amt-2020-7 Preprint. Discussion started: 20 February 2020 c … · 2020. 7. 8. · p!"!...

1

Inter-CalibrationofnineUVsensinginstrumentsover1

AntarcticaandGreenlandsince19802

3

ClarkWeaver1,2,PawanK.Bhartia1,DongL.Wu3,GordonLabow1,4,DavidHaffner1,44

1AtmosphericChemistryandDynamicsBranch,NASAGoddardSpaceFlightCenter,5Greenbelt,MD20771,USA.672EarthSystemScienceInterdisciplinaryCenter(ESSIC),UniversityofMaryland8CollegePark,MD20742,USA9103ClimateandRadiationLaboratory,NASAGoddardSpaceFlightCenter,Greenbelt,MD1120771,USA.12134ScienceSystemsandApplications(SSAI),Inc.,Lanham,MD20706,USA.1415Correspondenceto:[email protected] 16

https://doi.org/10.5194/amt-2020-7Preprint. Discussion started: 20 February 2020c© Author(s) 2020. CC BY 4.0 License.

2

Abstract1

Nadir viewed intensities (radiances) from nine UV sensing satellite instruments are calibrated 2

over the East Antarctic Plateau and Greenland during summer. The calibrated radiances from 3

these UV instruments ultimately will provide a global long-term record of cloud trends and cloud 4

response from ENSO events since 1980. We first remove the strong solar zenith angle 5

dependence from the intensities using an empirical approach rather than a radiative transfer 6

model. Then small multiplicative adjustments are made to these solar zenith angle normalized 7

intensities in order to minimize differences when two or more instruments temporally overlap. 8

While the calibrated intensities show negligible long-term trend over Antarctica, and a 9

statistically insignificant UV albedo trend of -0.05 % per decade over the interior of Greenland, 10

there are small episodic reductions in intensities which are often seen by multiple instruments. 11

Three of these darkening events are explained by boreal forest fires using trajectory modeling 12

analysis. Other events are caused by surface melting or volcanoes. We estimate a 2-sigma 13

uncertainty of 0.35% for the calibrated radiances. 14

15

1.Motivation16

In1980theNimbus-7spacecraftcarriedthefirstSolarBackscatterintheUV(SBUV)17

instrumentintolowearthorbittomeasuretotalcolumnozone.Sincethen,NOAAhas18

deployedasuitofSBUV-2instrumentsonboardtheNOAA-9,11,14,16,17,18and1919

spacecrafts.Sincetheywereallnadirviewingandthushadlimitedspatialcoverage,NASA20

alsodeployedasuiteofmappinginstruments:Nimbus-7TOMS(1980),EarthProbeTOMS21

andtheNadirMapper(NM)instrumentoftheSuomiNPPOzoneMappingProfilerSuite22

(OMPS,2012).Truetotheirdesign,theyhaveprovidedalong-termsatellitedatarecordof23


3

ozoneproducts;however,theyalsowereintendedtomeasuretheearth’sreflectivityinthe1

UVatwavelengthsinsensitivetoozone(331and340nm).Asidefromafewpublications2

(Hermanetal.(2013),Labowetal.(2011)andWeaveretal.(2015)),thisdatasethasnot3

beenfullyexploited.Ourultimategoalisalong-termrecordofaUVcloudproductthatcan4

bedirectlycomparedwithclimatemodels.Thispaperdetailsthefirststep:theinter-5

calibrationofradiancesfromthesuiteofnadirviewinginstruments.6

2.PreviouscalibrationofUVSatelliterecords7

ThebackboneofourdatarecordisthesuiteofeightSBUVinstrumentsstartingwiththe8

Nimbus-7in1980,andendingwithNOAA-19in2013.ThereafterweuseNadirMapper9

(NM)instrumentontheSuomiNPPOMPS.Eachinstrumentprovidesnarrowband10

backscatteredintensitiesnearthe340nmwavelength.Weusearadiativetransfermodel11

toaccountforthesmalldifferencesineachinstrument’scenterwavelength(seeAppendix).12

Regularsun-viewingirradiancemeasurements(Fsun)aremade,typicallyweekly,toprovide13

long-termcalibrationinformation.ThemeasuredintensitiesarenormalizedbyFsun,and14

multipliedbyp.ThroughoutthisstudyIreferstothesunnormalizedintensities.15

16

Westartwithintensitiesthathavealreadybeencalibratedtoaccountforinstrument17

effectssuchashysteresis(seeDelandetal2012),andthatarereportedintheLevel-218

datasetsforeachinstrument.ThefirstsevenSBUV/2datasetswerepreviouslycalibrated19

bycharacterizingtheinstrumentsovertheEastAntarcticPlateauicesheetusing20

LambertianEquivalentReflectivity(LER,HuangL.-K.etal.2003andHermanetal.2013).21


4

UsingaradiativetransfermodeltocalculateLERfromtheobservedintensitiesremoves1

muchofthesolarzenithangle(𝜽𝒐)dependence,butnotall;overtheicesheetsLERstill2

decreaseswith𝜽𝒐especiallyathigh𝜽𝒐.Whiletheydidanexcellentjobofcharacterizing3

thefirstsevenSBUV/2instruments,twoadditionalsensorsneedtobeintercalibratedto4

extendourrecordforward:theSBUV2onNOAA-19,andtheSuomiNPPOMPS.Ratherthan5

calibratetheseadditionalinstrumentswitharadiativetransfermodelusingLER,weusean6

empiricalapproachtoremovethesolarzenithangledependenceonintensity.Usingthese7

𝜽𝒐-normalizedintensities,weinter-calibratetheUVsensorsovertheEastAntarcticPlateau8

andtheGreenlandicesheets.9

3.Empiricallybasedinter-calibration10

SatelliteobservedNadir-viewedintensitiesovertheAntarcticandGreenlandicesheets11

haveanalmostlinearrelationshipwithsolarzenithanglethatiseasilyfittedwitha4-term12

polynomial.Figure1showstherelationshipoverbothicesheetsforallobservations13

sampledbytheSBUV2onNOAA-16.Withadriftingorbitandlonglifetime(2001-2014)14

NOAA-16sampledawiderangeofsolarzenithanglessowechooseitasourreference15

instrument.Thepolynomialfitusesallobservationsovertheinstrument’s15yearlifetime16

andsoprovidesamostprobableintensitythattheNOAA-16SBUV2wouldobservefora17

given𝜽𝒐.Ourcalibrationapproachistoremovethesolarzenithangledependencefromthe18

observedintensities(Iobs)byusingthereferencepolynomialfitsshowninFigure1.Wecan19

testifanobservedintensityishighorlowcomparedwiththeNOAA-16SBUV2reference20

bycalculatingafractionaldeviationintermsofintensity(𝜹𝐈)fromEquation1.For21

example,therightpanelofFigure1showsananomalouslylowintensitysampledovera22


5

darkscene(Iobsdark scene)observedatasolarzenithangle(q odark scene);itiscomparedwith1

theintensitythatNOAA-16wouldlikelyhaveobservedatthatsolarzenithangle(𝝃(q odark 2

scene)).ThedifferenceisdividedbyIobstoproduceafractionaldeviationinintensity𝜹𝐈.3

𝜹𝐈 = 𝐈𝐨𝐛𝐬,𝝃(𝜽𝒐)𝐈𝐨𝐛𝐬

Equation14

EachUVinstrumenthasitsownuniqueIobstoq orelationshipmainlybecausethe5

photomultipliertube(PMT)foreachinstrumenthasaslightlydifferentresponsefunction.6

TheunderlyingsceneUValbedo(averagedoveraninstrument’slifetime)couldbeslightly7

differentforeachinstrument,whichwouldalsochangetheIobstoq orelationship,butwe8

expecttheAntarcticplateaualbedotobestableovertime.TheSBUVPMTsaredesignedto9

haveazero-offsetbias,i.e.zerocurrentresponsewhentherearezerophotoncounts.Over10

AntarcticathepolynomialappearstohaveIobs=0atasolarzenithangleof90o,consistent11

withthisinstrumentdesign(Figure1).Oneneedstopayparticularattentiontomakesure12

theq ousedisexactlysimultaneouswiththeintensity,sincetheSBUVinstrumentshavea13

differentq oforeachwavelength.14

15

WealsoshowestimatesofIntensitycalculatedbytheradiativetransfermodelVLIDORT16

(VectorLInearizedDiscreteOrdinateRadiativeTransferpackage,Spurr,2006).Herewe17

assumeLambertiansurfacealbedoof.95,andRayleighatmospherewithsurfacepressure18

of663hPa.Thenumberofhalf-spacequadraturestreamsis40;thenumberofstokes19

vectorparametersis3.AtfirstglancetheVLIDORTsimulationappearstosimulatethe20


6

observations,buttheslopeofthesimulationisslightlydifferentthantheobservations1

(rightpanelofFigure1).WetriedusingtheItoq orelationshipsimulatedbyVLIDORTasa2

reference(insteadofusingNOAA-16)butthe𝜹𝐈wasstilltoodependentonq owhich3

complicatedtheanalysis.Ourmethodissimilarbutnotassophisticatedasthesun-4

normalizedradiancevalidationapproachdescribedinJarossetal(2008).Theyaccountfor5

thesnowBRDFwhichweomit.6

7

ThesuiteofSBUV/2instrumentsprovidesnadirobservationswitha170x170kmFieldOf8

View(FOV).ButtheOMPSMapperinstrumenthasasmallernominal50x50kmFOV,except9

atthetwomostnadirviewingpositions.HeretheFOVwidthsare20and30km(Seftorelal10

2017).Forconsistency,weonlyusedtheMapperviewingpositionsthatwerewithina11

nadir-centeredhypothetical170x170kmSBUVFOVandaggregatedtheirintensities(area12

weighted)priortocalculating𝜹𝐈.Foreachinstrumentwecalculatethesummertimeannual13

meanandplotthetimeseriesforbothicesheets(Figure2).14

4.Adjustingtheintensities15

Thepre-calibratedintensitiesSBUV2instrumentsonboardNOAA-17,-18and-19appear16

tobehighbiasedandNOAA-14lowbiasedcomparedtoourreference(Figure2).As17

describedbelow,acost-optimizationapproachisusedtoadjusttheintensitiesandreduce18

thesedisparities.Figure2onlyshowsthesummertimeaverage𝜹𝐈,butwhencalibrating19

instruments,itisinstructivetoexaminethe𝜹𝐈dependenceonq oforindividualyears.The20

leftpanelofFigure3ashowsthisfor2006whenthereferenceandthreeotherinstruments21

wereoperational.ThepositivebiasforNOAA-17and18isconsistentatallq o bins.All22


7

instrumentsshowasimilarskewed𝜹𝐈distribution,ateachq obin,towardlowvaluesof𝜹𝐈.1

Afteradjustment,thebiasesarenegligible(rightpanelFigure3a).2

3

Toadjustintensitiesforaspecificinstrumentamultiplicativefactor(c1)ischosensothat4

theadjustedintensitiesarealinearfunctionoftheoriginalintensities:Iadj=c1*Ioriginal+c0.5

Adjustingthemultiplicativefactor(c1)changesthegain,(intensityperobservedphoton6

counts)oftheinstrument.Tointer-calibrateallinstrumentswithrespecttoNOAA-16we7

useaminimum-costoptimizationalgorithmtosolveforasetofc1valuesthatminimizes𝜹𝐈8

disparitiesbetweentemporallyoverlappinginstruments.Thec1foreachinstrument,9

exceptthereference,isallowedtovary;Table1showsthegainchangesmadetoeach10

instrument.Notethatc1doesnotdependontime,sotheinterannualvariabilityofaspecific11

SBUVinstrumentremainsintactafterthecalibration.12

13

Onlythehighestqualityobservationsareusedfortheinter-calibration.Observationsare14

limitedtoq olessthan75obecauseathigherq

oozoneabsorptionandstraylighteffects15

becomesignificantandcontaminateresults.Furthermore,SBUVobservationsthathavea16

gratingdriveerrorandobservationsthatarelikelyimpactedbyPMThysteresisarenot17

usedtointercalibrate.18

19

ThegratingdriveselectsthewavelengthofaSBUVmeasurement.Sometimes,butnottoo20

often,thegratingdriveselectsthewrongvalueandtheintensitiesaremeasuredata21

wavelengthdifferentthantheSBUVinstrument’snominalwavelength.Inclusionof22

observationswithuncorrectedgratingerrorswillconfuseourresults,sinceouranalysis23


8

assumesthatintensitiestoderive𝜹𝐈areallatthesamewavelength.Fortunately,the1

gratingdrivepositionisarchivedsowecanapplyacorrection(seeAppendix);however,2

theyarenotusedintheintercalibration,butareusedinthelatertrendanalysis.Figure43

showsthesummertimeaverageempiricallyadjusted𝜹𝐈overbothicesheetsafterapplying4

thegainchangesinTable1.Solidcirclesexcludeobservationswithgratingdriveerrorsand5

opencirclesincludecorrectedobservations.Thereisclearlytightermatchbetween6

overlappinginstrumentscomparedwithFigure2.Buttherestillaredisparitiesbetween7

overlappinginstrumentsbetween1997and1999whenmultipleinstrumentssufferfrom8

gratingerrors.Itisdisconcertingthatourcorrectiondoesnotbringthemincloser9

alignment.10

11

BothNimbus-7andtoalesserextentNOAA-9,sufferedfromPMThysteresis.Theseearlier12

PMTswerenotabletoquicklyrespondtothe4ordersofmagnitudesignalchangesthat13

occurwhenthesatellitefirstcomesoutofdarknessoneachorbitandtheinstrumentsees14

itsfirstlight.ForNimbus-7hysteresiserrorsarebetween4and9%atfirstlightover15

AntarcticaandlessenasthePMTadjuststothebrightscenesovertheicesheet.Bythetime16

theNimbus-7reachesGreenlandthePMTisequilibratedandthereisnohysteresiserror.17

(MaximumhysteresiserrorsofNOAA-9are2%.).Theintensityobservationsfortheseearly18

instrumentshavebeencorrectedforhysteresis(Delandetal.,2001).Still,weinitiallywere19

unabletomatchNimbus-7withtheotherinstruments;therewasgoodagreementover20

AntarcticabutoverGreenlandNimbus-7wasabout1%higherthantheothers(Figure2).21

22


9

OurremedywastofirstcalibratetheSBUVinstrumentsonlyoverGreenlandwhere1

Nimbus-7isfreeofhysteresiserror.Asexpected,alltemporallyoverlappinginstruments2

agreedoverGreenland,butoverAntarcticaNimbus-7waslowbyabout1%comparedwith3

NOAA-9andNOAA-11.ThenwestartedremovingNimbus-7observations;firstthose4

within1minuteoffirstlight,then2minutes.Witheveryminuteofobservationsremoved,5

thedisparityoverAntarcticalessened.WeachievedthegoodagreementseeninFigure46

byremoving9minutesofNimbus-7observationsafterfirstlight.7

8

Figure5showstheq odependenceontheempiricallyadjusted𝜹𝐈forselectedyears.Allthe9

SBUVs,exceptforNimbus-7andNOAA-9,haveanalmostflat(<0.005)𝜹𝐈dependencewith10

q o. Aflatq

odependenceindicatesthatthePMTresponseissimilartotheNOAA-16.Over11

Greenland𝜹𝐈dependencewithq oisnotquiteasflat(Figure3b).Thesuppressionof𝜹𝐈at12

q o>57oandtimeafterfirstlight<9minutesisseenforallyearsofNimbus-7.Eventhough13

thesesuppressedobservations(q o>57o)werepreviouslycorrectedforhysteresis,14

artifactsremainandtheyarenotusedinanyanalysis.15

16

MultipleinstrumentsshowcoincidentreductionofdIoverAntarcticainJanuary199217

(Figure4)mostlikelyfromaerosolstransportedtotheAntarcticaftertheeruptionofMt18

Pinatubo6monthsearlierin1991(leftpanelFigure3c).TheApril1982eruptionofEl19

Chichonlikelycontributedtothecoincidentreductionin1983;otheranomaliesoccurin20

2001,2010and2013.Likewise,therearecoincidentreductionsindIoverGreenland.21

22


10

ToestimatetheuncertaintyintheSBUVintensityfrominstrumentcalibrationalonewe1

firstaveragethedIoverthecoincidentsatellitesforeachyear;thismergedtimeseriesis2

thegeophysicalcontribution.Absolutedeparturesfromthismergedtimeseries(Figure6)3

areattributedtoinstrumentcalibrationuncertainty.Twotimesthestandarddeviationof4

thefractionaldeparturesofalltheSBUVsandOMPS(usingbothicecaps)isabout0.0035.5

WeconcludethatannualaveragesofIhavea2-sigmauncertaintyof0.35%.6

7

5.GreenlandIceSheet8

ThealbedooftheGreenlandIceSheetisofinterestbecauseitcontributestochangesinthe9

surfaceenergybalanceandsurfacemelting.ThevariabilityofourUVdI recordis10

consistentwiththeMODISalbedodataset.Arecentstudypresentstimeseriesofthe11

surfacereflectanceovertheGreenlandIceSheetfromtheCollection5(C5)andC6MODIS12

datasets(Caseyetal.2017).WhiletheolderC5setshowsstrongdarkeningoftheicesheet13

since2000(notshown),C6hasnegligibletrendsthatarenotstatisticallysignificant.They14

reportsurfacereflectanceforthechannelclosesttoourUVchannel(MODISBand3,15

459nm)fordrysnowconditions(locationswithicesurfaceelevations>2000m)andfor16

wetsnowconditions(elevations<2000m).Foreasiercomparisonwehavetranscribedthe17

datafromtheirFigure4ontoourFigure4c.ManyofthesameepisodiceventsintheMODIS18

C6recordthatlimitmeasurementstowetsnowconditions(solidbluetraceFigure4c)are19

alsoseenbytheUVinstruments(Figure4bandc):darkeningintheNHsummerof2003,20

2010and2012.The2012darkeningwaslikelydrivenbyanomaloussurfacemeltingover21

Greenland.Satelliteestimatesofmelt-dayareafrommicrowavebrightnesstemperatures22


11

(Nghiem et al., 2012)andmasslossfromtheNASAGRACEinstrumentbothsuggeststrong1

surfacemeltingin2012.2

3

Surfaceorairbornelight-absorbingaerosolsthatoriginatefromborealforestfirescan4

explainsomeoftheotherreductionsofUVdIoverGreenland.The1995darkeningepisode5

islikelycausedbyforestfiresinCanada.Usingatrajectorymodel,WotawaandTrainer6

(2000)estimatethatCOemittedfromthelargefiresinwesternCanadareachGreenlandon7

1July(theirfigure2).Usingasimilartechnique,Stohletal2006estimatethatCOfrom8

AlaskanandCanadianfiresin2004reachedSummitGreenlandonabout16July.Their9

figure11showselevatedlevelsofobservedandtrajectory-modeledCOfrom16Julyto210

August.Finally,theglobaltravelsofsmokefromthe2003firesinSoutheasternRussiaare11

documentedbyDamoahetal.(2004)usingatrajectorymodelandMODISsatelliteimages.12

Theyestimatea24MayarrivaltimeoverGreenland(theirFigure2).Atime-seriesofdaily13

valuesofUVdIoverGreenlandshowabruptreductionsbytheSBUVinstrumentsoperating14

onthosedates(Figure8).Thereareotherdramaticdarkeningevents,likelycausedby15

eitherforestfiresmokeorsurfacemelting(e.g.2006and2008),thatwecouldnotfindin16

theliterature.17

18

WhiletheshorterC6recordshowsnoapparenttrend,ourUVrecordshowsaweak,though19

statisticallyinsignificantreductioninUVdI overGreenland:-0.05(0.06)decade-1at20

locationswithelevations>2000meters(Figure4b).Impuritiesinthesnowasdetectedby21

insituanalysisareconsistentwithourobservedtrend.Polashenski(etal2015)measured22

theconcentrationsoflightabsorbingimpurities(LAI)in67snowpitsacrossNorthWest23


12

Greenlandicesheetin2013and2014andcomparedthemwithstudiesthatanalyzedsnow1

fromthepast6decades.Increasesinblackcarbonordustconcentrationsrelativetorecent2

decadesweresmallandcorrespondedtosnowalbedoreductionsofatmost0.31,or~0.053

perdecadewhichissimilartoourUVsatelliteestimate.Thesnowstudiesalsorecord4

episodiceventsthatdarkenthesnow1-2%,similartothe1995,2003and2004darkening5

weseeintheSBUVsatelliterecord.6

7

6.Discussion/Summary8

9

TheEastAntarcticPlateauisthepreferredicesheetforperformingradiancecalibration.Its10

verylowtemperaturesandclearpristineconditions,exceptfortheoccasionalvolcanic11

eruption,allmaintainastablesurfacealbedowithtime.Incontrast,theinteriorGreenland12

icesheetisdarkenedeveryfewyearsbyair-borneparticlesfromBorealwildfiresorfrom13

albedochangescausedbywidespreadsurfacemelting.Sincewearenotdoinganabsolute14

calibration,butarelativecalibration(usingNOAA-16asareferenceinstrument),15

Greenland’salbedovariations(~2%)testhowwelltheSBUVinstrumentsrespondto16

changesinthealbedo.Moreover,includingitinourcalibrationanalysisenablesa17

characterizationofinstrumenthysteresiserrorsmainlywithNimbus-7overAntarctica.18

Onceremoved,itmatterslittlewhetherbothicesheetsoronlyAntarcticaareusedto19

determinethemultiplicativegaincoefficients(c1),theUVdI trendsoverbothicesheetsare20

almostthesame.21

22


13

Intensitiesatthe340nmwavelengthchannelobservedbyeightnadir-viewingSBUV1

satelliteinstrumentsandtheOMPSscanninginstrumentareintercalibratedoverthe2

AntarcticandGreenlandicesheets.Theapproachistocompareobservedintensitiesthat3

havebeennormalizedbysolarzenithangle.Aftertheinter-calibration,weestimatea2-4

sigmauncertaintyof0.35%basedontemporallyoverlappingsensors.Multipleinstruments5

respondinunisontoknowndarkeningeventsthatsometimescanbeexplainedbyvolcanic6

aerosols,sootfromborealforestfires,orsurfacemeltwater.Thesecalibratedintensities7

willbeusedtoderiveaUVcloudalbedorecordoverthetropicsandmidlatitudessince8

1980.9

10

Appendix-Accountingforsmallwavelengthdifferences11

Eachinstrumentprovidesnarrowbandbackscatteredintensitiesclosetobutnotexactlyat12

340nmwavelength.Forexample,theNimbus-7,NOAA-9andNOAA-14havenominal13

centerwavelengthsof339.90,339.75,340.05nmandFullWidthHalfMaximum(FWHM)of14

1.0,1.132and1.132nm,respectively.Theseseeminglysmallwavelengthdifferenceswill15

changeobservedintensitiesbyseveraltenthsofapercentathighsolarzenithangles.Using16

theVLIDORTRadiativeTransferModelwecreatea2-dimensionaltableofintensitiesat0.117

nmwavelengthresolutionandat10oSZAresolution.ALambertiansurfaceof0.95albedois18

assumed.ForeachinstrumentwedetermineasimulatedintensityIsimbyconvolvingthe19

instrument’sFWHMacrossthecenterwavelengthoftheinstrument.Toaccountforthe20

wavelengthandFWHMdifferencebetweenanon-referenceinstrument(e.g.Nimbus-7)21

andourreferenceinstrumentNOAA-16wemultiplytheobservedintensitiesfromNimbus-22

7byI(q o)simNOAA-16/I(q

o)simNimbus-7.Notethatthewavelengthcorrectionis23


14

dependentonsolarzenithangle.1

2

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16

1

2Table1.Gainc1andoffsetc0valuesusedtomakeadjustmentstoobservedintensitiesfor3UVsensinginstruments. 4


17

1Figure1.FigureA-1MeasuredIntensityat340nmfromtheNOAA-16SBUVversusSolar2ZenithAngleovertheAntarcticPlateau(blue)andGreenland(green).Eachpointisa3nadir-viewedobservationatthenativeFieldofView(170kmby170km)duringthe4summer(fifteendaysoneithersideofsolstice).Alsoshownisapolynomialfitanda5radiativetransfersimulation(red)assumingaLambertiansurfacealbedoof.95,aRayleigh6atmospherewithsurfacepressureof663hPa.NotethattheGreenlandintensitiesareoffset7fromtheAntarcticones.Therightpanelshowsazoomedinview(seetextfordetails).89


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1Figure2.Inter-annualvariabilityofpreviouslycalibrateddIfortheSBUVinstruments2(colored)andOMPSmapper(grey)overAntarcticaandGreenland.Theright-handaxis3showsthecorrespondingchangeinLER.4


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1

23Figure3.dIforallFOVsobservedovertheicesheetsplottedagainstsolarzenithangle(q

o)4forspecificyears.Thelargecirclesareaveragesof dIbinnedbysolarzenithangle.Figure53ashowsthepreviouslycalibrateddIontheleftandourempiricallycalibrateddIover6Antarcticaontherightfor2006.Figure3bissamebutoverGreenlandfor2006.Figure3c7showsourempiricallycalibratedvaluesoverAntarcticain1992andGreenlandin1995.89


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1

2Figure4.Inter-annualvariabilityofourdIfortheSBUVinstruments(colored)andthe3OMPSMapper(grey)overAntarctica(a)andinteriorlocationsoverGreenlandwithice4surfaceelevationsabove2000meters(b).Theright-handaxisshowsthecorresponding5changeinLER.Annualmeansplottedwithsolidcirclesonlyincludeobservationswith6correctgratingdrivepositions;opencirclesalsoincludethosewithgratingdriveerrors7thathavebeencorrected(seetext).Thelowestpanel(Figure4c)showsMODISCollection68reflectanceforBand3(459nm)atelevationsabove2000meters(drysnowconditions9dashedtrace)andbelow2000meters(wetsnowconditionssolidtrace).1011


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12Figure5.EmpiricallycalibratedforallFOVsobservedoverAntarcticaplottedagainstsolar3zenithangle(q

o)forselectedyears.ThetopfourpanelsshowthesuppressionofdIduring4thefirst7-10minutesafterNimbus-7seesitsfirstlightatthestartofaneworbit.Atfirst5light,time=0andq

o=90o.Thetimeafterfirstlight(minutes)isshownattopoffirstfour6panels.7 8


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1Figure6.sameasFigure4exceptthatmerged-satelliteaverageisremoved.2

3


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1Figure7.TimeseriesofempiricallycalibrateddIforallFOVsobservedoverGreenlandfor2selectedyears.BluearrowsindicateestimateddateswhenCOfromborealforestfires3reachGreenland(seetext).Colorschemeissameasotherfigures.4


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