A correction for total ozone mapping spectrometer profile shape errors ...

JOURNAL

OF GEOPHYSICAL

RESEARCH,

VOL. 102, NO. D7, PAGES 9029-9038, APRIL 20, 1997

A correction for total ozone mapping spectrometer profile shape errors at high latitude C. G. Wellemeyer, S. L. Taylor, and C. J. Seftor HughesSTX Corporation,Greenbelt,Maryland

R. D. McPeters

and P. K. Bhartia

NASA GoddardSpaceFlight Center, Greenbelt,Maryland

Abstract. The total ozonemappingspectrometer(TOMS) ozonemeasurementis derived by comparingmeasuredbackscatter ultravioletradianceswith theoreticalradiancescomputed usingstandardclimatologicalozone profiles.Profile shapeerrors occur in this algorithm at high opticalpath lengthswheneverthe actualverticalozonedistributiondifferssignificantly from the standardprofile used.Theseerrorsare estimatedusingradiativetransfercalculations and measurementsof the actual ozone profile. These estimatederrors include a shortterm componentresultingfrom day-to-dayvariabilityin profile shapethat givesrise to a standarddeviationof 10% in total column ozone amount, as well as a systematicerror in the long-termtrend at very high solar zenith angles.The trend error resultingfrom the long-termchangesin the ozoneprofile shapeis estimatedusingmeasurementsfrom the solar backscatteredultravioletinstrument.At the maximumretrieval solar zenith angle of 88ø, thesecalculations indicatethat TOMS long-termozonedepletionsmaybe overestimated by 5% per decade.For trend studiesthat are restrictedto latitudeslower than 60ø (a maximumof 83ø solarzenith angle),this error is reducedto no more than 1-2% per decade.Differential impact of the profile shapeerror at the variousTOMS wavelength pai,• 111UlCcttc• LIIdLl../ltOlll[3 high solarzenith angles.An interpolationmethod internal to TOMS is proposedto extract this information.It improvesthe retrieval at high solar zenith angle, reducingthe shortterm variabilityto a standarddeviationof 5%, and essentiallyeliminatesthe long-term error. The set of standardprofilesused in the algorithmare adjustedbasedon an analysis of empiricalorthogonalfunctionsderivedfrom a compositeclimatologyof Stratospheric Aerosol and Gas ExperimentII and balloonsondeprofile measurements. 1111tJ1111atltJll

111•.,

g.l,o

Ul

amounts are used in a radiative transfer calculation [Dave, 1964] in order to computeradianceswith which the measured The total ozonemappingspectrometer(TOMS) instrument radiancesare comparedin the ozonecomputation[McPeterset measured total column ozone continuouslyfrom its launch al., 1993]. A set of five fixed temperatureprofiles are used, onboardthe Nimbus 7 spacecraftin late October 1978 until it including one each for low, middle, and high latitudes and a ceasedto functionon May 6, 1993.These data were processed separatetemperatureprofile for each of the 125 and 175 Dobas version 6 usingan improved long-term instrumentcalibra- son unit (DU) ozone profileswhich characterizeozone hole tion in 1991 [Hermanet al., 1991a].A number of long-term conditions.The temperature profiles are used in the radiative ozone trend estimateshave been derived using this valuable transfer calculationsto take the temperature dependenceof data set [Stolarskiet al., 1991; Herman et al., 1991b]. The the ozone absorption cross sections into account. Pairs of latitudinal extent of these trend analyseswas limited by the wavclcngtnbare used in the ozone lculeYd• a•;ux•unn to iTiillonset of known algorithmicerrors in the TOMS retrievals at imize errors resulting from aerosolsand surface reflectivity highsolarzenith angles[Klenket al., 1982].Theseerror sources effects not taken into account in the radiative transfer calcuinclude sensitivityto the actual vertical distributionof ozone lation. Differencesbetween the assumedclimatologicalozone (profile shape),the temperaturedependenceof the ozone abprofile and the actual ozone profile lead to errors in total sorptioncoefficients, uncertaintyin the solarzenith angleitself, ozone determined using this schemeat high solar zenith anand small spherical effects on multiple scattered radiation gles.Longer wavelengthpairs are used at higher solar zenith (sphericalgeometryis appliedonly to the primaryradiation). Of thesepossibleerror sources,the profile shapeerror is the anglessince they are much less sensitiveto profile shape efprimary sourceof uncertaintyin the TOMS retrievals at high fects.The differentialimpact of the profile shapeerror at the variouswavelengthpairsindicates,however,that profile shape solar zenith angles. In the version 6 TOMS retrieval, climatologicalozone and information is present in the TOMS measurementsat high temperatureprofilesfor variouslatitude zonesand total ozone solar zenith angles.An interpolation procedure internal to TOMS is presentedbelow to extract this information by comCopyright1997 by the American GeophysicalUnion. paring interpair differencesresultingfrom alternate choicesof profile shape. Paper number96JD03965. 0148-0227/97/96JD-03965509.00 Also onboardthe Nimbus 7 spacecraftis the nadir-viewing Introduction

9029

9030

WELLEMEYER

ET AL.: A CORRECTION

Table 1. AtmosphericLayersUsed to Define TOMS Standard

Umkehr Layer

Profiles

Layer Pressure, hPa

Pressureat Altitude of Midpoint

10-12

0.000-0.990

......

9 8 7 6 5 4 3 2 1 0

0.990-1.980 1.980-3.960 3.960-7.920 7.920-15.80 15.80-31.70 31.70-63.30 63.30-127.0 127.0-253.0 253.0-506.0 506.0-1013

1.40 2.80 5.60 11.2 22.4 44.8 89.6 179.0 358.0 716.0

Altitude of Layer Midpoint, km 45.5 40.2 35.2 30.4 25.8 21.3 17.0 12.5 7.9 2.8

FOR

TOMS

SHAPE

ERRORS

controllingthe amount of radiation reachinghigher pressures where the bulk of the backscattersignalis generated[Daveand Mateer,1967].Becauseof this,the assumedheightof the ozone maximumbecomesimportant in TOMS measurementsmade at high path lengths. The error in a particulartotal ozoneretrievalfor a particular layer (1) can be calculatedusing some measurementof the localprofileto providelayerozonedifferences(AXl) whichare multipliedby the associated layersensitivity((O•/OX)l). The layer contributionsto the total ozone error are then summed over all layersto provide the final error estimate:

/xn =

(on/ox)/Xx, l

This approachis similarto one usedin a previousstudy[Klenk et al., 1982] and agreeswith error estimatesbasedon the full radiative

solar backscatteredultraviolet (SBUV) instrument,which makes measurementsat a seriesof shorter wavelengthsthat can be used to infer the ozone profile [Fleiget al., 1990]. The TOMS and SBUV instrumentsare coalignedso that at nadir, the SBUV providesa measurementof the actualvertical dis-

PROFILE

calculation

to within

better than 1% in total ozone.

This method is usedin conjunctionwith balloonsondemeasurements to estimate the variance

in individual

retrievals

due

to profile shapeeffects.CoincidentSBUV and balloonsonde measurements havebeen usedso that the upper level retrieval from SBUV can be used in conjunctionwith the balloon to tribution of ozone that is coincident to the TOMS nadir reprovide a complete profile for use in the calculation.The trieval. These profiles are used in radiative transfer calcula- resultsof this calculationfor measurementsat Hohenpeissentions to calculate corrected TOMS total ozone amounts for berg (48ø), Goose Bay (53ø), Churchill (59ø), and Resolute comparisonwith the resultsof the interpolationprocedure. (75ø) are consistentwith similar calculationsby Klenk et al. [1982] and indicatea standarddeviationof about 10% in the retrieved total ozone at solar zenith anglesof 86ø and higher. Profile Shape Error Estimates These errors are lessthan 1% at solar zenith angleslessthan The profile shape errors are estimatedusing a first-order 70ø.Becausethe SBUV is required in this type of calculation, Taylor seriesbased on differencesin layer ozone. The sensi- the resultsare restrictedto nadir-viewingconditions. tivity of retrieved total ozone to the differencein actual and This method has also been used to estimate errors in the assumedozone amount in a singlelayer is estimatedby recal- long-term ozone trendsderived from TOMS at high latitudes. culatingthe radiative transfer usinglayer ozone amountsper- Since the climatologicalprofiles used in computingTOMS turbed by 10% from the climatology.This calculationis done theoreticalradiancesare fixed, any long-termchangesin proseparatelyin each of the Umkehr layers defined in Table 1. file shape have the potential to introduce error in the longFigure 1 showsthe resultinglayer sensitivitiesas total ozone term trend derivedfrom TOMS at high solar zenith angles.In retrievalerror per layerozonedifference(percentper percent) this calculationthe SBUV-derived profile is usedby itself. As as a functionof centrallayerpressure(and estimatedaltitude) is shownin Figure 1, the sensitivitycalculationsindicatethat for a nominal high-latitude profile at a range of solar zenith the retrievedtotal ozoneis not sensitiveto profile shapeerrors angles.This figure indicatesa small oversensitivityof the retrieval algorithm to changesin the upper level profile shape and a somewhat smaller undersensitivityto changesin the 1 lower level profile. The algorithm performsvery well near the ozone peak where climatologicalvariationsare the strongest, 40 in this case,at layer 3. The undersensitivityof a wavelengthpair at the lower layers comesabout becausethe ozone-sensitive (higher ozone cross •o section)wavelengthdoesnot penetratethe entire ozoneprofile when the optical depth becomeslarge. The lowestpart of the ozone profile is implicitly derivedfrom the assumedclima2o• tology, and if this climatologyis not correct, ozone error relOO sults.This problemis partially mitigatedby usinglongerwavelengths at higher path lengths, but the results in Figure 1 lO include this correction. In layers 0 and 1, there is very little differential sensitivityin the pairs, becausenone of the TOMS lOOO o ozone wavelengthspenetrate these layersadequatelyto mea-0.1 0.0 0.1 0.2 sure troposphericozone [Klenket al., 1982].The tropospheric Totol Ozone Error(%)/LoyerOzone Difference(Z) ozone representsonly a smallpercentageof the total column, however,so that the error sensitivityin this region is small. Figure 1. Sensitivityof total ozone mapping spectrometer The oversensitivityto differencesin the upper level profile (TOMS) total ozonereportedat nadir to differencesin local has strong differential sensitivityin the pairs. At high solar ozone profile shapefrom the assumedclimatology.Sensitivizenith angles,the upper level ozone acts like an amplifier in ties increasein magnitudewith solarzenith angle.

WELLEMEYER

ET AL.' A CORRECTION

closeto the ozonemaximum.The profile shapeerrorsoccuras overestimatesof total ozone changesassociatedwith changes in the upper level profile, and underestimatesof total ozone changesassociatedwith lower level profile (tropospheric) changes.The use of SBUV data in this calculationtends to neglectthe effect of possibleunmeasuredincreasesin troposphericozone that would tend to offset the overestimateof total ozone decreaseassociatedwith long-term decreasesin upper level ozonemeasuredby SBUV. Becauseof thesecon-

FOR

TOMS

PROFILE

54 1

•

bands considered

solstice. Note that this error is maximum

in the winter

months

and becomesnegligiblein summer due to the smaller solar zenith angles.If trend studiesbased on the version 6 TOMS data are restricted to latitudes of 60ø or less, the maximum

error in derivedtrend is reducedto 1-2% per decade.Because this error is stronglydependenton optical path, we expectthe northern hemisphereto give larger error estimatesthan the southernhemisphere,where total ozone amountsand therefore opticalpath lengthsare significantlysmaller.Also, heterogeneous chemical depletions observed in the Antarctic "ozonehole," and more recentlyin the northern hemisphereas well, occur at 15-20 km, where TOMS sensitivityto profile

i

i

i

i

i

62 i

63 i

64 i

• • Dec. H don. •

Feb.

[3•D

Mar.

-6

77

78

79

80

81

82

85

84

85

86

87

Solar Zenith Angle (Degrees)

and the associated

monthlymean solar zenith anglesare indicatedin the figure. Also givenis the slopeof a leastsquareslinearfit appliedto the estimatederrors. Figure 3 showsthe slopesof similar regressionsappliedto three northern latitude bandsfor each of the monthsNovemberthrough March plotted versussolar zenith angle. (Theoretically,and in these results,the profile shape error increaseslinearly with the optical path length of the measurement,but this presentationof the resultsmay be interpreted more easily.) Shown on the upper abscissais the latitude at whichthe correspondingsolarzenith angleoccursat winter solstice.These results indicate that long-term TOMS trendsderivedat 63ønorth latitude, for example,will contain a seasonallydependentsystematicerror leading to an overestimate of long-termdecreasesof 2-4% per decadeat the winter

i

-2

-5

overestimateany negativeerror in ozone trend and therefore providesan upper limit for the derivederror estimate.Figure 2 showsmonthly zonal mean results of this calculation for The latitude

9031

December Latitude 57 58 59 60 61

56

!

ERRORS

o

siderations, the use of SBUV data in this calculation tends to

November.

55

i

SHAPE

Figure 3. Linear regressionslopesof estimatedprofile shape error in TOMS ozone for five separate months and three separate northern latitude bands plotted versus solar zenith angle.Correspondingwinter solsticelatitudesare givenon the upper scale.

shape errors is small (Figure 1). It is the classicalgas phase depletion taking place higher in the atmospherethat has the strongereffecton the retrieval. The uncertaintyin thesenorthern hemisphere trend error estimates is largely due to the uncertaintyin the long-term trend of the layer ozone amounts measuredby the SBUV Instrument.The long-term calibration of SBUV has been determined using an adaptation of the Langleyplot method basedon the equivalenceof zonal mean layer ozone amountsmeasuredat distinctlydifferent solar zenith angles.This method provides an accuracyof 2-3% per decade[Bhartiaet al., 1995].

TOMS Internal Profile Shape Correction The differential sensitivityof TOMS wavelength pairs to upper level profile shapeerrors impliesthat profile shapeinformation

is contained

in the TOMS

measurements.

The

sit-

uation is illustrated in Figure 4, which showsthe results of radiative transfer calculationsindicating the dependenceof TOMS measuredpair N valueson total ozone.The N value is a logarithmicform of the normalized radiance measuredby TOMS:

10

8 6

Symbol Lat.

$.Z A.

o o 57.5 A-- • 62.5 [3--El 67.5

78 ø 85 ø 86 ø

Trend Err.

0.5 g/decade 1.8 %/decade 4.1%/decode

N: where

I is the measured

- ] 00 Iago0 Earth

backscatter

(2) radiance

and F is

the measuredsolar irradiance.The pair N value is simplythe _ F4 __ difference between N values measured at a pair of wavelengths.The N value is usedbecauseits dependenceon ozone amount is roughly linear. Figure 4 showspair "N-omega" curves for middle- and high-latitude ozone profiles for the A-pair (312 and 331 nm), B-pair (317 and 331 nm), and C-pair -2 (331 and 340 nm) wavelengths for a sampleretrievalat a solar -4 zenith angle of 82ø. The differentialsensitivityof the pairs to differencesin profile shape representedby the middle- and 78 80 82 84 86 88 90 high-latitudeclimatologicalprofilesis clear in the figure. The Year measuredA-pair N value, for example, could be associated Figure 2. Estimatedprofile shapeerror in TOMS ozone due with total ozone amountsof about 240 or 265 DU, depending to long-termchangesin profile shapemeasuredby solarbackscatteredultraviolet(SBUV) instrument.Resultsshownfor 5ø, on whether the middle- or high-latitudeclimatologyis used. high-latitudezones in the northern hemispherefrom 1979 to Note that the possiblerange of derivedozonevalue is reduced 1990 for the month of November with the approximate solar for the B-pair wavelengths. In the version 6 TOMS algorithm, the latitude of the meazenith angle indicated.Trend errors are estimatedby linear surement is used to interpolate between the results of the regressionof the time series.

9032

WELLEMEYER

ET AL.: A CORRECTION

FOR

TOMS

PROFILE

SHAPE

ERRORS

Interpolation Scheme for Determining Profile Mixing 120

I I _ Solar Zenith Angie = 82 ø

_ Reflectivity =51% _ Surface Pressure = 0.7

I

I

•

•

I

I

.•

at

•

_

•

lOO -

_

803•__ .............//• .,,,,,•... M ...... dA-pair N-value• _ _

•

_

z

•

60

-

o

.•-.../ •..... ....M ..... ed B-pair Nival. ue 40

_

High-Latitude(75•)

20 -

•

_

•

j

:.

-

•

•

C Pair

•............................::

- i ••asuredC•(r•-value

: 100

1 50

200

250

300

350

400

450

500

550

600

Ozone (Dobson Units)

Figure 4. TOMS internal correctionprocedurefor profile shapeerrors.A m•ing factor is determinedfor middle-and high-latitudeprofilessuchthat the measuredradiances(expressed asN values)at the A-pair and B-pair wavelengthsare consistentwith the solutionozone amount.The solidlinesrepresentthe theoretical pair radiancescalculated using middle- and high-latitude climatologiesfor the various pairs of TOMS wavelengths. The dottedlinesindicatehowthe measuredradiances(asterisks on the ordinate)are interpreted to providea measurementof total ozone(asteriskon the abscissa). A uniquesolutionexistswhere the same m•ture of the middle- and high-latitudeclimatologiesfor the A-pair wavelengthsand the B-pair wavelengths are consistent with a singletotal ozonevalue (intersectionof dottedlines).

middle-latitude(45ø) and high-latitude(75ø) climatologies. The resultingmixing fraction definesa linear combinationof the two climatologiesrepresenting an estimate of the local profile.As one mightexpect,the heightof the ozonemaximum is the chief differencein the TOMS climatologyat low, middle, and high latitudes,with the high-latitudeprofilesexhibitingthe lowestozonemaximum[Bhartiaet al., 1985].In the real atmosphere,however, a great deal of mixing occursbetween the middle- and high-latitudeair masses,and at high path lengths where profile shape dependenceexists,a climatologywith a fixed latitudinal dependenceis inappropriate.The improved interpolationproceduredeterminesthe singlemixingfraction that can explain the measuredradiancesat the A-pair and B-pair wavelengthswith a singletotal ozonevalue.This mixing factor defines the linear combination of middle- and highlatitude profile shapesthat is consistentwith the measured

affectedby profile shapeerrors at the path length illustrated, but at higherpath lengthsthe B-pair and C-pair wavelengths exhibit behavior analogousto that of the A-pair and B-pair wavelengthsin Figure 4, and a similarprocedurefor correcting the C-pair ozone can be definedusingthe B-pair N value. Figure 5 showsthe improvementassociated with thisA-pair and B-pair mixingprocedure,where the SBUV-basedprofile shapecorrectionresultshave been used as "truth." The data are from a one-orbit-per-weeksampledata set for the period 1983-1985. The comparisonis restrictedto nadir by the requirement that the SBUV profile information be available. The upper plot showsthe uncorrectedTOMS profile shape error relative to ozone data correctedusingthe SBUV-based method.Thesedifferencesare plotted as a functionof optical path lengthcomputedas (1 + sec(90) x fl. The lower plot showsthe result of the applicationof the TOMS internal mixradiances. ing procedure.These resultsindicate significantimprovement More explicitly,we use the version6 latitude-basedmixing in TOMS retrieval in the opticalpath length regionfrom 2 to to determinean initial total ozoneestimateand then compute 4. The TOMS internal method agreeswith the SBUV-based A-pair N valuesfor the middle and high latitudesseparately. method to better than 1% total ozone in the mean and to Then the mixing factor is calculatedas within about 5% in standard deviation. Nmid - Nmeas

f = Nmid_ Nhig h

(3)

The B-pair ozone amountsare usedto calculatethe corrected total

ozone

amount

as

fl : •')-'mid Jr-f(•"•high•"•mid) This calculation

is iterated

if the corrected

(4)

total ozone amount

Becauseinformationis limited in this procedure,we assume that the true profile is somelinear combinationof the middleand high-latitude climatologicalprofiles. The two measurements,B-pair N value and A-pair N value, are usedto derive two piecesof information:total ozoneand mixingfraction.The residualnoisewith respectto the SBUV-basedmethodisprobably the result of a combinationof the limitations of this two-dimensionalinterpolation procedure and signal-to-noise considerations.It is clear in Figure 4 that at lower ozone

is significantlydifferentfrom the initial estimate. As can be seen in Figure 4, the C-pair is not significantly amounts(or solarzenith angles)we losesensitivity to profile

WELLEMEYER

ET AL.: A CORRECTION

shape.At 175 DU, for example(path length = 1.43), there is no differencebetweenthe middle- and high-latitudecases,but at 200 DU (path length= 1.64)we beginto seea smallsignal. We have limited the data in Figure 5 to path lengthsgreater than 2.0, becausefor lower path lengthsthe procedureappears to be limitedby signalto noise.At path lengthsgreaterthan 3.0 or 4.0 (about 450 DU in Figure 4), the A-pair "N-omega" curveisbecomingindiscriminate.As suggested above,a similar procedurebasedon the C-pair and B-pair wavelengthscan be usedin this higher path length range. Figure6 showsthe time seriesof the mixingfactor derivedin this developmentaltesting of the internal mixing procedure. Mixing factors of zero indicate selection of pure middlelatitude shape climatology,and values of one indicate pure high-latitudeclimatology.In the many caseswhere the mixing factor is greater than one, the indication is that the profile shapeis of someextremeclimatologyof higherlatitude than is characterizedby the existingstandardprofiles.This is not surprisingbecausethe existingstandardprofileswere not developed for use in an interpolationprocedureand are representative of the mean high-latitudeprofile [Bhartiaet al., 1985].In

lOO

'

'

'

i

FOR

TOMS

PROFILE

SHAPE

ERRORS

9033

High/Mid-Profile Selection Scheme 4

-2

m I mmmmmI I I I I I 83.0

83.5

i mmmmmI 84.0

I

I

I

I

84.5

Year

FiBure 6. M•inB fractiondeterminedusinBthe TOMS internal correctionprocedureat hiBh solar zenith anBlcsand latitudes (path lenBths•-4) plotted as a functionof time for a sampleprocessinB overa •-ycar period.A m•inB factorof zero indicatespure middle-latitudeclimatolo•, and a m•inB factor of one indicatespure hiBh-latitudcclimatolo•. M•inB factors biBherthan one representextrapolationsbeyondthe cxistinB hi,h-latitude ozone profiles.

A

50

order to adjust the standard profiles to avoid this type of extrapolationin applicationof the method, a reanalysisof the standardprofileswasundertakenbasedon the SAGE II ozone profiles in an effort to define a more extreme high-latitude climatology.This analysisis describedin the next section.

-50

,

80 ø

85ø

87ø

88ø

(SZA at350DU)

Ozone X (1 + SEC ({9))

Empirical Orthogonal Function Analysis of Ozone Profile Climatology

In order to optimizethe profile shapeinterpolationprocedure describedin the previoussection,the empiricalorthogo100 nal functions(EOF) of an externalozoneprofile climatology have been calculatedand analyzed.Ozone profilesfrom the SAGE II over the period from launchin October 1984 through June 1991 when the eruption of Mount Pinatubo began to • 50 impact the SAGE II ozone retrieval are used in this study Q x x x [McCormicket al., 1989]. The standardprofilesof TOMS are definedin Umkehr layers(Table 1), so the SAGE II profiles vx _ x ,• are convertedto pressurecoordinatesusingthe National Me'• •x • X teorologicalCenter(NMC) temperatureprofilesprovidedwith .'• • X X XX•• X XX • x x x the archiveddata and integratedinto Umkehr layers.To pro' --•• xx x. x x• vide a consistentdata set, only retrievalswith good data down • xxv x• XxXw• • X throughlayer2 (approximately180mbar,or 12.5km) are used x in the study.The depth of the SAGE II retrieval is limited by -50 80ø 85ø '87 ø 88• (S• at350DU) z 0 • 10 15 clouds.From the completeset of 27,110 profiles,23,433 are selectedusingthis criterion. ozone x (1 + SEC (e)) To provide statisticallyconsistentlower layersfor SAGE II profiles, a set of 4912 balloonsondeprofiles in the period wkh a co•½•fion procedurebasedon •oinddcnt SBUV meaNovember 1978 through 1987 for 20 groundsitesdistributed suremento• the lo•al pm•l½. (a) Unsolicited TO•S mcasu•½about the globe are used (Table 2). The ozone amount in meritsminus SBUV •o•m•tcd mcasummcms.(b) •o•m•tcd layers0 and I (0-10 km) are regressedagainstthe layer 2 ß O•S measurements minus SBUV ½o•½½tcd measurements. ß h½s½di•½•cn½½s a•½ plotted as a •unction o• optical path (10-15 km) ozone amount to correlatethe balloonsondeclimatologywith the SAGE II profiles.This is done separatelyin an•l½s•o• a nominal ozone amount o• 350 •U a•c also shown. each of three broad latitude zones from 00-30ø, 30ø-60 ø, and ,

ß

A

.

,

i

,

x

.

.

x

x

x

x

i

....

i

9034

WELLEMEYER

ET AL.: A CORRECTION

FOR

TOMS

Station

Number

7

64

10

9

Latitude

Longitude

31.63

130.60

28.63

59

14 21 24

ERRORS

i

i

150

of

Sondes

12

SHAPE

i

Table 2. Inventory of BalloonsondeMeasurements Number

PROFILE

77.22

43.05

142 346 205

140.13 - 114.10 -94.98

._o

Kagoshima

o

Sapporo

50

o E

Tateno Edmonston Resolute

145.10 9.05

"..'•:

..... .,."::'"::::

-5O

58 41

-38.03 39.08

40 65

1 2

43.55 43.78

5.45 -79.47

Haute Provence Toronto

Aspendale Cagliari-Elmas

Goose Bay

76

334

53.32

-60.38

77

291

58.75

-94.07

Churchill

99 101 107 174

1366 88 224 669

47.80 -69.00 37.87 52.22

11.02 39.58 -75.52 14.12

Hohenpeissenberg Syowa Wallops Island Lindenberg

187 197 205 215 219

12 196 14 310 19

18.53 44.37 8.29 47.48 -5.42

73.85 - 1.23 76.57 11.07 -35.38

Poona Biscarrosse Trivandrum Garmisch Natal

221 242

164 298

20.58 15.50

.•';,•:•

0

._

26 38

52.24 50.11

._• 100

New Delhi

141.33

36.05 53.55 74.72

Station Name

-9O

i

i

-60

-30

0

30

60

'

'

60

• ß_•

4o

ß•

20

o

0

'

'

'

i:!,•:i:3.5.'!ii; '•.'•:::•.:. ..........

;:'

'2):';?'.•'i•[•' ' '•'"%:•::'" '.•.............."•'•::'(":"•;'::";!'•::":" ";:

ø - 20 ':::" '":' ........ ...... ,. ,,:•?.........

,-4o

Legionowo Prague

90

Lotitude

m-60 -80

-90

' ':t...,.• ,•

, lB

....

-60

-50

0

50

60

90

Latitude

600-90ø latitude. The results of these regressionsare used to compute representativelayer amounts to be added to the SAGE II profiles. A random-numbergenerator is used to providevarianceabout theselayer amountsequivalentto the varianceunexplainedby the regressions.For layer 0 a simple mean is used,and for layer 1 a linear fit is applied. The resultingclimatologyis fairly representative,exceptthat the SAGE II only reacheslatitudeshigher than 70ø twice per year, and the balloonsondeclimatologyis overrepresented in the northern hemisphere.As will be shownbelow, the profile shapeis highlyvariable at middle and high latitudes,so the SAGE II data are probablyrepresentativeof almostall possible ozoneprofile shapes.The balloonsondedata showa hemisphericasymmetryin troposphericozone, and becauseit is overrepresentedin the northern hemisphere,it is givingan integratedtotal ozoneamountin the resultingclimatologythat is slightlyhigh. The troposphericozone amountsthemselves

Figure 8. Coefficientsfor (a) the primary and (b) the secondary coefficientsrepresenting the composite SAGE II/ balloonsondeclimatologyplotted as a functionof latitude.

are not actuallyusedin the studyexceptto providea realistic integrated total ozone, and for this role they shouldbe adequate.

The empiricalorthogonalfunctions(EOF) are the principal components[Kotzet al., 1981]or eigenfunctions of the covariance matrix of the layer ozone amounts of the composite SAGE II/balloonsonde ozone profiles. They are the ordered set of orthogonalfunctionsthat most efficientlyrepresentthe variancein layer ozoneamountsin the compositeclimatology. Any of the individualprofilescan be representedas a linear combination

of the EOF:

Xi-- • Cija j

(5)

J

_

whereX• is theith ozoneprofile,Co is thejth coefficient for the i th profile,anda• is thej th eigenfunction.

FirstEigenfunction Second Eigenfunction ---

Third Eigenfunction

Figure 7 showsthe first three EOFs. The primary EOF representsa nominalozoneprofile,and the primarycoefficient

ß

\..

scales the total ozone. The correlation

10

/!

.,..

_

lOO

1000

.......

0

i

0

.........

10

i

20

i

i

i

i

,

i

i

i

,

30

Normalized

Figure 7. The first three empirical orthogonal functions (EOF) for the compositeSAGE II/balloonsondeclimatology. The first two eigenfunctions explain95% of the variancein the climatology.

between total ozone and

the primarycoefficientis 0.87. The secondaryEOF in Figure 1 changessignat the peak of the primary EOF. Variation in the secondarycoefficientmodifiesthe height of the ozone maximum in the resultantprofile. Approximately95% of the variance in total column ozone amount is explainedusing the primary and secondaryEOF. Figure 8 showsthe primary and secondarycoefficientsplotted versuslatitude. A rough correlation between latitude and these coefficientscan be seen, but

more strikingis the large variability in the profile as characterized by the EOF in the middle and high latitudes. For retrievalsat high solar zenith angleswhere profile shapedependenceexists,a profile shapeselectioncriterionthat is more responsivethan latitude is clearlyrequired.The profile inter-

WELLEMEYER

ET AL.: A CORRECTION

polationproceduredescribedin the previoussectionprovides this capability,but to reducethe need for extrapolationin this procedure,standardprofileswhichbetter spanthe set of possibleprofilesare needed. Figure 9 showsthe compositeclimatologyas well as the standardprofilesfrom versions6 and 7 in the two-dimensional eigenspacedefinedby the EOF analysis.The individualpoints are the compositeSAGE II/balloonsondeclimatology.Connectedby dashedlines are the version6 standardprofilesfor low, middle, and high latitude, while the solid lines are the version7 standardprofiles.(The version6 low-latitudeclimatologyis obscuredby the solidline representingversion7.) The version6 low-latitudeclimatologyonly containsthree profiles for total ozone ranging from 225 DU to 325 DU in 50-DU steps[McPeters et al., 1993].(Total ozoneis correlatedwith the primarycoetticient.) The existingversion6 standardprofileshave been adjusted to produce version 7 profiles that better span the external climatologyin eigenspace.Explicit useof the eigenfunctionsis not possible,however,becausecertainrestrictionsare placed on the behavior of the standardprofiles. For example,one could use a fractional multiple of the secondaryeigenfunction to adjust a given profile along the ordinate in Figure 8 easily and accurately.However, such an adjustmentwould change the upper level profile and the tropospheric amounts and would result in differencesin those regionsfor different total ozone amountswithin a singlelatitude region. To build sucha dependenceinto the standardprofileswould producedependenciesin the TOMS retrievalsthat are not supportedby the observation.To avoid this, the peak height is moved up or down as indicated by the secondaryeigenfunctionwithout changingthe upper level shape or troposphericamount. By makingsuchadjustmentsit is possibleto create a set of version 7 standard profiles that span the eigenspacedefined by the combinedclimatologyand exhibit consistentbehavior in the upper and lower regions.The version7 high-latitudeprofiles havemoveddownwardin Figure 9, indicatinglower heightsfor ozonemaxima.When the mixingproceduredescribedearlier is appliedusingthisclimatology,the occurrenceof mixingfactors greater than one is reducedsignificantly. Additional standardprofileshavebeen createdin the equatorial region at 375, 425, and 475 DU. Although total ozone

FOR

TOMS

PROFILE

SHAPE

ERRORS

amountsin this range do not normallyoccurin the equatorial region,this is done to provideendpointsfor profile mixing at higher ozone amounts encounteredin middle latitudes. Profiles from the compositeclimatologyare binned in small cells in the eigenspaceof Figure 9 and averaged.Theseprofilesare then modified to conform to the algorithmicrequirements mentioned above and then adjusted slightly to obtain their final locationsin eigenspace.The resultingprofiles,aswell as

High Latitude Ozone Profiles ß

,

ß

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10

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A 1000

-

.

.

ß

.

i

....

i

50

o

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Dobson

Mid Latitude

Ozone

Profiles

Total Ozone Amounts

1

125D.u. 175 D.U.

•

X

225 D.U.

•,. •

275 D.U. 325 D.U.

'•N'• •'•

575 D.U. 425 D.U. 475 D.U. 525 D.U.

•'• •. \ \ •'•,,•

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Units

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....

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Total Ozone Amounts 225 275 525 375 425 475

D.U. D.U. D.U. D.U. D.U. D.U.

10

g -20 g

150

Units

/

.:½ii:•.•?t ..... :5'-'"'; ';"'5..• ......,

O.U. D.U. D.U. D.U. D.U. D.U. D.U. D.U. D.U. D.U.

100

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125 175 225 275 325 37õ 425 475 525 575

Dobson

60 f' ' ]

....

Totol Ozone Amounts

o

40

9035

Stondord Profiles

f.D--40 --60

--80

.:?...,;5i•.•:i?'•:? }:': ' :'":;"?' ?'"'

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Figure 9. Version ? and version6 TOMS standardprofiles, 0 .50 100 150 as well as the compositeSAGE II/balloonsondeclimatology Dobson Units (points),plottedin the eigenspace definedby the primaryand secondaryEOF. Version ? profileshave been adjustedto bet- Figure 10. Version7 standardozoneprofilesfor (a) high,(b) ter spanthe climatology. middle,and (c) low latitudes.

9036

WELLEMEYER

ET AL.:

A CORRECTION

FOR

TOMS

PROFILE

0.1

SHAPE

ERRORS

High Latitude Temperature Profiles Total Ozone Amounts

1 -

10.0

_

125 D.U. 175 D.U. 22,5 D.U. 27,5 - ,57,5 D.U.

1.0

10 .

100.0 125 D.U.

D.U.

lOO 1000.0

, -1.0

I

,

,

,

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•

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0.0

Correlation

,

1

0.5

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Figure 11. Vertical profile of correlationbetweenlayer temperature and total ozone amount. SAGE II coincidentNational MeteorologicalCenter temperaturesare correlatedwith the integratedtotal ozonefrom individualprofilesin the composite SAGE II/balloonsondeclimatology.

1 ooo

,

I

,

i

160

,

I

,

,

180

,

I

ß

ß

200

ß

i

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.

.

i

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260

'

i

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.

280

.

.300

Mid Lotitude Temperoture Profiles ß

i

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i

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Total Ozone Amounts I 125 175 225 275 325 375 425 475 525

the rest of the version7 ozoneprofiles,are shownin Figure 10 and Table

3.

-

D.U. D.U. D.U. D.U. D.U. D.U. D.U. D.U. D.U.

10 We note that the interpolationproceduredescribedin the 575 D.U. previoussectiondoes not take place in the eigenspaceillustrated in Figure 9. It is a two-dimensionalinterpolation done by interpolationalong the separateclimatologies(connected 125 D.U. lOO by solid lines in Figure 9) whichvary in total ozone and between two of the climatologieswhich vary in height of the ozone maximum. These coordinatesare not necessarilyorB thogonal,thoughthey are seento be roughlysoby examination 1 ooo of the primaryand secondaryEOF in Figure7. As discussed in 160 180 200 220 240 260 280 .300 the previoussection,the interpolationis limited to two dimenKelvin sionsby availabilityof information. The troposphericozone amounts from the balloonsonde Low Latitude Temperature Profiles climatologyexhibit a hemisphericasymmetry.The possibility of using separatesets of standardprofilesin the two hemi225 D.U. 275 D.U. spheresis rejected,however,becausethe impact of the tropo325 D.U. 375 D.U. sphericozone on the total ozone retrieval is minimal. The 425 D.U. 475 D.U. troposphericozone amount representsonly 3% of the total 10 columnamountand is partly measuredby TOMS [Klenket al., 1982]. Building in a small hemisphericasymmetrybasedon externaldata sourcesis probablyan unwarrantedcomplication to the TOMS retrieval and the interpretation of the TOMS 225 D.U. 475 D.U. 100 data.The useof symmetrictropospheresin the retrievalresults in small errors that are more easilyinterpretedby the user.A smallconcessionto the asymmetryis made, however,by using c lower tropospheric amounts in the case of the 125- and 1000 175-DU profilesbecausethey are only found in the southern 160 180 200 220 240 260 280 .300 hemisphere.A somewhatsmallertroposphericamountis also Kelvin used in the 225-DU profile. If total ozone amountsof this magnitude are' encounteredin the northern hemisphere,a Figure 12. Version 7 standardtemperatureprofilesfor (a) small error will result [Klenket al., 1982]. high,(b) middle,and (c) low latitudes. ,

I

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ß

New Climatological Temperature Profiles The climatologicaltemperatureprofileshave also been recomputedfor usein the version7 algorithm.CoincidentNMC temperatureprofiles are reported with the SAGE II ozone profiles[McCormicket al., 1989].The temperatureof the layer centerdeterminedin unitsof the logarithmof pressureis used to represent each Umkehr layer. These temperatureshave been averagedby total ozone amount in the three separate

i

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latitude regionsin order to give a separateclimatologicaltemperatureprofilefor eachozoneprofile.Theseresultsare quite similar to the version6 climatologyexceptfor the total ozone dependencein the vicinity of the ozone maximum.Figure 11 showsa profile of layer temperature correlation with total ozone indicating a maximum of about 0.5 near the ozone maximum.Becauseof this, the total ozonedependenttemperature profiles have been adopted in version 7 to take into

WELLEMEYER

Table 3.

TOMS

ET AL.: A CORRECTION

FOR TOMS PROFILE

SHAPE ERRORS

9037

Version 7 Standard Ozone Profiles

Umkehr Layer Profile

0

1

15.0 15.0 15.0 15.0 15.0 15.0

9.0 9.0 9.0 9.0 9.0 9.0

6.0 8.0 10.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0

9.5 9.5 10.0 14.0 14.0 14.0 14.0 14.0 14.0 14.0

2

3

4

5

6

7

8

9

>9

5.0 6.0 10.0 21.0 37.0 54.0

7.0 12.0 31.0 53.0 81.0 108.0

25.0 52.0 71.0 88.0 94.0 100.0

62.2 79.2 87.2 87.2 87.2 87.2

57.0 57.0 57.0 57.0 57.0 57.0

29.4 29.4 29.4 29.4 29.4 29.4

10.9 10.9 10.9 10.9 10.9 10.9

3.2 3.2 3.2 3.2 3.2 3.2

1.3 1.3 1.3 1.3 1.3 1.3

5.0 7.0 9.0 12.0 14.0 16.0 18.0 22.0 26.0 30.0

4.0 8.0 12.0 15.0 26.0 39.0 54.0 72.0 91.0 110.0

6.0 12.0 18.0 29.0 45.0 64.0 84.0 107.7 127.7 147.7

8.0 26.0 44.0 58.0 74.7 85.7 97.7 101.0 108.0 115.0

31.8 41.9 52.1 63.7 66.9 71.1 71.7 72.6 72.6 72.6

28.0 33.6 39.2 40.6 41.7 42.5 42.9 43.0 43.0 43.0

20.0 22.3 24.5 24.5 24.5 24.5 24.5 24.5 24.5 24.5

11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1

3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7

1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

7.0 8.0 9.0 12.0 15.0 20.0 25.0 32.0 41.0 49.0

18.3 22.8 27.6 34.0 46.8 61.2 76.2 91.0 107.1 123.2

7.6 22.0 45.7 66.9 82.6 93.8 104.9 117.1 128.1 142.2

8.2 26.9 41.0 54.2 65.2 75.2 84.2 93.0 101.0 111.0

28.6 32.3 35.0 36.0 41.7 45.9 51.4 55.8 60.2 60.6

22.0 26.8 28.8 28.8 28.8 32.5 35.6 37.5 38.2 38.8

12.4 15.0 15.4 15.4 17.2 18.7 20.0 20.9 21.7 22.5

7.7 8.0 8.3 8.9 8.9 8.9 8.9 8.9 8.9 8.9

2.5 2.5 2.9 3.4 3.4 3.4 3.4 3.4 3.4 3.4

1.2 1.2 1.3 1.4 1.4 1.4 1.4 1.4 1.4 1.4

Low latitude

225 275 325 375 425 475 Middle

latitude

125 175 225 275 325 375 425 475 525 575

High latitude 125 175 225 275 325 375 425 475 525 575 Values

are in Dobson

units.

Table 4. TOMS Version 7 StandardTemperature Profiles Umkehr Layer Profile

0

1

2

3

4

5

6

7

8

9

>9

283.0 283.0 283.0 283.0 283.0 283.0

251.0 251.0 251.0 251.0 251.0 251.0

215.6 215.9 216.5 216.0 216.0 216.0

200.7 203.5 207.0 210.0 213.0 216.0

210.7 211.9 213.6 216.0 217.0 219.0

221.6 222.5 223.0 224.0 224.5 225.0

231.1 231.1 231.1 231.1 231.1 231.1

245.3 245.3 245.3 245.3 245.3 245.3

258.7 258.7 258.7 258.7 258.7 258.7

267.4 267.4 267.4 267.4 267.4 267.4

265.4 265.4 265.4 265.4 265.4 265.4

237.0 260.0 273.0 273.0 273.0 273.0 273.0 273.0 273.0 273.0

218.0 228.0 239.0 239.0 239.0 239.0 239.0 239.0 239.0 239.0

196.0 201.7 213.3 217.1 219.1 220.2 220.9 221.5 222.3 225.0

191.0 198.0 207.5 212.2 216.6 219.0 220.7 222.5 224.8 227.0

193.0 202.1 211.7 214.9 217.0 219.0 221.0 222.7 225.5 227.0

210.0 214.3 219.1 220.4 220.8 221.9 223.7 224.4 225.8 227.0

227.6 227.6 227.6 227.6 227.6 227.6 227.6 227.6 227.6 227.6

239.4 239.4 239.4 239.4 239.4 239.4 239.4 239.4 239.4 239.4

253.6 253.6 253.6 253.6 253.6 253.6 253.6 253.6 253.6 253.5

263.9 263.9 263.9 263.9 263.9 263.9 263.9 263.9 263.9 263.9

262.6 262.6 262.6 262.6 262.6 262.6 262.6 262.6 262.6 262.6

237.0 260.0 260.0 260.0 260.0 260.0 260.0 260.0 260.0 260.0

218.0 228.0 228.0 228.0 228.0 228.0 228.0 228.0 228.0 228.0

196.0 201.7 209.7 222.6 222.6 222.6 222.6 222.6 222.6 222.6

191.0 198.0 208.5 223.4 223.4 223.4 223.4 223.4 223.4 223.4

193.0 202.1 212.5 223.8 223.8 223.8 223.8 223.8 223.8 223.8

210.0 214.3 222.0 226.5 226.5 226.5 226.5 226.5 226.5 226.5

223.3 223.3 228.0 231.6 231.6 231.6 231.6 231.6 231.6 231.6

237.1 237.1 237.1 237.1 237.1 237.1 237.1 237.1 237.1 237.1

251.6 251.6 251.6 251.6 251.5 251.5 251.5 251.5 251.5 251.5

262.4 262.4 262.4 262.4 262.4 262.4 262.4 262.4 262.4 262.4

265.6 265.6 265.6 265.6 265.6 265.6 265.6 265.6 265.6 265.6

Low latitude

225 275 325 375 425 475 Middle

latitude

125 175 225 275 325 375 425 475 525 575

High latitude 125 175 225 275 325 375 425 475 525 575 Values

are in Dobson

units.

9038

WELLEMEYER

ET AL.: A CORRECTION

FOR TOMS

PROFILE

SHAPE

ERRORS

accountthe temperaturedependencein the ozoneabsorption Canada.The TOMS data were producedand archivedby the Ozone coefficients. This effect is estimated to be of the order of 1-2%

Processing Team at NASA's GoddardSpaceFlight Center.This study hasbeen done in supportof the Laboratoryfor Atmospheresat the

by analyzingthe variability of ozone weightedtemperatures. NASA Goddard SpaceFlight Center under NASA contractNAS5Seasonalvariability in the temperatureprofile has not been 29386. taken into accountexplicitlyin this study.The standardtemperatureprofilesare createdby binningthe SAGE temperaReferences ture profilesby total ozoneamountderivedby integratingthe individualcompositeprofiles.The resultingprofilesare shown Bhartia,P. K., D. Silberstein,B. Monosmith,andA. J. Fleig,Standard profilesof ozone from groundto 60 km obtainedby combining in Figure 12 and Table 4. satellite and ground based measurements,in AtmosphericOzone, edited by C. S. Zerefos and A. Ghazi, pp. 243-247, D. Reidel,

Norwell, Mass., 1985.

Conclusions

Profile shapeerrorsin derivedtotal ozonecausedby differences between the actual vertical distribution

of ozone and the

Bhartia, P. K., S. L. Taylor, R. D. McPeters,and C. Wellemeyer, Applicationof the Langleyplot method to the calibrationof the SBUV instrumenton the Nimbus7 satellite,J. Geophys.Res.,100,

2997-3004, 1995. climatologicalprofile used in the retrieval are the primary Dave, J. V., Meaningof successive iterationof the auxiliaryequation sourceof error in version 6 TOMS measurementsat high in the theory of radiative transfer,Astrophys.J., 140, 1292-1303, 1964. latitude. They producea random error of 10% standarddeviof ation in TOMS ozone at the highestsolar zenith anglesat Dave,J. V., andC. L. Mateer,A preliminarystudyon the possibility estimatingtotal atmosphericozonefrom satellitemeasurements, J. which retrievalsare made (86ø-88ø).The error in long-term Atmos. Sci., 24, 414-427, 1967. trend derivedby TOMS at thesevery high solarzenith angles Fleig,A. J., R. D. McPeters,P. K. Bharfia,B. M. Schlesinger, R. P. associatedwith long-termchangesin the ozoneprofile meaCebula,K. F. Klenk, S. L. Taylor, and D. F. Heath, Nimbus7 solar suredby SBUV are estimatedfor the northern high-latitude backscatter ultraviolet(SBUV) ozoneproductsuser'sguide,NASA Ref. Publ., 1234, 19-25, 1990. regionsasa 5% per decadeoverestimation of ozonedepletion. For trend studies in which the latitude is restricted to less than

60ø, however,this error is no greaterthan 1-2% per decade. Becausethiserror is stronglydependenton opticalpath, it is smaller in the southern hemisphere where total ozone

Herman, J. R., R. Hudson, R. McPeters, R. Stolarski, Z. Ahmad, X.-Y.

Gu, S. Taylor, and C. Wellemeyer,A new self-calibrationmethod appliedto TOMS and SBUV backscatteredultraviolet data to determine long-termglobal ozonechange,J. Geophys.Res.,96, 75317545, 1991a. Herman, J. R., R. McPeters, R. Stolarski, D. Larko, and R. Hudson,

amounts, andtherefore optical pathlengths, aresignificantly Globalaverageozonechangefrom November1978to May 1990,J.

smaller.Also, heterogeneous chemicaldepletionsobservedin the Antarctic

"ozone

hole"

Occur near the ozone maximum

Geophys.Res.,96, 17,279-17,305, 1991b. Klenk, K. F., P. K. Bhartia, A. J. Fleig, V. G. Kaveeshwar,R. D.

McPeters, and P.M. Smith, Total ozone determination from the where TOMS sensitivityto •rofile shapeerrorsis small.It is backscattered ultraviolet(BUV) experiment,J. Appl. Meteorol.,21, the classicalgas phase depletion taking place higher in the 1672-1684, 1982. atmospherethat has the strongereffecton the retrieval. Kotz, S., N. L. Johnson,and C. B. Read (Eds.), Encyclopedia of StaAn internalcorrectionmethodhasbeendevelopedbasedon tisticalSciences, vol. 2, pp. 82-86, JohnWiley, New York, 1981. the differentialsensitivityof alternatewavelengthpairsto pro- McCormick,M.P., J. M. Zawodny,R. E. Veiga,J. C. Larsen,andP. H. Wang, An overviewof SAGE I and SAGE II ozone measurements, file shapeerrors at high solar zenith angles.This method rePlanet.SpaceSci., 37, 1567-1586, 1989. ducesrandomerrorsdue to profile shapefrom 10% standard R. D., et al., Nimbus-7total ozonemappingspectrometer deviationto 5% standarddeviationand reducesthe long-term McPeters, (TOMS) data productsuser'sguide,NASA Ref. Publ.,1323, 19-24, drift.

A new ozone and temperatureclimatologyhas been devel-

opedin supportof the correctionmethod.The climatology is basedon the ozone profiles measuredby SAGE II and the

1993.

Stolarski, R. S., P. Bloomfield, R. D. McPeters, and J. R. Herman,

Total ozonetrendsdeducedfrom Nimbus7 TOMS data, Geophys.

Re•.Lett.,18(6),1015-1018, 1991.

coincident NMC temperature profilesreportedwiththeSAGE P. K. Bhartia and R. D. McPeters,NASA GoddardSpaceFlight II ozonedata.Thisnewozoneclimatology maybe lessrepre- Center, Mail Code 916, Greenbelt, MD 20771. (e-mail: bhartia sentativein a mean sensethan the previousversion6 clima- @wrabbit.gsfc.nasa.gov; mcpeters@wrabbit'gsfc'nasa'gøv) C: J. Seftor, S. L. Taylor, and C. G. Wellemeyer, Hughes STX tology,becausethe profileshave been adjustedto represent Corporation,Greenbelt, MD 20771. (e-mail: [email protected]. com; more extreme situations,particularlyat high latitudes. staylør@høss'stx'cøm;cgw@høss'stx'cøm) Acknowledgments.The authorsgratefullyacknowledge the wdrk of Leslie Moy in producingFigure 4. The balloonsondedata usedin this study were obtained from WMO Data Archive in Downsview,

(ReceivedJuly 15, 1996;revisedDecember10, 1996;

accepted December 10,1996.)