温度反演经典文章(6)

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

largeuncertaintiesthatcanleadtoanevenlargererrorontheLSTretrievedfromMIRmeasurements.Therefore,whilethein-troductionoftheMIRchannelsmaybene ttheretrievaloftheLSTincertaincases,itcanalsointroduceevenlargeruncer-taintiesinothers.

(6)HowtophysicallyinterprettheresultsoftheLSTmeasurement

remainsacrucialquestion.AsnotedbyPrataetal.(1995),thedef-initionofthesurfacetemperaturemaydependstronglyonthetypeofapplicationandthemethodofmeasurement.BecausethesurfacetemperatureTsinEq.(4)isde nedusingtheradianceemittedbyasurface,thistemperatureiscalledtheradiometrictemperature(ortheskintemperature)thatcorrespondstothera-diationemittedfromdepthslessthanthepenetrationdepthofagivenwavelength(Becker&Li,1995;Norman&Becker,1995).ThepenetrationdepthisusuallywithinafewmillimetersintheTIRregion(Wan,1999).Thisradiometrictemperaturephysicallydiffersfromotherde nitionoftemperatures,suchasthethermo-dynamictemperaturede nedforamediuminthermalequilibri-umandmeasuredbyathermometer.Forhomogeneousandisothermalsurfaces,theradiometricandthermodynamictemper-aturesarereportedtobeequivalent.Asthethermodynamictem-peratureisactuallyhardtomeasureinreality,evenforwater,theradiometrictemperatureisoftentheonlypracticalmeasureforthehomogeneousandisothermalsurface.However,mostsurfacesarenotinequilibriumandforheterogeneousandnon-isothermalsurfaces,thesetwotemperaturesaredifferent.Consideringthatthespatialresolutionofthecurrentonboardsys-temsvariesapproximatelyfrom10 2to10km2,theremaybeseveralsurfacetypeswithdifferenttemperaturesandemissivitieswithinonepixel,whichcomplicatesthephysicalunderstandingoftheLSTvaluesretrievedfromspaceandtherelationofthera-diometrictemperatureatlargescalestoothertemperaturesusedindifferentapplications.Todate,noconsensushasbeenreachedonthede nitionoftheLSTforheterogeneousandnon-isothermalsurfaces,butthede nitionbyBeckerandLi(1995),whichdependsonthedistributionsoftheLSTandtheLSEwithinapixel,ismeasurablefromspaceandisthusrecommendedforLSTretrievalfromspace.

(7)ValidationofLSTretrievedfromspacebornemeasurementsat

thescaleofthesensor'spixelsisalsochallenging.Validationisproblematicduetothedif cultyofconductinginsituLSTmea-surements,andinobtainingrepresentativeLSTdataatthescaleofasinglepixel.Generally,temperaturesoverthelandsurfacesvarystronglyinspaceandtime(Prataetal.,1995),anditisnotunusualfortheLSTtovarybymorethan10Koverjustafewcentimetersofdistanceorbymorethan1Kinlessthanaminuteovercertaincovertypes.Appropriatelyscalingthesatellite-derivedLSTstothosemeasuredatgroundlevel,especiallyatlargescaleisalsodif cult.ThestrongspatialheterogeneityandtemporalvariationoftheLSTlimitsground-basedvalidationtoseveralrelativelyhomogeneouslandsurfaces,suchaslakes,de-serts,anddenselyvegetatedlandusingdirectcomparisonsofinsitutemperaturemeasurementswithtemperaturesretrievedfromthesatellitedata(e.g.Hooketal.,2005,2003,2007).Acom-plementaryapproachistousesiteswhicharehomogenousintermsofemissivityusingtheradiancebasedvalidationapproach(Hulleyetal.,2009).Sanddunescanbeoneexampleofthistypeofsiteswhicharereferredtoaspseudo-invariantsites(Hulleyetal.,2009).Furthermore,howtoperformarepresentativemea-surementoftheLSTofacomplexheterogeneoussurfaceisalsoacriticalquestion.Scalingmethodsshouldbedevelopedtorelatethemeasurementsatdifferentscalesandhelpvalidatethere-trievedLST(Liuetal.,2006;Wu&Li,2009).Besidesthedif cultiesmentionedaboveintheretrievalofLSTfromspace,theaccuracyoftheLSTdataalsodependsontheperformanceof

thecloudmaskusedtoisolateclearskydataandonthequalityoftheTIRdata,i.e.,thestabilityofthespectralresponsefunctiongi(λ),theSNRandtheaccuracyoftheradiometriccalibration.3.EstimationofLSTfromspace

Overthepastseveraldecades,LSTestimationfromsatelliteTIRmea-surementshassigni cantlyimproved.Manyalgorithmshavebeenpro-posedtotreatthecharacteristicsofvarioussensorsonboarddifferentsatellitesandutilizingdifferentassumptionsandapproximationsfortheRTEandLSEs.Thesealgorithmscanberoughlygroupedintothreecategories:single-channelmethods,multi-channelmethods,andmulti-anglemethods,providedthattheLSEsareknownapriori.IftheLSEsarenotknown,thenthealgorithmscanbecategorizedintothreetypes:stepwiseretrievalmethod,simultaneousretrievalofLSEsandLSTwithknownatmosphericinformation,andsimultaneousretrievalwithunknownatmosphericinformation.3.1.LSTretrievalwithknownLSEs

3.1.1.Single-channelmethod

ThesingleTIRchannelmethod,alsocalledthemodelemissivitymethod(Hooketal.,1992),usestheradiancemeasuredbythesatellitesensorinasinglechannel,chosenwithinanatmosphericwindow,andcorrectstheradianceforresidualatmosphericattenuationandemissionusingatmospherictransmittance/radiancecodethatrequiresinputdataontheatmosphericpro les.LSTisthenretrievedfromtheradiancemeasuredinthischannelbyinvertingtheRTEgiveninEqs.(3)and(4),providedthattheLSEiswellknownorestimatedinadvance(Chédinetal.,1985;Hooketal.,1992;Lietal.,2004a;Mushkinetal.,2005;Ottlé&Vidal-Madjar,1992;Price,1983;Susskindetal.,1984).Accuratedeter-minationoftheLSTusingthismethodrequireshigh-qualityatmospher-ictransmittance/radiancecodetoestimatetheatmosphericquantitiesinvolvedinEqs.(3)and(4),goodknowledgeofthechannelLSE,anac-curateatmosphericpro le,andacorrectconsiderationofthetopo-graphiceffects(Sobrinoetal.,2004b).

Generally,theaccuracyofatmospherictransmittance/radiancecodeisprimarilylimitedbytheradiativetransfermodel(RTM)usedinthecodeandbytheuncertaintiesinatmosphericmolecularabsorptionco-ef cientsandaerosolabsorption/scatteringcoef cients(Wan,1999).ThemostpopularatmosphericRTMs,suchastheseriesofMODTRAN(Berketal.,2003)and4A/OP(Chaumatetal.,2009),havebeenwidelyusedtoperformatmosphericcorrectionsand/ortosimulatesatelliteTIRdata.AfewstudieshaveshownthattheaccuracyofthedifferentRTMsrangesfrom0.5%to2%withinknownatmosphericwindows,suchas3.4–4.1μmand8–13μm,leadingtouncertaintiesintheretrievedLSTof0.4Kto1.5K(Wan,1999).Itisworthnotingthattheincompletechar …… 此处隐藏：4995字，全部文档内容请下载后查看。喜欢就下载吧 ……