EMPIRICALPOLARIZATIONDISTRIBUTIONMODELS:UPDATE

DANIEL GOLDIN (SSAI/NASA-LARC), CONSTANTINE LUKASHIN (NASA-LARC), WENBO SUN (SSAI/NASA-LARC)

DanielGoldin POLARIZATION:RECAP

• PolarizationstatefullyspecifiedbydegreeofpolarizationP,angleofpolarizationc andtotalintensityI (alternatively,maybespecifiedbyStokesparametersI,Q,U)

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Imager’ssensitivitytopolarization

Totalreflectance

Polarizedradiance

Totalradiance where

CLARREO’sownaccuracy UncertaintiesinmandP

• (Mean)reflectanceneedstobecorrectedforpolarizationeffects:

• Uncertaintyduetopolarizationcontributestouncertaintyinreflectance:

• m isafunctionofc.SunandXiong[2007]haveshowncyclical dependenceforMODIS.TheexactdependencewouldbeestablishedbyCLARREOinstrument.Fornow,thisdependenceisfoldedintodm(usemeandm)

DanielGoldin EMPIRICALPDMSFROMPARASOL

• PARASOLwastheonlyinstrumentthatprovidedpolarizationmeasurementsonorbit

• 3wavelengthsavailablefromPARASOL:470,670and865nm

• Goal:constructP andc PDMsforthe3bandsforvariousscenetypes(IGBPs,clear-sky,cloudy,aerosols)

• UseinterpolationtoconstructP andc PDMsbetweenthe3bands

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DanielGoldin RECENTRESULTS

• Finished2Dfitsforthehighestpolarizationscenetypes

• PDMfits,advantages:

• Aswithanyfits,PDMfitsusefultosmoothoutstatisticalfluctuationsandfillgapsindata

• Compact(only7-8parameters)anduniversal(appliedtoanyscenetype)• Robust,evenforlow-statisticsPDMs

• Usefits(meansandfiterror)toempiricalP andc PDMsinthefinalmodule

• Implementedaworkingversion(C++/C)ofthePDMmoduletoretrievedegreeofpolarizationP andangleofpolarizationc,withcorrespondingstd.dev., based on PARASOL data

• Workingonlowerpolarizationscenes(cloudyscenetypes)

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DanielGoldin PDMCLASSIFICATION

• Computedtotalmeansandstd.devs forPPDMs

• Initiallyconsideredhighestpolarizationscenarios:

• Pickedshortestavailablewavelength(l =490nm)

• SZA=40(closetothetypicalrangeofBrewster’sangles)

• ConsideredPDMsatleast2std.dev.awayfrom0

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IGBP Surface Type Pmean

P std. dev.

1 Evergreen needle-leaf forest 0.19 0.11

2 Evergreen broad-leaf forest 0.26 0.07

3 Deciduous needle-leaf forest 0.14 0.11

4 Deciduous broad-leaf forest 0.20 0.11

5 Mixed forest 0.16 0.12

6 Closed shrubland 0.18 0.12

7 Open shrubland 0.17 0.10

8 Woody savannas 0.17 0.12

9 Savannas 0.23 0.06

10 Grasslands 0.18 0.09

11 Permanent wetlands 0.16 0.13

12 Croplands 0.20 0.08

13 Urban and Built-up 0.24 0.09

14 Cropland Mosaics 0.21 0.09

15 Permanent snow and ice

16 Bare soil and rocks 0.16 0.06

17 Water Bodies 0.31 0.08

18 Tundra

19 Fresh Snow

20 Sea Ice

DanielGoldin PPDMFITFUNCTION

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Gaussiansdescribing(mostly)glintregion

basedon1st orderscatteringapproximation(awayfromglint)

• Performac2 fitonP PDM:

DanielGoldin PPDMFITFORIGBP=9(l =670nm): MEANS

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DanielGoldin PPDMFITFORIGBP=9(l =670nm):STD.DEVS

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70%C.L.(≈1s)uncertainty

DanielGoldin PPDMFITFORIGBP=10 (l =490nm):MEANS

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DanielGoldin PPDMFITFORIGBP=10(l =490nm):STD.DEVS

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DanielGoldin PPDMFITFORIGBP=14(l =490nm):MEANS

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DanielGoldin PPDMFITFORIGBP=14(l =490nm):STD.DEVS

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DanielGoldin PPDMS:SUMMARY

• 2Dfitsprovidegood(DP ≈+/-0.1)approximationtoPPDMsmeansandstd.devs.

• FinalempiricalPPDMsrecorded(TBD)intheformofthefitcoefficientstof(f,q)orbinnedvaluesoff(f,q) andper-binfituncertainties

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DanielGoldin c:SINGLESCATTERINGAPPROXIMATION

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• Performac2 fitonc PDM:

DanielGoldin PPDMFITFORIGBP=14(l =865nm,50o <SZA<60o )

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Second/third orderscatteringEffectswhenSZA≈VZA

DanielGoldin c PDMS:SUMMARY

• Singlescatteringisagoodapproximationtowithin+/-4o forempiricalc inthe50o <RAZ<310o region

• IntheregionRAZ=0o/360o,SZA=VZA:

• PARASOLhaslowresolutioninc• Singlescatteringnotagoodapproximationthere:higherorderscatteringmodel

needed• Singlescatteringfityieldsreasonablemeanvaluesbutdoesn’tyieldreasonablefit

uncertainties,soplan(TBD)tousehybridapproachforfinalrecordedc values:

• UsePARASOLforbinswithdata,css approximationforbinswithnodata

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DanielGoldin TOBEDONE

• Workonlowerpolarizations(clouds)andotherIGBPs

• Implementice&watercloudsPDMs

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DanielGoldin

BACKUP

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DanielGoldin HIGHcUNCERTAINTYREGIONS

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c PDMhigh-uncertaintyregion:RAZ>310,VZA>30

c PDMcentralregion:50<RAZ<300

• RegionsRAZ=0o/360o,SZA=VZAhaveStokesparamer≈0o,closetoPARASOL’sresolution

• ForsinglescatteringQ=0isundefined,needhigherorderscattering:

DanielGoldin PUNCERTAINTIES

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• Plotshowsintercal.uncertaintyinreflectancevs.degreeofpolarizationassumingtheintercalibratedimager’ssensitivitytopolarizationm=0.03anddmsettothreedifferentvalues

• ConsideringCLARREO’sowntargetuncertaintyduetopolarizationdr0 =0.15%,onecanconcludethat:

• valuesofP <0.1,maybeconsideredbelownoisethreshold.Thus:

• duetosharpdrop-offinP(e.g.,nextslide),somePDM’sfor670and865nmcanbeneglected

• snow-coveredsurfacescanbeneglected(IGBP=15,19and20)

• Lowslopeindicatesintercal.P isinsensitivetostd.devs<≈0.1.

• PDMuncertaintieshaveroughlythenecessaryprecision

dm =100%dm =20%dm =10%

Clear sky ocean (IGBP = 17)

dm =100%dm =20%dm =10%

Evergreen Broadleaf Forests (IGBP = 2)