OMPSNadirMapperLevel2Description
1.PurposeandScope
ThepurposeofthisdocumentistoprovideanoverviewoftheOMPS-NPPNadirMapperLevel2dataproductformatforitsusers.TheNMTO3-L2APPretrievestotalcolumnozonefrommeasurednormalizedradiancescontainedwiththeL1Bfile.
2.NadirMapper
TheOMPSnadirinstrumentiscomposedoftwospectrometersthatsharethesametelescope.AdichroicfilterdownstreamofthetelescoperedirectsphotonsintoeithertheNMortheNadirProfiler(NP)spectrometer.Thetelescopeitselfhasa110°totalacross-trackfieldofview(FOV),resultingin2800kminstantaneouscoverageattheEarth’ssurface;thisissufficienttoprovidedailyfullglobalcoverageattheequatorfortheNMsensor.Thetelescopeincludesapseudodepolarizer[McClainetal.,1992]designedtominimizethesystem’ssensitivitytoincomingpolarization.Thedichroicfilterisoptimizedtoreflectmostofthe250–310nmlighttotheNPspectrometerandtransmitmostofthe300–380nmlighttotheNMspectrometer.Oncesplit,thelightfromtheNMspectrometerisdispersedviaadiffractiongratingontoonedimensionofatwodimensionalcharge-coupleddevice(CCD)locatedatthespectrometer’sfocalplane.Theseconddimensionreflectsthecross-trackspatialcoverageprovidedbytheslitapertureandoptics.TheCCDconsistsof340pixelsalongthespectraldimensionand740pixelsintheacross-trackspatialdimension.Measurementsmeetingthe300–380nmwavelengthrangespecificationrequiredbytheNMsensorareobtainedbyilluminating196ofthe340pixelsinthespectraldimension.Intheacross-trackdimension,708pixelsareilluminated.Fornominaloperations,thepixelsignalsaresummedinto35separate“macropixel”FOVs;allbutthetwoouterFOVscontain20pixelspermacropixel;theleftoutermostmacropixelcontains26pixels,whiletherightoutermostcontains22.SincethereadoutoftheCCDissplitinthecenter,measurementscomprisingthecentralFOVareactuallysplit(althoughnotsymmetrically).Ratherthanrebinningthesemeasurementsingroundprocessing,theyremainsplit,resultingin36cross-trackFOVs.Inthiscase,thecentraltwoFOVscomprise12pixels(30×50km)and8pixels(20×50km),respectively.Becausemacropixelsareconstructedinprogrammableflightelectronics,theOMPSnadirtemporal(along-track)andspatial(across-track)resolutionsarehighlyconfigurable.High-resolutionmeasurements,approximately10km×10kmatnadir,havebeenroutinelycollected
1dayperweekforthefirst2yearsofthemission.Toremainwithinthetelemetrybandwidthconstraints,asetofonly59wavelengthswasselected;thisselectionstillallowsretrievalsoftotalcolumnozoneandotherquantities(suchasSO2).3.0AlgorithmBackground
ThebasicalgorithmusedfortheV2.1datasetisNASA’sV8.5totalcolumnretrievalalgorithm,whichuses2wavelengths(317.5and331.2nmundermostconditions,and331.2and360nmforhighozoneandhighsolarzenithangleconditions).Thelongerofthetwowavelengthsisusedtoderiveeffectivecloudfraction(fc)basedontheMixedLambertEquivalentReflectivity(MLER)model[Ahmadetal.2004]thatwasdevelopedtomodeltheeffectofcloudsonRayleighscattering.Whenfcbecomeslessthanzeroorwhenthereissnow/ice,weassumethatnocloudispresentandusetheLambertEquivalentReflectivity(LER)modeldescribedbyAhmadetal.[2004]toderivetheclearscenereflectivityR.Whenfcexceeds1,weassume100percentcloudcoverandderivedcloudreflectivityusingtheLERmodel.Giventhefc/R,theshorter(strongerozone-absorbing)wavelengthisusedtoderivetotalozone.
AnimportantdifferencebetweentheV8.5algorithmandpreviousversions,aswellwiththearchivedTOMSdataset,istheassignmentofeffectivecloudheight.Ithasbeenassumedinthepreviousalgorithmsthattheabsorptionofbackscatteredsolarradiationessentiallystopsatthecloud-toplevelwhenthecloudsareopticallythick.Toestimatethetotalcolumnamount,the“un-measured”columnbelowthecloud-top(computedusingclimatology)isaddedtothemeasuredcolumn.RecentanalysisoftheOMIdatainconjunctionwithCloudSatradardata[Vasilkovetal.2008]indicatesthatthisassumptionisinvalid.MiescatteringcalculationsusingCloudSatdataindicatethatinallcloudyscenes,includingdeepconvectiveclouds,theUVradiationreceivedatthesatelliteissensitivetotheozonecolumnbelowthenominalcloud-toppressurereportedbythermalinfraredsensorssuchasMODIS.Analysisshowsthatphotonsactuallypenetratesomedistanceintoacloud.InV8.5,weuseaclimatologyoftheOpticalCentroidCloudPressure(OCCP)inferredfromRotational-RamanScattering(OMCLDRR)toderivethetotalozonecolumn.TheclimatologywasdevelopedusingOCCPretrievalsfromtheOzoneMonitoringInstrument(OMI)onboardtheAurasatellite.SincethepressurecorrespondingtoOCCPisusuallysignificantlybelowthecloud-toppressureclimatologyassumedintheV8algorithm,theV8.5derivedcolumnamountshavedecreasedoverclouds.Themagnitudeofthedecreasedependsoncloudfraction,location,andsolarzenithangle.
Theeffectivecloudfraction(fc)derivedfromtheMLERmodelisusedtoestimatetheCloudRadianceFraction(CRF).CRFcharacterizesthefractionofmeasuredradiationthatisscatteredbyclouds.MiescatteringcalculationsindicatethattheclearandcloudyozonecolumnsweightedbyCRFprovidesavalueveryclosetowhatonewouldfromtheplaneparallelMie
cloudmodelwiththeindependentpixelapproximationtoaccountformixedscenes.TheadvantageoftheMLERmodelisthatonedoesn’tneedindependentknowledgeofgeometricalcloudfractiontocalculatetheozonecolumnaccuratelyincloudyscenes.
ThealgorithmalsocalculatestheabsorbingAerosolIndex(AI)fromtheradianceresidualsat360nm.TheAIisusefulfortrackingglobaltransportofsmokeanddust,foritcantracktheseaerosolsaboveandthroughclouds,aswellasoversnow/icecoveredsurfaces.VariousstudieshaveindicatedthatAIisverynearlyproportionaltotheaerosolabsorptionopticaldepthat360nm.However,theproportionalityconstantvarieswiththealtitude(ofthecenterofmass)oftheaerosollayer-thelowerthealtitudethesmallertheconstant.MostaerosolshavestrongerabsorptionintheUVthaninthevisible,includingmineraldustfromdesertsandcarbonaceousaerosolscontainingorganicandblackcarbon.SincetheAIisalsoaffectedbythespectraldependenceofsurfacealbedocausedbysea-glintandwater–leavingradiance,andsincethereareresidualerrorsintheMLERmodelinestimatingRayleighscatteringinpresenceofclouds,werecommendthatonlytheAIvalueslargerthan+1shouldbeusedforaerosolstudiesandareascontaminatedbysea-glintshouldbeavoidedcompletely.Sinceabsorbingaerosolscausetheozonederivedfromthebasicozoneretrievalalgorithmtobeoverestimated,aparametricrelationshipbasedonAIisusedtocorrecttheinitialretrievedozonecolumn.Thisrelationshipalsoappearstoremovealargeportionoferrorscausedbysea-glint.
Otherthanthethreeprimarywavelengthsmentionedabove,theOMTO3algorithmusesadditionalwavelengthsforqualitycontrolanderrorcorrectioninmorerestrictedgeophysicalsituations.Theseincludecorrectionforozoneprofileshapeerrorsatlargesolarzenithanglesusing312.6nmmeasurements,andthedetectionofstrongsulfur-dioxidecontaminationusingmultiplewavelengthpairs.ForamoredetaileddescriptionofthealgorithmpleaserefertotheAlgorithmTheoreticalBasisDocument(ATBD)on
http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/viewInstrument.php?instrument=13.
NotethatV2.1willbethelastversiontousethisalgorithm;thenextreleaseofNMTO3-L2willutilizeanew,improvedretrievalalgorithmbasedontheRogersOptimalEstimationmethod.ThenewalgorithmwasrecentlyusedtoreprocesstheOMIdatasetaswellasdatasetsfromtheNimbus-7TotalOzoneMappingSpectrometer(TOMS)andtheEarth-ProbeTOMSinstruments.
4.What’sNew?
V2.0isthefirstversionofthedatasetreleasedthroughtheGESDISC.ThepreviousV1.0datasetwasavailablethroughNASA’sOMPSscienceteam’swebsite:
https://ozoneaq.gsfc.nasa.gov/omps
ChangesfromV1.0toV2.0include:
Nonsciencerelatedchanges
NomenclatureandNamingConvention
1) ThenamingconventionfortheL1BdatasethasbeenchangedfromTC_SDR_EV_NASAtoNMEV-L1B:
a. TC(TotalColumn)hasbeenreplacedbyNM(NadirMapper).b. NOAAnomenclature(SDR)hasbeenreplacedbyNASAnomenclature(L1B).
2) Allcapitalizationofnameswithinthefilehasbeenreplacedbycamelcasing.3) Underlinesinallnameshavebeeneliminated.
Sciencerelatedchanges
1) TheV2.1datasetusesV2.0ofNMEV-L1Basinput.TheV2.0L1Bprovidesimprovedradiancecalibration.
2) SurfaceCategoryhasbeenaddedtotheAncillaryDatagroup.ItisbasedontheInternationalGeosphere–BiosphereProgramme(IGBP)surfaceclassification.
3) GroundPixelQualityFlagsandInstrumentQualityFlagshavebeenaddedtotheGeolocationGroup
4) ATrendingDatagrouphasbeenaddeda) QualityFlagsFrequencyprovidesthepercentfrequencyofoccurrenceforeachofthe
definedAlgorithmQualityFlags
TheV2.0filecontainsswath-basedradiancedataforthedaylitpartofoneorbit.Therearetypically36cross-trackmeasurementsperswathand400swatchbasedobservationsperorbit.
5.FileNamingConvention
TheOMPSNadirMapperdataproductsusethefollowingfilenameconvention:OMPS-satellite_sensorproduct-Llevel_vm.n_observationDate_productionTime.h5
Where:
• satellite=NPP• sensorproduct=NMTO3• level=2• m.n=algorithmversionidentifier(m=major,n=minor)• observationDate=startdateofmeasurementsinyyyymmddformat
o yyyy=4-digityearnumber[2012-current]o mm=2-digitmonthnumber[01-12]o dd=2-digitdaynumber[01-31]
• productionTime=filecreationstampinyyyymmmddthhmmssformat
o hhmmss=productiontime[localtime]
Filenameexample:
OMPS-NPP_NMTO3-L2_v2.1_2012m0403t103340_o02243_2017m0223t154258.h56.FileFormatandStructure
NMTO3-L2datafilesareprovidedintheHDF5format(HierarchicalDataFormatVersion5),developedattheNationalCenterforSupercomputingApplicationshttps://www.hdfgroup.org/.ThesefilesusetheSwathdatastructureformat.Thetop-mostlevelintheHDF5hierarchyofNMTO3-L2filescontainsthreedifferentdirectories,oneforeachtypeofpixel-dependentdata:CalibrationData(containingdatausedtocreatecalibratedradiances),GeolocationData(containingdatatogeolocateeachpixel,aswellasspacecraftlocationandpointinginformation),andScienceData(containingthecalibratedradiances,qualityflags,anderrorterms).Thesethreegroupsaredescribedinmoredetailinthenextsection.7.DataContents
The data fields most likely to be used by typical users of the NMEV-L1B product are listedbelow.
Parameter Group Date GeolocationFieldsLatitude GeolocationFieldsLongitude GeolocationFieldsBandCenterWavelengths CalibrationDataSolarFlux CalibrationDataSolarFluxWavelengths CalibrationDataColumnAmountO3 ScienceDataUVAerosolIndex ScienceDataReflectivity331 ScienceData
The first OMPS NMEVmeasurements used to create the NMEV-L1B product were taken onJanuary 28, 2012. Data for February-March 2012 have numerous gaps due to variations ininstrument. Regular operationsbeganonApril 2, 2012. Note that theOMPSNadirMapperconductedhigh-resolutionmeasurementsapproximatelyonedayperweekfromApril2012toJune2016.
NMTO3-L2includesthefollowingdimensionterms:Name long_name Value
DimAlongTrack Along-trackdimension 400DimCrossTrack Across-trackdimension 36DimWavelength 12DimCorners 4DimPressureLevel 11 Metadata inNMEV-L1Bdata files includesattributeswhosevalue is constant forall filesandattributeswhosevalue isunique toeach individual file. Table3.2.1summarizes theseglobalattributes.
GlobalAttribute Type DescriptionAPPName String SoftwarenameAPPVersion String SoftwareversionArchiveSetName String ArchivesetnameforprocessingArchiveSetNumber Integer*8 ArchivesetnumberforprocessingConventions String Nameofconvention(s)formetadataDATA_QUALITY Integer QualityofthedataDOI String DOIvalueDayNightFlag String IdentifydayornightmeasurementsEquatorCrossingDate String DateofequatorcrossingEquatorCrossingLongitude Real*4 LongitudeofequatorcrossingEquatorCorssingTime String TimeofequatorcrossingFormat String DatafileformatLocalGranuleID String FilenameLongName String FullproductnameOrbitNumber Integer*8 FirstorbitnumberofdayPGEVersion String Softwareversion(sameasAPPVersion)ProductDateTime String TimeoffilecreationRangeBeginningDateTime String StartingdateandtimeofdataRangeEndingDateTime String EndingdateandtimeofdataSAMP_TBL Integer NumberofsampletableusedtotakedataSAMP_TBL_VER Integer VersionofthesampletableusedtotakedataShortName String ShortproductnameVersionID Integer*4 VersionIDforthisproductVersionNumber String Versionnumberforthisproductacknowledgement String Acknowledgementofdataproducercomment String Anyadditionalcommentscontributor_name String Nameofdatacreatorcontributor_role String Roleofdatacreatorcreator_email String e-mailaddressofdatacreatorcreator_institution String Organizationofdatacreatorcreator_name String Nameofdatacreatorcreator_type String Typeofdatacreator(e.g.person,organization)date_created String Dateoffilecreationhistory String Historyoffile
id String Shortproductnameinstitution String Producerofdatainstrument String Instrumentmakingmeasurementsinstrument_vocabulary String Sourceofinstrumenttermskeywords String Identifyingkeywordskeywords_vocabulary String Sourceofkeywordsusedinmetadatalicense String Sourceofdatainformationregulationsmetadatalink String WebaddressformetadataDOInaming_authority String Organizationprovidingnaminginformationplatform String Platformformeasuringinstrumentprocessing_level String Levelofdataproduct(e.g.L1B,L2)program String Typeofmeasurementprogramproject String Nameofprojectpublisher_email String e-mailaddressofdatapublisherpublisher_institution String Organizationofdatapublisherpublisher_name String Nameofdatapublisherpublisher_type String Organizationtypeofdatapublisherpublisher_url String URLofdatapublisherreferences String Referencematerialfordataproductsource String Sourceofmeasurementdatasummary String Anyadditionalsummarytime_coverage_end String Endingdataandtimeofdatatime_coverage_start String Startingdateandtimeofdatatitle String Titleofdataproduct8.Products/Parameters
CalibrationDataGroupDatasetName Description Dimensions Units
APrioriLayerO3 Aprioriozoneprofile DimAlongTrackDimCrossTrack
DobsonUnits
CloudPressure Opticalcentroidpressure DimAlongTrackDimCrossTrack
SurfaceCategory Surfacecategory DimAlongTrackDimCrossTrack
TerrainPressure Terrainpressure DimAlongTrackDimCrossTrack
Atmospheres
CalibrationDataGroupDatasetName Description Dimensions Units
Wavelength WavelengthsinputfromL1B(note:L1Bwavelengthsinterpolatedtomatchthesevalues)
DimWavelengths nm
GeolocationDataGroupDatasetName Description Dimensions Units
GroundPixelQualityFlags Ground pixel quality flag (bit packed) DimAlongTrackDimCrossTrack Nounits
InstrumentQualityFlags Bit-packederrorflagsforeachpixelDimAlongTrackDimCrossTrack Nounits
Latitude Groundpixellatitude DimAlongTrack,DimCrossTrack
Degrees
LatitudeCorner GroundpixellatitudecornersDimAlongTrack,DimCrossTrack4
Degrees
Longitude Groundpixellongitude DimAlongTrack,DimCrossTrack
Degrees
LongitudeCorner GroundpixellongitudecornersDimAlongTrack,DimCrossTrack4
Degrees
RelativeAzimuthAngle DifferencebetweenviewingandsolarazimuthanglesDimAlongTrack,DimCrossTrack Degrees
SecondsinDayTimewhenswathmeasurementwastaken(innumberofsecondssincebeggingoftheday) DimAlongTrack, Seconds
ViewingAzimuthAngle SatelliteazimuthofeachpixelDimAlongTrack,DimCrossTrack Degrees
ViewingZenithAngle Satellitezenithangleofeachpixel DimAlongTrack,DimCrossTrack
Degrees
SolarAzimuthAngle Solarazimuthofeachpixel DimAlongTrack,DimCrossTrack
Degrees
SolarBetaAngle AnglesbetweenageocentricSunvectoranditsprojectionontheNPPorbitalplane
DimAlongTrack NoUnits
SolarZenithAngle Solarzenithangleofeachpixel DimAlongTrack,DimCrossTrack
Degrees
SpacecraftAltitude Spacecraftaltitude DimAlongTrack MetersSpacecraftLatitude SpacecraftLatitude DimAlongTrack DegreesSpacecraftLongitude SpacecraftLongitude DimAlongTrack DegreesSubSatelliteSolarZenithAngle Solarzenithangleatsubsatellitepoint DimAlongTrack Degrees
Time TAI93timeofmeasurement(numberofsecondssince1January1993)
DimAlongTrack Seconds
UTC_CCDSDS_A Twenty-sevencharacterUTCdate-and-timestring DimAlongTrack NoUnits
GroundPixelQualityFlags SAA,eclipse,Sunglint,terraintypeflags DimAlongTrack,DimCrossTrack NoUnits
Definitionofbit-packedGroundPixelQualityFlags0-3 Unused 4-5 SAAFlag WARNING Indicateslocationofspacecraftw.r.t.SAA
0=outsideSAAboundaries1=<5%ofnominalmaximumSAAeffect2=between5%and40%ofnominalmaximumSAAeffect3=>40%ofnominalmaximumSAAeffect
6-19 Unused 20 ManeuverFlag WARNING Indicatesaspacecraftattitudemaneuverwasinprogressduringthe
measurement21 AttitudeThresholdFlag WARNING Indicatesanyofthe3geodeticspacecraftattitudeEulerangles
exceedsadefinedthreshold22-31 Unused
DefinitionofInstrumentQualityFlags0-7 Unused 8 EclipseFlag WARNING Indicatesgroundpixeliswithinumbraorpenumbraofthemoon9-15 Unused
ScienceDataGroupDatasetName Description Dimensions Units
AlgorithmFlags DimAlongTrack,DimCrossTrack Nounits
ColumnAmountO3 FinalTotalcolumnozoneamountsDimAlongTrack,DimCrossTrack DobsonUnits
Layerefficiency DimAlongTrack,DimCrossTrack Nounits
MeasurementQualityFlags DimAlongTrack,DimCrossTrack
Nounits
NValue Nvalue[-100*log10(normalizedradiance)] DimAlongTrack,DimCrossTrack
Nounits
NValueAdjustment SoftcalibrationappliedtoNvalues DimAlongTrack,DimCrossTrack
Nounits
O3BelowCLoud Estimateofozoneamountbelowcloud DimAlongTrack,DimCrossTrack
DobsonUnits
QualityFlags DimAlongTrack,DimCrossTrack NoUnits
RadiativeCloudFraction DimAlongTrack, DimCrossTrack
Fractionbetween0and1
Reflectivity331 331nmreflectivity DimAlongTrack, DimCrossTrack Fraction
Reflectivity360 360nmreflectivity DimAlongTrack, DimCrossTrack Fraction
SO2index EstimateofSO2contamination DimAlongTrack, DimCrossTrack DobsonUnits
Sensitivity SensitivityofNvaluetochangeinozone DimAlongTrack, DimCrossTrack
1/DobsonUnits
StepOneO3 Estimateoftotalozonefromstep1ofalgorithm
DimAlongTrack, DimCrossTrack DobsonUnits
StepTwoO3 Estimateoftotalooznefromstep2ofalgorithm
DimAlongTrack, DimCrossTrack DobsonUnits
UVAerosolIndex AerosolIndex DimAlongTrack, DimCrossTrack Nounits
dN_dR SensitivityofNvaluetochangeinreflectivity DimAlongTrack, DimCrossTrack
dN_dT SensitivityofNvaluetochangeintemperature
DimAlongTrack, DimCrossTrack 1/Degrees
Fc Radcl radgr 9.References
PawanK.BhartiaandCharlesW.Wellemeyer,OMIAlgorithmTheoreticalBasisDocument,VolumeII,Chapter2,https://eospso.nasa.gov/atbd-category/49.
Kramarova,N.,E.Nash,P.Newman,P.K.Bhartia,R.McPeters,D.Rault,C.Seftor,P.Q.Xu,andG.Labow(2013),MeasuringtheAntarcticozoneholewiththenewOzoneMappingandProfilerSuite(OMPS),Atmos.Chem.Phys.,14,2353-2361,doi:10.5194/acp-14-2353-2014,2014.