温度反演经典文章(12)

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Z.-L.Lietal./RemoteSensingofEnvironment131(2013)14–37

thattheassumptionofthetime-invariantLSEsimplicitlyrequiresthesurfacetobehomogenousandhaverelativelystablesoilmoisture.The rstrestrictionistoalleviatetheLSEvariationcausedbypixelsizesandbyviewingangles,whilethesecondistoavoidtheLSEchangeswithsoilmoisture,suchastheoccurrenceofprecipitationanddew.

TheprimaryadvantageoftheTTMisthatitmakesnoassumptionaboutthespectralshapeoftheLSEs,onlythattheyaretime-invariant.Thismethodhasasimpleandstraightforwardformulation;however,theretrievalaccuracyisnotalwaysguaranteedbecausethe2Nequa-tionsarehighlycorrelatedandtheirsolutionsmaythusbeunstableandverysensitivetoinstrumentnoiseanderrorsintheatmosphericcorrections(Casellesetal.,1997;Gillespieetal.,1996;Watson,1992).Becauseaccurateatmosphericcorrectionsaredif culttoperformwithoutsimultaneousatmosphericpro lemeasurements,theuseofapproximatepro lescouldleadtolargeuncertaintiesintheLSTandLSEretrievals.PeresandDaCamara(2004)foundthatincreasingthenumberofobservationsand/orthetemperaturedifferenceimprovedtheretrievalaccuracy,butthisimprovementislimitedbythehighcorrelationbetweenTIRmeasurements.

Inadditiontotheproblemsmentionedabove,thismethodrequiresaccurategeometricregistrationofimagesacquiredattwodifferenttimes(Gillespieetal.,1996;Watson,1992).Similartotheday/nightTISIbasedmethod,theimpactofmis-registrationontheLSTandLSEer-rorsissmallforhomogeneousareasbutlargeforheterogeneousareas(Wan,1999).AchangeinthesatelliteVZAcausesachangeintheLSE,consequentlyviolatingtheassumptionoftime-invariantLSEsandde-creasingtheaccuracyoftheTTM(Lietal.,2013).

3.2.2.2.Physics-basedday/nightoperationalmethod(D/N).Inspiredbytheday/nightTISIbasedmethodandTTMmethod,WanandLi(1997)furtherdevelopedaphysics-basedD/Nmethodtosimulta-neouslyretrieveLSTandLSEsfromacombineduseoftheday/nightpairsofMIRandTIRdata.ThismethodassumesthattheLSEsdonotsigni cantlychangefromdaytonightandthattheangularformfactorhasverysmallvariations(b2%)intheMIRwavelengthrangeofinteresttoreducethenumberofunknownsandmaketheretrievalmorestable.Toreducetheeffectoftheresidualerrorofatmosphericcorrectionsontheretrieval,twovariables,theairtemperatureatthesurfacelevel(Ta)andtheatmosphericWV,areintroducedtomodifytheinitialatmosphericpro lesintheretrieval.Withtwomeasure-ments(dayandnight)inNchannels,thenumbersofunknownsareN+7(NchannelLSEs,2LSTs,2Ta,2WV,and1angularformfactorintheMIRchannels).Thus,tomaketheequationsdeterministic,Nmustbeequaltoorgreaterthanseven.

Generally,thephysicalD/paredwiththeTTMandTISImentionedpreviously,thisD/Nmethodishighlightedbyseveralfacets:

(1)ThecontributionofsolarirradiationtotheradianceoftheMIR

channelsindaytimesigni cantlydecreasesthecorrelationsamongtheequationsandmakesthesolutionmorestableandac-curate.UnliketheTISImethodthat rstobtainsthebi-directionalre ectivityofthepixelandthencalculatesLSEandLSTseparate-ly,theD/NmethodretrievessimultaneouslyLSTandLSEandavoidsthepropagationoferrorfromstepwiseretrieval.Inaddi-tion,theD/NmethodscanaccuratelydetermineLSTsandLSEseventhoughtheLSTsareequalatthetwotimes(dayandnighttimes);whiletheTTMwithonlyTIRmeasurementsrequiresig-ni cantdifferenceinthetemperatures.

(2)TheaccuracyoftheretrievedLSTsandLSEsisstronglyimproved

byintroducingtwovariables(TaandWV)toaccountfortheun-certaintiesintheinitialatmosphericpro les.Asaresult,theac-curacyoftheatmosphericcorrectionisnotrequiredtobeashighasthatofTISIandTTM.

(3)TheD/Nmethoddoesnotrequire12-hourintervalmeasure-ments(dayandnight).Aslongasthesurfaceemissivitydoesnotchangesigni cantly,daytimeandnight-timedatacollectedoverseveraldaysarealsoappropriate.However,similartotheothermulti-temporalmethods,theD/Nmethodstillsuffersfromthecriticalproblemsofgeometrymis-registrationandvariationsintheVZA.Wan(1999)aggregatedtheMODISpixelstoincreasethescalefrom1kmto5kmor6kminordertoovercomethemis-registrationproblems.Meanwhile,16VZAsubrangesareusedtoensurequalityofdayandnightVZAsubranges(Wan&Li,2010).Asetofnewre nements(Wan,2008)wereimplementedtorejecttheworstsolutionsandforbetterLSTre-trievalseveninlessidealconditionssuchasundertheeffectsofnear-bycloudsandheavyaerosols,differentsurfaceemissivityvaluesintheMIRand8.75μmchannelsduringthedayandnightduetoeventsofrain,snowandnighttimedew(giventherelativelyhighemissivityvaluesinbands31and32lessaffectedbytheseeventseveninaridregions).TheimprovementsincludethecombineduseofTerraandAquaMODISdata,settinglargerweightsonthedaytimedata,fullyin-corporatingtheviewing-angledependentGSWmethodintotheD/NalgorithmasaclosecomponentandrelatedconstraintsonLSTdiffer-ences,usingthevariablesofemissivitiesinbands31and32,WVandTaintheiterationsofsolutionoftheD/Nalgorithmandeffectivelyin-creasingtheweightsonthehighestqualitydataofbands31and32.MoredetailsontheMODISD/Nmethodcanbefoundintheliterature(Wan,2008;Wan&Li,1997,2010).

3.2.2.3.Graybodyemissivitymethod(GBE).ThismethodassumesthattheLSEhasa atspectrum,i.e.,theLSEisindependentofthewave-length,forwavelengthslargerthan10μmtoreducethenumberofunknownsandstabilizetheretrievalalgorithm(Barducci&Pippi,1996).

ThemainadvantageoftheGBEisthatnoadditionalassumptionabouttheshapeoftheemissivityspectrumisrequired,excepttheas-sumptionthatitis atinsomewavelengthinterval.Intheory,theLSTandLSEscanbesimultaneouslyretrievedaslongasatleasttwochan-nelshavethesameLSE(notnecessarilythegraybody)inthewave-lengthintervalofinterest.However,thelimitationsofthemethodareevident.TheapplicationoftheGBEmethodtospace-basedmea-surementsrequiresaccurateatmosphericcorrectionsintheTIRchan-nelsandatleasttwochannelswiththesameLSE.SimilartotheTTM,thismethodisverysensitivetoinstrumentnoiseanderrorsintheat-mosphericcorrectionsbecausetheTIRmeasurementsarehighlycor-related.Moreover,requiringspectrally atLSEsoftenhamperstheuseoftheGBEinmultispectralTIRdataunlessatleasttwochannelswiththesameLSEcanbeidenti ed.ThisproblemcanbemoreorlessovercomewithhyperspectralTIRdata,becauseitiseasierto ndatleas …… 此处隐藏：4749字，全部文档内容请下载后查看。喜欢就下载吧 ……