GEOPHYSICAL
RESEARCHLETTERS,VOL. 20, NO. 14, PAGES1419-1422,
JULY 23, 1993
A COMPARISON
OFOBSERVED(I-•LOE) AND MODELED(CCM2)
METHANE AND STRATOSPHERIC WATER VAPOR
PhilipW. Mote1,James
R.Holton
1,James
M. Russell
III 2,andByronA. Boville
3
Abstract.Recentmeasurements
(21 September
- 15
October1992)of methane
andwatervaporbytheHalogen
Occultation
Experiment
(HALOE)ontheUpperAtmosphere
Research
Satellite(UARS) arecompared
with modelresults
forthesameseason
froma troposphere-middle
atmosphere
versionof the NationalCenterfor Atmospheric
Research
(NCAR)Community
ClimateModel(CCM2). Severalimportantfeatures
of thetwoconstituent
fieldsarewellreproducedbytheCCM2,despite
theuseof simplified
methane
photochemistry
in theCCM2 andsomenotabledifferences
between
themodel's
zonalmeancirculation
andclimatology.
Observed
featuressimulated
bythemodelincludethefollowing: 1) subsidence
overa deeplayerin theSouthern
Hemisphere
polarvortex;2) widespread
dehydration
in thepolar
vortex;3) existence
of a regionof lowwatervapormixing
ratiosextendingfrom theAntarcticintotheNorthernHemisphere
tropics,
whichsuggests
thatAntarctic
dehydration
contributes
to midlatitude
andtropicaldryness
in thestratosphere.
Introduction
horizontal
resolution
of approximately
3.75' x 3.75', andextends from the surface to 76 km.
A satisfactory
modelof middleatmospheric
watervapor
mustincludethewatervaporsourcefrommethaneoxidation.
We haveaccomplished
thisby carryingmethaneasa tracer
andincludinga parameterization
for methaneoxidation.
Methane
isitselfa long-lived
•xacer
usefulfordiagnosing
the
transportcirculationof the model andfor modelvalidation
usingsatellite
observations.
Therateconstants
forthesimple
methanechemistryincludedin themodelwerederivedfrom
a two-dimensional
6hemistry-transport
model(Garciaand
Solomon1983)andarefunctions
of latitude,heightand
month.
Oxidation
of methane
produces
• watermolecules
per
methane
moleculeoxidized,where[l mustlie between1.5
and2.0 (Remsberg
et al., 1984).Boththemodeling
results
of
Le Texieret al. (1988)andan analysis
of LIMS (H20) and
SAMS(CI-I4)observations
(Hansen
andRobinson,
1989)
indicate
that[l increases
withheightin thestratosphere,
reachingvaluescloseto 2 at thestratopause.
At thisaltitude
the photodissociation
of methaneis mostefficient.For this
With thelaunchof theUpperAtmosphere
Research
Satellite(UARS) in September1991andthereleaseof a new
versionof theNationalCenterfor Atmospheric
Research
(NCAR)Community
ClimateModel(CCM2) in September
1992,newgenerations
of bothstratospheric
constituent
data
andmodelresultsarenowbecoming
availablefor comparison.Contrasting
of UARS observed
constituent
fieldsto
reason,andbecause
someH2 is ultimatelyconverted
to wa-
model simulated fields is a valuable aid to model validation.
andthedynamicalfieldsweretakenfrom an earlierrun of the
Specifically,
comparison
of simulated
watervapormixing
ratiofieldswithobserved
fieldsprovides
animportant
testof
modelphysics
andnumerics
since,owingto itsverylarge
gradients
nearthetropopause
anditscondensation
sink,water.
vaporis a difficultconstituent
to modelaccurately.Likewise,
themodelresultscanbeusedasanaidin interpreting
theobservations
since,unlikeobserved
fields,modeldynamical
fieldsarecompletelyknown.
CCM2in whichthewatervaporhadnotyetreached
equilibrium,andthestratosphere
wasquitedry(lessthan2 ppmvof
H20 abovethemiddlestratosphere).
Thebeginning
dateof
theintegration
wasSeptember
1. Theozonedistribution
and
theseasurface
temperature
werespecified
basedonmonthly
means(interpolated
to eachday)takenfromclimatology.
Themethane
mixingratioin thelowertroposphere
washeld
fixedat 1.5ppmv,a valuesomewhat
belowtoday's
global
tervapor,weset[l = 2 everywhere.
Thiswill tendtoproduceanoverestimate
of stratospheric
watervapor,butlikely
by not morethan 10%.
The model was initialized with a zonal mean methane mix-
ing ratio takenfrom the two-dimensional
modelof Garcia
andSolomon
(1983).Theinitialdistributions
of watervapor
meanof 1.72ppmv(IPCC, 1990).
Aftera four-year
runatlowervertical
resolution
to "spin
Modelandexperiment
design.
up"themethane
andwatervapor,theverticalresolution
was
The CCM2 is a globalspectral
modelthathasa numberof
changes
fromprevious
NCARmodels.
It employs
a semiLagrangian
scheme
forconstituent
transport
andimproved
radiative,convective,
andcloudparameterizations.
A com-
increased
to75 levels(L75)withroughlylkm spacing
throughout
themiddleatmosphere.
TheRayleigh
friction
profile(usedtocrudely
parameterize
unresolved
dragforces
pletedescription
maybe foundin Hacket al., 1992. For this
studywe utilizea "middleatmosphere"
version,whichhas
teractthetendency
fortheSouthern
Hemisphere
polarnight
jet to become
excessively
strong.
Also,to diminishtheex-
in theupperstratosphere)
wasmodifiedsomewhat
to coun-
cessive
dehydration
in theSouthern
winterpolarstratosphere
(wherethemodeltemperatures
arecolderthanobserved),
the
1Department
ofAtmospheric
Sciences,
Univ.ofWashington minimum
saturation
vaporpressure
wassettocorrespond
to
2NASALangleyResearch
Center
itsvalueat 181K(0.65ppmvat 100mb).Because
saturation
3National
Center
forAtmospheric
Research
mixingratioisproportional
to saturation
vaporpressure
dividedbyambient
pressure,
thischange
hasthegreatest
imCopyright
1993bytheAmerican
Geophysical
Union.
pactatlowpressures,
effectively
restricting
thedehydration
Papernumber93GL01764
0094-8534/93/93GL-01764503.00
regionto a thinnercolumnat lower altitudethanwould other-
wisebethecase.Thismodification
haslittleornoimpact
on
1419
1420
Moteetal.: Comparison
of observed
andmodelled
CH4andH20
mixingratiosin thelowerstratosphere,
eitherat thepolesor
in the tropics.
Observational data source
HALOE usesthe gasfilter correlationradiometertechniquefor measurement
of CH4, HC1,HF, andNO, andbroad
bandradiometryto measureH20, 03, andNO2 [Russellet
al., 1993a].Theexperimental
approach
is solaroccultation,
whichprovides
anessentially
self-calibrating,
highlyprecise
measurement
sincetheexoatmospheric
solarsignalis usedto
normalize
radiance
transmitted
through
theatmosphere
at any
givenaltitude.Comparisons
of HALOE CH4 andH20 measurementsat the 20 km level with in situ data collected from
theER-2 aircraftshowagreement
to within 13% [Tucket al.,
1993a].OtherHALOE comparisons
withground
based,balloon,androcketdataagreeto withintheerrorbarsof thedata
sets.
UARS is operating
in a 57ø,585km orbit,whichprovides
coverage
from80øSto 80øN.HALOE samples
twolatitude/
longitudepointsperorbit,onesunrise
andonesunset,
for a
totalof 30 measurements
perday. Its spatialresolution
for a
singlemeasurement
is approximately
6 kmhorizontally
(perpendicular
to thetangent
track)by2 kmvertically,and
about200km alongthetangent
track.Longitudinal
sampling
occursat theorbitspacing
of 24ø. Because
of theslowprecessionof thesatellite's
orbit,HALOE samples
anygiven
latitudeonlyonceduringa thirty-three
daysweepfromone
poleto theother,it should,consequently,
be bornein mind
that the meridional sections shown below for HALOE
do not
representtruetime averages,but arecomposites
in whichthe
solaroccultation
measurements
madea slowsweepfrom
60'N to 80'S.
Results.
HALOE watervaporandmethanecrosssectionsfor
SouthernHemispherespringareshownin Figuresl a and2a.
Thesecrosssectionsareconsistent
with previousobservations, and are describedelsewhere[Russellet al., 1993b;
Tucket al., 1993b].In thisnotewe focuson somedynamically significantaspectsof the measurements,
whicharealso
clearlypresentin theCCM2 simulation.
In spiteof thedifficultiesinherentin three-dimensional
tracermodeling,andthe
additionaldifficultiesof modelingwatervapor,the CCM2
reproducesimportantlarge-scalefeaturesof the zonalmean
distributions:the generaldecreaseof methaneandincrease
of watervapormixingratioswithheight,andthesteepslopes
of themixingratioisopleths
in thesubtropics
of bothhemispheres.
We hereconfineourcomparison
to theperiodSeptember
21 to October15, 1992,shownin theHALOE datain Figures
1 and2. The sametimeperiodhasbeentakenfrom thebeginningof thefifth yearof integration
of theCCM2, soit
shouldbekeptin mindthatthetransition
to highervertical
resolution
tookplaceonlya few weeksbeforethetimeperiod
underconsideration.
Duringthefourthyearof integration
atL35 resolution,
thelowerlimit onthesaturation
vapor
pressure
minimumdiscussed
abovewasemployed,
sodehydrationduringthefourthSouthern
winter,justbeforethetime
Methane. BothHALOE data(Figure la) andCCM2
output(Figurelb) showsmoothupwardbulgesin themixing
ratio isoplethscenteredat Northernlow latitudes,reflecting
the meantropicalascentandextratropicaldescentof the
Brewer-Dobsoncirculation[Brewer, 1949]. Above about
5hPa,the HALOE methanecontourshavea doublepeaked
structurein the subtropics
andflat isoplethsnearthe equator,
whichis knownto be associated
with theequatorialsemi-annualoscillation[Gray andPyle, 1986;Choi andHolton,
1991]. By contrast,
theCCM2 methaneisopleths
retaintheir
curvedshapewell into theupperstratosphere,
with no evidenceof a doublepeak. On eachsideof theregionof tropical ascentis a narrowzonewith largemeridionalgradientsof
methane;the isoplethsof HALOE methaneare somewhat
steeperin the SouthernHemispheremiddlestratosphere
than
thosein the CCM2, thoughthe slopesareaboutthesamein
theNorthernHemisphere.Betweenapproximately
20øSand
50øSbothHALOE andCCM2 showevidenceof mixingby
planetarywaves,whichresultsin flatterisopleths;
theyare,
indeed,almostcompletelyflat in theCCM2.
The Southernpolarregionis of particularinterestsince
thisis the seasonof intenseozonedepletion.Subsidence
within the polarvortexis evidentin bothfigures,whichshow
thatmethane-poorair hasbeentransported
downwardinto the
lowerstratosphere.
Suchair, mesospheric
in origin,extends
deeperin theHALOE methanedistribution,
whichindicates
thattheregionof substantial
subsidence
extendsto lower
altitudein theobserved
polarvortexthanin themodel.
Watervapor. The watervaporfieldsof bothHALOE
(Figure2a) andCCM2 (Figure2b) sharemanyfeatures
with
themethanefields.Theseincludetheupwardbulgein the
tropics,thedoublepeakin HALOE, steepsubtropical
isopleths,flatterisoplethsin midlatitudes
(particularlyin the
CCM2), anddepressed
isoplethswithintheSouthernpolar
vortex. But, unlikemethane,watervaporhasa sinkin the
polarlowerstratosphere,
andthissinkproduces
broadregions
of dramaticdehydration
in boththeHALOE andCCM2 watervaporfields. The dehydration
causedby theformationof
polarstratospheric
cloudsis evidentlyof roughlythe same
magnitudein bothcases,althoughit clearlyoccursovera
muchdeeperlayerin the CCM2.
Thisdehydrated
air originatingin thepolarvortexis dispersedequatorward,
whereit is caughtin theupwardflow of
the Brewer-Dobson circulation so that the altitude of mini-
mummixingratioincreases
towardthetropics. The factthat
methanevaluesincreasesubstantially
towardthe equator
(particularly
in theHALOE data)alongthewatervaporminimum surface indicates that there must be substantial wave
mixingwith lowerstratospheric
air fromlow latitudesalong
thissurface.Suchmixingwill of courseaffectmethanefar
morethanwatervaporbecausethecoldtropicaltropopause
will leadto dehydration
of air crossingfrom the tropical
troposphere.
This differingbehaviorof methaneandwater
vaporin thelowerstratosphere
of theSouthern
Hemisphere
suggests
thattransport
fromthetropicsto mid-latitudes
may
be rather slow, which is consistentwith the existenceof a
periodconsideredhere,was lessthanin earlierSouthern
substantial
potentialvorticitybarrierin thetropics(see,e.g.,
TrepteandHitchman,1992).
In the CCM2, watervapormoreeasilypassesfrom troposphereto stratosphere
in subtropical
latitudes,asevidentby
theupwardintrusionof highermixingratios(yellow). From
winters in the simulation.
Novemberto March (not shown),a local minimum in water
1421
Moteet al.' Comparison
of obseiwed
andmodelled
CH4andH20
MALOE
CClV•
10
10
o.6•
0.2
0.0
-80
-57
-33
- 10
I.A•E
1•
37
60
-80
-57
(DEG)
-33
- 10
I.A•E
13
37
(DEG)
(a)
Co)
Fig. 1. Zonalmeanmethanemixingratio(ppmv)for theperiodSeptember
21 to October15, (a) HALOE data
(sunsetsonly) for 1992; (b) CCM2 outputfrom a 75-levelversionof the model. Color scalethe samefor both
plots.
HALOE
CCM2
'-I "•.....
-1 .... I ' !
'1 .....•
i
I
I
I
I
:.:.. 8.0
7.4
ß
•.0
2.0
-57
-•
-10
13
37
60
LA'ITHJDE (DEG)
-80
-57
-•
-10
13
LAiTIIJDE
(DEG)
37
(a)
Fig. 2. As in Fig. 1, butfor watervapormixingratio.
vapormixingratio appearsat 85 hPafrom about10øSto
10*N dueto lower temperatures
andgreaterdehydration;
but
outsidethoselatitudes,theCCM2 remainsconsiderably
more
moistin the lower stratosphere.
Near0.2 hPa,HALOE indicatesslightlyhigherwater va-
pormixingratiosthandoestheCCM2. Thiscouldbedueto
thelow valueof tropospheric
methaneusedin themodel.
Above0.2 hPa,mixingratiosdecrease
dueto thephotodissociation
of watervapor,a process
notincludedin theCCM2
simulation.
Discussion and conclusions.
The pole-to-equator
distributionof dry air in the lower
stratosphere
in SouthernHemispherespringwasfaintly suggestedin Octobermeansfrom SAGE II [Rindet al., 1993],
andwasalsosuggested
in limitedaircraftprofilestakenduring theAirborneAntarcticOzoneExpedition[Kelly et al.,
1989], andin theGCM studyof theAntarcticozoneholeof
Cariolleet al [1990].But its full extenthasnotpreviously
been documented,and it is remarkable for a few reasons.
First,it suggests
thatconsiderable
transport
takesplaceout
of thelowerportionof thepolarvortexduringearlyspring.
In mid-Octoberthevortexis stillquitevigorous,andit is
generallythoughtthatthe largepotentialvorticitygradient
preventssignificant
transport
outof thevortex(see,e.g.,
Schoeberl
et al., [ 1992]),althoughTuck [ 1989]hasargued
thatthereis considerable
exchangeof air acrossthevortex
boundary.Admittedly,thepotentialvorticityconstraint
is
muchweakerin thelowerstratosphere
(say,below50 hPa),
butit is stillsomewhat
surprising
thatsufficientdry air canbe
transported
outof thevortexto completelyfill theSouthern
Hemisphere
lowerstratosphere
at thisseason.
Second,theseresultsindicatethatair dehydrated
by polar
stratospheric
cloudsin theSouthern
polarregioncaninfluencethewatervapordistribution
in thetropics.The HALOE
data show a distinctminimum at about 10*S, 50 hPa, which is
very nearthe "hygropause."That thisis relatedto Antarctic
dehydration
is apparentfromplotsof thequantity2'[CI-I4]+
[H20], a goodtracerof dehydrated
stratospheric
air, shown
in Tucket al. (1993b). AlthoughAntarcticdehydration
probablydoesnotcontroltropicallower stratospheric
mixingratioson an annualbasis,it apparently
playsanimportantrole
duringsomemonths,andmayprovideanexplanation
for the
existence
of a watervapormixingratiominimumwell above
thetropopause
overPanamain September
[Kley et al., 1982].
In theCCM2, in whichtemperatures
in theSouthern
polar
regionare colderandresidualcirculationweakerthan
observed,
theeffectsof dehydration
persistall year(figures
not shown). SAGE II data showeda hint of it in October,but
1422
Moteet al.: Comparison
of observed
andmodelledCH4andH20
nonein January.As laterHALOE resultsare processed,
it
will be interestingto seehow the watervapordistributionin
the SouthernHemispherelowerstratosphere
evolves.We
intendto presentmoredetailedanalysesin a longerpaper.
Acknowledgments:
We wishto thankAdrianTuck for helpful commentson a preliminaryversionof the manuscript,and
MatthewHitchmanandan anonymous
reviewerfor detailed
suggestions
for improvement.This workwassupported
by
NASA contractNAS5-26301 (UW), NASA Grant W18181
(NCAR), andNASA GlobalChangeFellowshipNGT 30076
(UW).
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P.O. Box 3000, Boulder, Colorado, 80307.
J.R.Holton andP.W. Mote, Departmentof Atmospheric
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J.M. RussellIII, AtmosphericSciencesDivision,NASA
LangleyResearchCenter,Hampton,VA, 23665
Received:April 6, 1993
Accepted:April 30, 1993