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SPRINGTIME STRATOSPHERIC WATER VAPOUR IN THE SOUTHERN HEMISPHERE

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SPRINGTIME STRATOSPHERIC WATER VAPOUR IN THE SOUTHERN HEMISPHERE Powered By Docstoc
					                     RESEARCH
           GEOPHYSICAL      LITTERS,VOL.20, NO. 12,PAGES             18,
                                                        1235-1238,/UNE 1993


                                         WATER
                    SPRINGTIMESTRATOSPHERIC         IN
                                              VAPOUR THE SOUTHERN
                                     HEMISPHEREAS MEASUREDBY MLS

                     Harwood Cart L. Froidevaux Jarnot
                  R.S.      •, E.S. •,            •, I•.F.  •, W.A.    •,      s,
                                                                    LahozC.L.Lau
                      G.E.Peckham         •, P.D.        •,      s,
                                 s,W.G.P•ead RicaudR.A.Suttie J.W.Waters   •

   Abstract. The effectsof the break-upof the antarc-            AlthoughHaO in the antarctic vortex has been stud-
tic vortex on the water vapour distribution are studied                from        of       systems,
                                                             iedpreviously a variety observing    MLS
    MLS measuren•ents
using                          made
                  of watervapour   during                    allowsthe evolution of broad-scalefeatures of the ff20 dis-
          1991
September and November      1991. In earlyNovember tribution to be observed  daily overan unprecedented areal
at 22 hPa a moistareais foundwithin the polarvortex, and vertical extent. Here we make use of the southward-
consistentwith an observed descent of order 10 km and                          in          with potentialvortic-
                                                             lookingobservations conjunction
strong        cooling. the vortex
      radiative       As             erodes(beginning ity (PV) based windandtemperature
                                                                   on                 analysesfromthe
of November1991), parcelsof moist air become detached UK Meteorological
                                                                      Office        and
                                                                            (Swinbank O'Neill, 1992)to
fromthe edgeof the vortex and mix rapidly (within 2-3        discuss               of
                                                                     someaspects the circulationduringthe break-
days)with driermid-latitudeair. Whenthe vortexbreaks         up of the antarctic vortex in spring, showingevidenceof
               largerparcels moist fromboth
up(mid-November),          of    air                                 in                    at
                                                             descent the vortex,erosion the vortexedgeandtrans-
the edgeand the inner vortex migrate to mid-latitudes.       port of vortexair to low latitudes.
These parcels have a. longerlifetime than thoseproduced
by vortexerosion,  probablybecause    they are correlated                       The vertical   descent

withhigherpotentialvorticitygradients.   The break-up of
                                                                                                                 in
                                                                 In the antarcticvortexin the lowerstratosphere win-
                           by
the vortex is accompanied a mean adiabaticequator-
wardtransportresultingin a significant           in
                                         increase mid-                     of
                                                             ter, a process dehydration   and denitrificationis believed
stratospheric water vapour valuesat mid-latitudesin late     to take placeas a resultof the formationof polar strato-
spring.
                                                             spheric                                     to
                                                                      clouds.This allowsactive chlorine be released
                                                             which results in the "ozone hole". By early November
                      Introduction                           1991, however,   whenMLS began an extendedperiodof
                                                             lookingsouth,the main area of the vortex was found to
   The Microwave Limb Sounder (MLS) carriedby the            be moistat 22 hPa (the highest pressures which at
UpperAtmosphere   ResearchSatellite(UARS)launched            the profileshavesignificant informationin the currentre-
on 12 September1991 measuresconcentrations severa!
                                         of                  trieval software). Figure 1 showsH20 at 22 hPa on 8th
species importance the middleatmosphere,
       of           in                      princi-          Novemberi991 (hereafterdates shownas e.g. 911108).
pallyClO, OaandH20 (Waters, 1989).The satellite
                                              yaws           The moist area is found to be well correlated with the
aroundat intervalsof approximately one month. This           highestvaluesof-PV as discussed  further below. The
leads a periodin whichthe region
     to                             sampled MLS
                                          by                                                              a
                                                             highmixingratiosin the vortexare presumably result
is from • 80øS to ~ 34øN followedby one in which it is                bringingmoist air from above. Severalstud-
                                                             of descent
from ~ 34 øS to • 80 oN.                                     iesof zonalmeancirculationand heatingrates havefound
              we         on          from
   in thispaper concentrate measurements the          cooling             in             in
                                                             and descent polar regions spring(e.g. Gille
MLS H2¸ channelat 183 GHz. The MLS measurements       and Lyjak, 1986). It is alsowell known(e.g. Schoeberl
of watervapourhavea horizontal verticalresolution et al., 1992),that verticaldescent
                                and                                                    (downward     isen-
                                                                                                across
                                                             at                 at         of
                                     With the present tropes) ~ 50 hPaoccurs the edge the polarvortex
of • 400 km and ,.• 4 km respectively.
version theretrieval
       of             software
                             (V0003),theprecisionand  in winter and early spring. Sincein November1991 the
accuracy individual
       for         profiles 46hPaare0.5ppmv
                          at                and              vortex is shifted from the pole by at least 10ø, evidence
15%           while
   respectively, at 4.6hPathey 0.3ppmv
                                 are        and              that vertical descent has taken place is clearly visiblein
    The data for each24 hourperiod
1,5%,                             centred !200
                                        on                             vs              (r/
                                                             a longitude r/ cross-section = /,•.(0),0 = potential
UT havebeenlinearly           onto
                   interpolated a fixedlatitude-                        as        2a
                                                             temperature) in figure in whichMLS H20 values   for
longitude Ascending descending
       grid.     and       portions the
                                 of                          911108havebeenlinearly interpolatedin the verticalonto
orbitweretreated        and
                separate!y thenaveraged.                                and
                                                             isentropes averaged    within a 70øS-60øS latitudebin.
                                                                         are         of
                                                             Alsoshown estimates the Erte! PV (figure2b) mid
                                                             the net diabaticheatingrates(figure2c) averaged within
                                                             the same latitude bin.
             University,
....•'Edinbt[rgh    UK                                                                         by
                                                                 The heatingratesare calculated the methodof
             Laboratory,
 JetPropulsion         USA
 a Herlot-Watt        UK
             University,                                     Haigh(1984)  using             of
                                                                               MLS retrievals tiaO, O• andtem-
                                                                     as                on              the
                                                             perature input.Asa check thesensitivity input
                                                                                by
                                                             fieldswereperturbed typicalMLS uncertainties. The
       1993
Copyright bytheAmerican
                     Geophysical
                            Union.                                       to          rate
                                                             perturbation theheating was_<10% thestrato-
                                                                                                   in
                                                             sphere.
Paper    93GL00832
    number                                                               isentrope
                                                                 Ona given       through   of
                                                                                        most thestratosphere
0094-8534/93/93GL-00832503.00                                        2,     is       of                with
                                                             in figure there a region highH•O correlated
                                                          1235
1236                                                                          ter
                                                        Ilarwoodet a,l.:MLS Wa, Vapour

                                                                                   of
                                                                           or •wa.11' the vortex can be definedby the valueof its
                                                                           steepestgradientand labeled(Tucket al., 1992)a.s'con-
                                                                                    (a.nyparcelof air insidea givenPV contour
                                                                           servative'                                          is
                                                                           witlfin the vortex) or 'liberal' (a.nyparcel of air outsidea
                                                                               PV      is      the      tn     3,
                                                                           given contour outside vortex). figure lightly
                                                                           shadedareasrepresentthe 'wall' of the vortex betweenthe
                                                                           'liberal'contourvalueo|'-10x10-SKm-2kg-•s-• andthe
                                                                                    tive'
                                                                           'conserva. contour                x -'5
                                                                                                valueof -12.,5 10 Km-2kg-•s-•.
                                                                           The heavily       areas         the
                                                                                       siva,deal tel)resent moistregion where
                                                                           I120 valuesexceed,5.1 l)pmv.
                                                                                                        correla,tion
                                                                              The spatial an(1teinl)ora.1          betweenthe moist
                                                                               and     r      ca.•      seen
                                                                           areat thepola.vortex beclearly           3.
                                                                                                           fromfigure
            stereogra.pl•ic o1'
Fig. 1. Pola.r                      at
                       map H20 (ppmv) 22l•Pa                               Initially l,]•evortex is coherentaa•doffset somewhatfrom
on 911104.The ma.pextendsto 30øS.                                                                    it      tes,
                                                                           the pole. As tingeproceeds, elo•ga. splitsinto two
                                                                                               into     pieces.Oul.
                                                                           andfinally fi'a.gments several                     in
                                                                                                                  si(!etl•e ma,
high-PV. It is seenthat the radiativecooling(negative                                                      ir
                                                                           vortex, there are sotne•l•oist a. parcelswhich are corre-
net heating             71
            rate) above = 6.6 is greater  withi• thevortex                 1,tl, with PV a.reas
                                                                              ecl              havingt]•esigna. of the interior
                                                                                                               ture              or
                 of
(rate of change temperature 3 K.day a.t71 6.8)
                                 ,-0        -•      =                            of
                                                                           •wa,11' tire vortex. On 91110,1 one sucl• ex-vortex moist
than outsideit (,-0 1.5 K.da,y-' a.t r/ = 6.8) and 11•a.tit                                l,ed
                                                                           air parcelis 1oca, i• the region30-3.5øSand 10-90øW
increases with 'q. The inferredverticaldisplacement the
                                                     o1'                   (over South America). Th(, move•nentof this feature west-
water isopleths la.rge,
                 is       beingfrom r] = 6.9 to 6.5, which                 ward along the 35øSlatitude circle to southern Australia
             to
corresponds ,-- 10 kin. In comparison                et
                                           Schoeberl al.                               with the observed
                                                                           is consistent               winds. Despitea reduction
                                   ,-•             with
(1992)founda.valueof 2-3 k•n at 71 6, consiste•lt our                      in thesizeof thevortex               in
                                                                                                 andthedilIiculty establishing
calculationof a smallernet heating rate at that l•eigltt.                  the lifetimeof such       (Tucket al., 1992),its lifetime
                                                                                              features
                                                                           is estimatedto be muchgreater than two weeks. In view
                    Movement        of the moist area                                            we
                                                                           of thesecorrelatioits, use the measured11.20   fieldsin
                                                                           an attempt to qum•tify the impact of vortex erosionand
    To understand          better   the movement   of tim moist area.
                                                                           break-up at mid-latitudes.
and the pola.rvortex we have interpolatedthe lX•LS H•O
              a.t       of
lnea.surements a series daysin November1991and su-                                                     k-up of the vortex
                                                                                        Erosionand brea.
perimposedt,                          ted
            hem on PV maps calcula. fi'om tire [IKMO
assimilateddata set (see figtire 3). We obtain essentia.lly                     It is widely believedthat stronggradientsot' PV a.t
similar PV distributions            based on NMC   data.       Note that   the edge of the polar vorticesinhibit tra.nsfer into tl•e vor-
                            on
whereas the PV is 1)a•se½l analyses for 1200 tIT, the                      tex (Mcintyre, 1989). However,the srna.ll-scale    I•orizontal
II20 fields are effectivelya 24 hour average. The 'edge'                                   at
                                                                           mixingoccuring tl•e vortexedgeisstrongenougl• peelto
                                                                                        of                              air.
                                                                           out fila.•nents tile vortexto tl•c surrounding Thisef-
                                                                                                  by         and
                                                                           fecthasbeenn•odeled .luckes Mclntyre(1987)and
                     7.2                                42                 reported by Kelly el al. (1989) for the sout]•en• vortex
          (ø)        7.0                                                   and by Tuck el, a.1.(1992) for the northern vortex.
                                                                              During the period 911104 to 911108 a filament of moist
                                                                                                   and ,-, 20øE) from the main wet
                                                                           air is extruded(,-, ,10øS
                     6.6                                                   region. The PV analysisstronglysuggests    tl•a.t this is a
                                                        27•.
                     6.4                                22
                                                                           pieceof the vortexedgewhichis beingpulledoff. There-
                                                                           after, trace of this moisl filament is lost. This could be
          (b)        7.0                                                   due to a lack of resolutionand/or rapid horizontalmix-
                                                                           ing which dilutes any wet area.soutside the vortex into
                •    •.8
                                                                           the surroundingdrier air. "l'helifetime of these southern
                     6.6
                                                        27•.               henrispherespringfeaturesis estimatedto be 2-3 daysin
                     6.4                                22                                                for
                                                                           contrastto a lifetimeof 2 weeks the air parcelmoving
                                                                           along the 3.5øSlatitude mentioned above. Tills is consis-

                     7.0                                                                            at
                                                                           tent with PV gradients the vortex 'xvall'beingweaker
                                                                           in late springthan in early spring.
                                                                               Murphy et al. (1989) showthat erosionof 11•evortex
                     6.6                                27 &                            titude represents surface
                                                                           by 3-4ø in la.               a           loss ,-, ,50
                                                                                                                area.   of      %.
                                                        22                                    ify
                                                                           In order to qt•a.•l. the effect of the erosionand break-up
                       180W90W GM 90E 180E                                           ()•
                                                                           processes l,l•exortex during the period 911IO1-91112,5,
                                     Longitude
                                                                                    ca.        fron• 90øSto the equatortim stirface
                                                                           we l•a.ve lc•la.l.e(1
                 vs                 for
Fig. 2. Longitude In(O) cross-sections the lat-                                                 wil,
                                                                           area lossa.ssociatt,d l• the moistregionwl•ere 1:I20is
                for      of
itudebin 70øS-60øS 911108 a) fI:•0 mixingratio                                                        nd
                                                                           greatel' titan ,5.1 ppmv, a. the surfacearea lossesa.ssoci-
      b)     -s          -•)           ting
(ppmv), PV/(10 K.m'.kg-•.s andc),,et hea,                                  a.te(l with tl•e 'liln'ra.l' and 'conservative' definitions of the
rate, J/(K.da.y-•).                                                        vortex.
                                                   etal'      Vapour
                                             Harxvood •[LSWaler                                                                                    1237


                               4 Nov 1991        6 Nov1991            B Nov 1991                    10 Nov 1991




                               12 Nov 1991      14 Nov 1991           16 Nov 1991                   18 Nov 1991

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

                                                                                                     4
                                                                                                  ¾"W ....
                               19 Nov 1991      21 Nov 1991           23 Nov 1991                   25 Nov 1991


                                                                 ..    ?" • ....               / .... • ...
                              .....     .....                                                      .... ....


                       ofPV            and         K         1991.
      Fig.3. Timesequence (UKMO-derived)H20at 650 forNovember Themaps onaare
      polar          projection 30øS 90øS. lightly
           stereographic     from  to    The          area    a representation
                                                 shaded provides           ofthe
           of         by     regions
      'edge' thevortex marking     where  PV/(10 K.m2.kg-l.s) lies
                                               -s        -1             -12.5and-10.The
                                                                  between
            shaded marks
      heavily     area         where
                         regions                5.1
                                    I-[.•0exceeds ppmv.

   From 911104 to 911108 we find • decrease in stirface          911104 and 911125. The distributions are broadly con-
areaof 4.1 x 106km=(• 10 %) for the 'liberal'vortex,             sistent with the co•cept that air is dried as it enters the
        •                                 vortexand
2.1 x 10 km• (• 10 %) t•r the •conservative'                                  at                                  of
                                                                 stratosphere the tropical tropopausebecause the low
        •
4..5x 10 km2 (• 20 %) for the moistregion. The in-               temperatures,and that it progressively   moistensthrough
termedlate region encompa.ssi•g
                              mixing ratios ,1.8 to 5.1          methaneoxidation(e.g. Joneset al., 1986) as it is carried
ppmvexpands 9.4 x 106km=(•,, 70 %). The loss-rate
          by                                                                                  tion and subjected to lateral
                                                                 round in the diabatic circula.
for the period 911118-911125(post break-up)in the area           mixing by planetary-scale waves.
of the moist region a.
                     lmost doubles,decreasing area
                                              the                    The stronggradie•ts around 60øSin the zonally-avera-
                                  the
by 5.7 x 106km• (,--,35 %), whereas intermedia.re-
                                            te                                                               to
                                                                 ged H20 fieldson 910923, which correspond the 'edge'
gion         by
     expands 4.9 x 106km (• 15 %). Thusthe to-
                             =                                                          vortex (see figure 1,), fi'om r/ =
                                                                 of the zonally-a.vera.ged
tal area is conservedto within 15 % in the later period;
however, this is not the casein the earlierperiod. This
                                                                                                 7.2
suggeststhat horizontalmixing is dominant   duringvor-
             but
tex break-up that diabatic           are
                              effects of moresignifi-
caace earlier. The discrepancy        the
                              between PVa.nd moist
                                                                                   o..     % 6.8
             may               to
are• changes be attributable thedifferent     observ-
ingtechniques                  conservation
              and/orthedifferent            properties                             .,-)          6.6

          •nd           and
of tracers PV (Haynes Mcintyre,       1987).Further-                                             6.4

      the       in
more, change areas     couldindicatethat t hemotionis
                                                                                   F• (b)        7.0
divergent          and
          (Butchart Remsberg,    1986).It is notpossi-
                       for
ble to extend estima.tes area-lossbasedon ti•e previous
                                                                                    >
                                                                                    O      'E' 6.8
                                                                                            _
PV valuesinto the laterperioda.s            define
                                theyno longer                                      z

thevortex       A
         edges. consideration          the
                              ofwhether dramatic
risein the rate of lossof moist are• is due to an accelera.ted
     to        scales,    ormovement
mixing unresolved convergence     off
            is     the   of
thisisentrop• beyond scope thisletter.
                                                                                       >                                                32    o,
                Effect on zonal mean fields                                        7       -                               ø
                                                                                   u•            6.6                                    27 &
                                                                                   t"q                        "
              of
   The movement wet air from polarto lniddlela.t.i-                                              6.4 • _            , .-"                22
    caused theequatorward
tudes    by            transport
                              i•(luced the
                                     by                                                             90S           60S     3OS    Eq   30N
                                                                                                                       LetRude
     break-up
vortex            detectable
            produces             in
                          changesthezonal
means, to which we now turn.                                                                            of
                                                                  Fig. 4. Latitudevs in(O) cross-sectionszonal-mean
        4
  Figure shows meridional
             the        cross-sections
                                  oftiao                                                                       and
                                                                  H•O mixingratio (ppmv)for a)910923, b)9] 110.1 c)
                                                                 911125.
     ratio
mixing zonally      along      for910923,
              averaged isentropes
1238                                            Harwoodet al.' M LS Water Vapour


6.5 to 71= 6.8 ma, provideevidenceof strong descentat
                    y                                                                       References

the vortex edgeand limited mixing across     it.                     Butchart, N., and E.E. Remsberg, The area of the strato-
                       for
   The cross-section 911104 corresponds the firstto                      spheric                            for
                                                                                 polar vortexas a diagnostic tracer trans-
                      in
map in the sequence figure3 and thus shows situa-  the                                       surfa,
                                                                         port on an isentropic ce, J. A•mos. Sei., .•$,
tion prior to the transport of vortex air to lowerlatitudes.
The mid-latitudedry air (<_4.7,5ppmv) at r/ = 6.5 is in                   J.C.,andL.V. Lyjak,Radiative
                                                                     Oille•                                and
                                                                                                     heating cooling
stark contrastto the moisthigh latitude air (> 5 ppmv).                  ratesin the middle          J.
                                                                                           atmosphere, A•mos.Sci.,
By 9111215   (figure4c), after the break-upo[ the vortex,                2215-2229, 1986.
there is no longera.n/-/20 gra.dient   polewardfi'om30øS                   J.D., Radiative
                                                                     Ha.igh,                     of
                                                                                          heating the lowerstratosphere
                     dry
and the previously mid-la,titude      regionat t?= 6.5 has               and the distribution of ozone in a two-dimensional
                                                     _>
beenfilled by wet high latitude a.irto reachva.lues 4.7,5               model,•uart. J. Roy. Met. Soc,110, 167-185,1984.
ppmv. This rapid equatorwardtransport inducedby the                  Haynes,P.H., and M.E. Mcintyre, On the evolutionof
       of        is                 direction
break-up thevortex •husin theopposite                                    potentia.1
                                                                                  vorticity thepresencealia.ha.tic
                                                                                          in           of      hea.ting
the prevailingmeandiabaticdrculation.                                                                J.
                                                                         and frictionalor otherforces, Atmos.
                                                                         828-841, 1987.
                         Conclusions                                      R..L.,et al., The watervapour
                                                                     Jones,                                  of
                                                                                                       budget the strato-
                                                                        sphere studied   usingLIMS and SAMS satellitedata,
   MLS measurements H20 in spring1991in the south-
                    of                                                  Quart. J. Roy. Met. Soc., 112, 1127-1143. 1986.
              interpolatedonto isentropes
ern hemisphere,                          have shown                  Juckes,M.N., and M.E. Mcintyre,A high-resolution one-
tha• i) significant     occursat the centreand edge
                  descent                                               layermodelof brea.king  planetary    in
                                                                                                         waves the stra.to-
of the southern    vortex which moistened       its interior   be-       sphere, Nat'ure,328, 590-596, 1987.
tween 650 and 900 K, 5) the tempora.1 spatial evo-
                                    and                              Kelly, K.K., et a.l., Dehydration in the lower Antarc-
lutions of the moist areas detected        a,t 650 K in Novem-                         during
                                                                         tic stratosphere    lt•tewinterandearlyspring,
                                           of
ber 1991 correlatewell with differentphases the vortex                                   Res., 9•, 11,317-11,357,
                                                                         1987, J. Geophys.                      1989.
erosion and break-upand iii) the break-upof the vortex               Mcintyre, M.E., On the Antarctic ozone hole, j. Atmos.
at 650 K genera,ted mean equa,torwa, transport,in the
                    a                 rd                                 Terrest. Phys., 51, 29-43, 1989.
opposite directionto the prevailingmeandiaba,tic
                                               circula-                                             of        a.nd
                                                                     Murphy,D.M., et a,l.,Indicators tra.nsport vertical
tion.   This moistens the mid-la.titudes    in the zonal mean.           motion from correlations between in situ measure-
Moist air parcels detected  outsidethe vortexcorrela.tewith             mentsin the AirborneAntarcticOzoneExperiment,
air parcels  havingthe signature the 'edge'a,ndinterior
                                  of                                    d. Geophys.Res., 9d, 11,669-11,685,1989.
of the vortex. Someex-vortexair parcelsgenerA,ted        by          Schoeberl,                           of
                                                                               M.R., et al., The structure the polarvortex,
the erosionof the vortex in November 1991 dilute very                   J. Geophys.   Res., 97, 7859-7882,1992.
rapidly (2-3 da,ys) with the surrounding  drier air and any                    R., nd
                                                                     Swinbank, a. A. O'Neill, A stratosphere-troposphere
impact on the mid-la.  titude zonal-mean 1'120 distribution             data simulation   system,          28
                                                                                                  prej>rint, August1992.
is difficult to detect. Others, however,mea.sured    around          Tuck,A.F., et al., Polarstratosphericcloudprocessed air
35øS,have a muchlongerlifetime (>_ 10 days). These                      and potentialvorticity in the Northern hemisphere
                             of
canbe attributedto an erosion the vortexoccurringin                     lower                at
                                                                              stratosphere mid-latitudes     duringwinter,j.
earlyspringwherethe PV gradienta.tthe vortexedgeis                      Geophys.   Res., 97, 7883-7904,1992.
                                         mixing may
greaterthan in late spring,thus horizonta.1                          Waters, J.W., Microwave                   of
                                                                                                limb-sounding earth's up-
be expectedto be lessefficient. A further effect of the                 per atmosphere,   Atmospheric             23,
                                                                                                       Research, 391-410,
      and break-upof the vortex a.s
erosion                                     by
                                   detected MLS is                       1989.
a movementof air at 0 -- 650 K which is both H20-rich
                                                                       E.S. Cart, R.S. Harwood, W.A. Lahoz, P.D. Ricaud,
and Os-poor(not shown  here), overinhabitedareas:over
South America,in early November 1991 a. over South
                                       nd                                                     EdinburghUniversity,Scot-
                                                                     Department of Meteorology,
                                                                     land UK EH9 3JZ.
Australia in late November 1991.
                                                                        L. Froidevaux,R.F. Jarnot, W.G. Read, J.W. Waters,
                                                                                  Laboratory,
                                                                     Jet Propulsion                 Institute Tech-
                                                                                           California        of
                    We
   Acknowledgements. thank ma.ny              who
                                     colleagues                      nology,Pasadena.,CaliforniaUSA 91109.
               to
havecontributed the MLS projectand in particularto                      C.L. Lau, G.E. Peckham,                       of
                                                                                                R.A. Suttie, Department
the H20 measurements'NASA, the UARS projectoffice;                   Physics,                      Scotland
                                                                             Heriot-Watt University,        UK EH14 4AS.
         at
colleagues JPL, EU, H-WU and RAL; A. O'Neill for
             ana.
meteorological lyses.The work in the UK was funded                                  ReceivedFebruary 1, 1993;
by SERCand NER.C,and in the US by NASA.                                              acceptedMarch 1, 1993.

				
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