温度反演经典文章(20)

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

AsatisfactorymodelisexpectedtoretrievetheLSTfromacom-binationofbrightnesstemperaturesmeasuredatdifferentfre-quenciesandpolarizationmodes.

(2)DevelopmentofamodeltoderivetheskinLSTfrompassivemi-crowavedata.Aswellknown,theLSTretrievedfrommicrowavedataisdifferentfromthatderivedfromTIRdata.Theformerre- ectsanaveragevalueofthesoiltemperaturefromthelandsur-facetoaparticulardepth(dependingonthefrequencyusedtoretrieveLST)underneaththesurface,whereasthelatteristheskintemperaturewithseveralmicronsofdepth.TocombinethesetwotypesofLSTandextracttheskinLST,amodelmustbedevelopedtoextracttheskinLSTfromtheLSTderivedfrompassivemicrowavedatawiththeaidofLSTsderivedfrompas-sivemicrowavedataatdifferentfrequenciesandofthethermalconductivityequationappliedtosoil.

(3)Developmentofamicrowave-TIRfusionmodel.Aneffective

modelthatfusestheLSTsretrievedfromTIRandpassivemicrowavedatamustbedevelopedinthefuturetoproducehigh-resolutionspatialLSTdatainallweatherconditions.ThekeyproblemtoberesolvedishowtorecovertheLSTatthespa-tialresolutionofTIRdatawhenamicrowavepixelisfullyorpartlycloudy.5.6.MethodologyforangularnormalizationofLST

AsreportedbyLagouardeandIrvine(2008),Lagouardeetal.(1995,2004),Chehbounietal.(2001)andLietal.(2004b),theLSTvarieswithVZA,anditsangularvariationforthree-dimensionalsurfacesresultsprimarilyfromtheangularvariationofthepixelemissivityandtherel-ativeweightsofdifferentcomponents(e.g.,vegetationandbackgroundsoil)withdifferenttemperaturesinanon-isothermalpixel.Thediffer-enceintheLSTmeasuredinnadirandoff-nadirobservationscanbeaslargeas5Kforbaresoilsandeven10Kforurbanareas.Becausemostpolar-orbitsatellites(e.g.MODIS,AVHRR)scanthelandsurfaceinthecross-trackdirectionwithdifferentVZAsvaryingfrom–65°to+65°,angle-dependentvariationsintheretrievedLSTareinevitable,makingtheLSTsofdifferentpixelsinthesameorbitincomparableandcausingerroneousresultsinapplication.ThiseffectmustalsobeconsideredforLSTsobtainedfromdifferentsensorsoratdifferenttimes.Therefore,itisverycrucialtonormalizethesatellite-derivedin-stantaneousLSTsatvariousVZAstoareferenceVZA(e.g.,atnadir).

Onemethodofperformingangularnormalizationonsatellite-derivedLSTsistosimplyattributetheangularvariationofthemea-suredeffectivetemperaturederivedfromarea-weightedemittedradiancestothedirectionalbehaviorofthepixelemissivity,aspro-posedbyLietal.(1999).However,thedirectionalemissivityde nedinthismannerisusuallynotmeasurablefromspace,andtheassump-tionthatthereisnodownwardenvironmentalthermalradiancemaycausesomeunexpectederrorsinthenormalizedresult.Anothertech-niquefornormalizingthesatellite-derivedLSTreliesonasimpli eddirectionalthermalRTMthatconsidersthecomponenttemperaturesandfractionswithinthepixel.Newmethodscanbeginbyparameter-izingthedirectionalthermalRTMwiththeminimumnumberofun-knownsbasedonthedirectionalfractionofvegetationcoverandthecomponenttemperaturesofpixels.Then,themethodshouldes-tablishrelationshipsbetweenthedirectionalradiativetemperaturesobservedfromdifferentdirections.Theoff-nadirLSTcanbenormal-izedtoareferencedirection(e.g.,atnadir)bydeterminingthefrac-tionsofvariouscomponentsandthecorrespondingcomponenttemperaturesortheirratiosfrommulti-angleormulti-channelobser-vations.Thefractionofcomponentsunderaspeci cviewinganglecanbecalculatedusingthebidirectionalre ectancedistributionfunc-tion(BRDF)modelinthevisibleandnearinfraredspectralregions.AlthoughangularvariationsintheLSThavebeendemonstratedorsimulatedatthepixelscaleintheliterature(Pinheiroetal.,2006,2004;Rasmussenetal.,2011,2010),thereisnoanypracticalway

toperformangularnormalizationofsatellite-derivedLSTsduetothecomplexityofthisnormalization.Thisissuethereforerequiresfurtherinvestigationinthefuture.Tovalidatethenormalizationmodel,angularmeasurementsofthethermalradiationatgroundlevelmustalsobeconducted.

5.7.Methodologyfortemporal(time)normalizationofLST

Itiswellknownthattheacquisitiontimes(UTC)ofallthepixelsinoneimagearenearlythesamebutthelocalsolartimesaremuchdif-ferent.Forinstances,inapolar-orbitsatelliteimagesuchasaMODISimage,thedifferenceinthelocalsolartimebetweentheeastandthewestpixelsalongthescanninglinecanbeupto1.5h,whichmeansthattheeastpixelsareexposedtosolarirradiationapproximately1.5hbeforethewestonesiftheskyisclear.Asaresult,theLSTprod-uctsderivedfromthesamesatellitecannotbecomparedifthediffer-encesinthelocalsolartimesofthepixelsaresigni cant.ThisphenomenonalsoaffectsLSTproductsacquiredbydifferentsatellitesatdifferenttimesandsigni cantlylimitstheapplicabilityofthein-stantaneousLSTproducts.Itisthereforenecessarytotemporallynor-malizethesatellite-derivedLSTstothesamelocalsolartime.Thediurnaltemperaturecycle(DTC)modelshowspromisingabilitytonormalizetheLSTtoanytimeofacloud-freeday.However,onlyDTCmodelswithsixparametershavebeendevelopedtodescribethediurnalvariationoftheLSToncloud-freedays(Göttsche&Olesen,2001;Jiangetal.,2006;Schädlichetal.,2001).Becausepolar-orbitsatellitesgenerallypassagivenlocationonlyonceortwiceperday(fourtimestotalforMODISTerraandAqua),eitheranewDTCmodelwithaminimumnumberofunknownparameters(lessthan4)oracombinationofpolar-orbitandgeostationarysatel-litesmustbedevelopedinthefuturetotemporallynormalizethepolar-orbitsatellite-derivedLSTs.Consideringthatgeostationarysat-ellitesobservethesamelocationwithhightemporalfrequency,LSTsderivedfromgeostationarysatellitedatacanbeusedtodetermineatypicalDTCmodel.AssumingthattheLSTsderivedfrompolar-orbitsatellitedataexhibitthesamediurnalpatternasthosederivedfromgeostationarydata,theonce-ortwice-dailyLSTsderivedfrompolar-orbitsatellitedatacanbeinterpolatedtoanytimeofacloud-freedayutilizingtheDTCmodeldevelopedusinggeostationarydata.However,onapartlycloudyday(nocloudcontaminationwhenthepolar-orbitsatelliteoverpasses),theseDTCmodelswillnotbeap-plicable,andalocalLSTvariationmodelshouldthereforebedevel-opedinthefuturewiththeaidoftemporalinformationprovidedbygeostationarysatell …… 此处隐藏：5038字，全部文档内容请下载后查看。喜欢就下载吧 ……