave periods in the ocean segment, 15 in the case Figure 1, the uncertainty,sL,intheestimateofthemeanequals0.16m,closetowhatweseeovermanysimilaroceansegments.Thisproblemisnotinstrumental;itisjustmadeapparentbytheabilityofICESat-2toresolvewaves.Tolowerthisuncertainty,weareexploringusingharmonicanalysisofsurfaceheightovereachoceansegmentanduseofthezerowave-numberamplitudetorepresentSSH(D.Percival,personalcommunication,2019)asawayofremovinglargeperiodicvariationsinheightasacauseofuncertainty.AfurtheradvantageofthisapproachwillbethatitwilladdameasureofwavespectralpropertiestoATL12.Inanyevent,wewillhavetoderivemeasuresoftheeffectivenumberofdegreesoffreedomasanATL12outputforeachoceansegmentinordertoproperlyaccountforuncertaintiesintheATL19griddedSSHproduct.

Issue4.ErroneousDownlinkBands

Inordertoconservebandwidthovertheocean,theFlightScienceReceiverAlgorithms(FSRA)onboardICESat-2selectphotonsinadownlinkbandcurrently30-minverticalwidth(SeeATL03UsersGuide).ThisbandiscenteredoverwhereFSRAdetectsthehighestconcentrationsofphotons.Aslongasthebackgroundphotoncountislowatnightorinclearconditionsthisbandwillbecenteredontheoceansurfacenearthegeoid.Butindaylightwithscatteredclouds,thedownlinkbandcanshifttounrealisticheightsordepthsfor200-pulsemajorframes.Toedittheseerroneousdownlinkbands(EDB),ASAS5.1selectsabandthatincludesphotonswithasignalconfidencegreaterthanorequalto1.Signalconfidenceequalto2,3,4correspondtolow,medium,andhighconfidencethatthephotonisasurfacereturn.Signalconfidenceequalto1isappliedtotheremainingnoconfidencephotonsfillingaband±15maroundthehighconfidencephotons.Thisconfidencelevelfilter,alongandconsideringphotonsonlywithin±15mofthegeoidusuallyfiltersouttheerroneousdownlinkbands,buttheremaybeoccasionswherethisdoesn’tworkandsomephotonsfromEDBleakintoconsideration.

ToaddresstheseissuestheFSRAteamhasrecentlyrunanexperimentalFSRAonboardICESat-02foraboutaday.Amongotherthings,theexperimentalFSRAexpandedtheverticalwidthofthedownlinkbandandnowallowsthedownlinkofphotonsinsomesortofbandwhenitfindsnoconcentrationofphotonheights.WehavetotesthowtheASASATL12coderespondstothisFSRAchangeandadapttoit.

σ L =σ h Ndf( )−1/2

Issue5.SeaSurfaceHeightsAmongBeams

Ourcal/valcomparisonswithCryoSat-2(Figs.4and5)havebeenperformedforthecenterstrongbeamonly.WehavenotdoneadetailedstudyoflevelingSSHacrossthethreestrongbeams,butintwogranulesweusedforcomparisonsbetweentheASAS5.1codeanddevelopmentalMatlabcode(e.g.,Fig3),wefoundbeam1was4cmlowerthanthecenterbeamandbeam3was4cmhigherthanthecenterbeam.WewillbecomparingresultsamongthethreestrongbeamsandCryoSat-2overwiderareasandtimesinthenearfuture.

Issue6.BathymetryTestandSurfaceType

Asmentionedinthenotes,futurerevisionsaretoincludeabathymetrictesttoinsuretheATLprocessingcoversonlyoceanwaters.InthecurrentATL12Rel.001,theusermayfinddataactuallycollectedoverlow-lyinglandsurfacesthatareincludedintheocean-landboundarybufferzone.Theusercangainanappreciationofhowmuchnon-openoceanisincludedinanoceansegmentbylookingatthegtxx/ssh_segments/stats/surf_type_prcntoutputvariablethatgivesthepercentageoftheoceansegmentcoveredbyeachsurfacetypecoveredinanoceansegment.(Beadvisedthatthesurfacetypefrom1to5denoteland,ocean,seaice,landice,andinlandwateraslistedinATL03,butthedescriptioninATL12RTel001ismisstatedbyreversingtheorderoflandiceandseaice).Inthefuture,ASASwillincludeoceandepthfromtheGEBCOstaticgriddedworldoceanbathymetryandwillprocessdataasoceandatafordepthsgreaterthanathresholddepthsuchas10m.

Issue7.SSBCalculation

WeestimatetheSeaStateBias(SSB)duetovariationsinsamplingoversurfacewaves.TheEMseastatebiasoccursforexampleifmorephotonstendtobereturnedfromwavetroughsthanfromwavecrests.ThisSSBisequivalenttothecovarianceofphotonreturnrateandseasurfaceheightdividedbytheaveragephotonreturnrate.Weestimatethiswiththereturnratesandheightsaveragedin10-malongtrackbins.TheSSBestimateshavebeensmallerthanwehaveexpected~-1.2%ofSignificantWaveHeight(SWH)forourdevelopmentalcodeandabouthalfthatfortheATL12data.TypicalradaraltimetersshowanSSBofabout–4%andwehavefoundICESat-2tohavenoSSBbelowSWH=2m,but-18%f=ofSWHabove2m[Morisonetal.,2018].Therefore,wewillinvestigatethedifferencesintheMatlabandASAScodecomputationsofSSBandalsocomparetheratebasedSSBtocomparisonofphotonaverageheightstoaveragesof10-mbinaveragesthatshouldbelesspronetoSSB.

Issue8.Highfreeboardsamplesduetoseastate

RelatedtotheSSBCalculationissue,ATL07resultsneartheiceedgesometimesshowreducedSSHneartheiceedge.Ithasbeenhypothesizethatthisisduetostrongphotonreturnsfromquasi-specularreturnsfromicecrystalscollectedinthetroughsofwaves.WeseesomeevidenceofthesedepressionsintheATL12dataalongtheEastGreenlandiceedgeandwilltestthishypothesiswithourSSBcalculation.

Issue9.LowertransmittedenergyinBeam3(StrongBeam)

AccordingtotheATL07/10results,thetransmitenergyofBeam3(Strongbeam2Ror2L,dependingonorientationoftheICESat-2observatory)isapproximately80%thatofBeam1andBeam5.Thus,thesegmentlengthsandphotonreturnratemaybedifferentfromtheothertwostrongbeams.

Issue10.CloudFlagUsage

Thecomputationofthelayer_flag,whichcombinestheinformationincloud_flag_atm,cloud_flag_asrandbsnow_conintoaconsolidatedflagforindicationofcloudcoverage,wasimplementedincorrectlyinthecurrentrelease(Rel.001-ASAS5.1).DONOTUSE.InconnectionwithaddressingIssues1,3,and5wewillinvestigateusingtheATL09cloudcoveragedatatofilterthealtimetrydatainfutureversionsofATL12.

Issue11.OutputofbackgroundPhotonRate

AlsoinconnectionwithaddressingIssues1,3,and5,wewillbeginoutputtingphotonbackgroundrate.TheOceanATBDtable5.2fortheATL12outputparametershasclarifiedthedefinitionofbackgroundrate(backgr_seg)andspecifiesitasanoutputparametertothegtx/ssh_segments/statsgroup.

Issue12.BinBoundaryDefinitions

Currently,theATL12binsarecenteredonevencentimeters,e.g.for±15m,centersat15.00m,14.99m,…etc.However,thearrayofbinedgesisonlyaslongasthearrayofbincenterssothelastbinendsupbeinghalfaswideasitshouldbe.Thelengthofthearrayofbinedgeswillbeexpandedbyone.Reference: Morison J., R. Kwok, S. Dickinson, D. Morison, C. Peralta-Ferriz, and R. Andersen (2018), Sea State Bias of ICESat in the Subarctic Seas, IEEE Geoscience and Remote Sensing Letters, 15 (8), 1144 - 1148, DOI: 10.1109/LGRS.2018.2834362