Analysis of the Algerian severe weather event in November 2001 and its impact on ozone and ...

14 Pages·2003·12.02 MB·English

teb301533/teb078 Tellus.cls October1,2003 14:29
Tellus(2003),55B,993–1006 Copyright(cid:2)C BlackwellMunksgaard,2003
PrintedinUK.Allrightsreserved
TELLUS
Analysis of the Algerian severe weather event in
November 2001 and its impact on ozone and nitrogen
dioxide distributions
∗
ByW.THOMAS1 ,F.BAIER2,T.ERBERTSEDER2 andM.KA¨STNER2,1Institutfu¨rMethodikderFern-
erkundung (IMF), and 2Deutsches Fernerkundungsdatenzentrum (DFD), Deutsches Zentrum fu¨r Luft- und
Raumfahrt(DLR),P.O.Box1116,D-82234Wessling,Germany
(Manuscriptreceived1July2002;infinalform14April2003)
ABSTRACT
AnanalysisofthesevereweathereventinNovember2001overthewesternMediterraneanispresented
focusingonsatellite-basedtracegasmeasurementsfromtheGlobalOzoneMonitoringExperiment
(GOME)onboardtheEuropeanRemoteSensingSatellite(ERS-2).Thisstudyissupplementedby
asynopticanalysisandsimulationsofthethree-dimensionalstratosphericchemicaltransportmodel
ROSE.ArcticairmassesmovedrapidlyfromScandinaviatotheIberianpeninsulaandweremixedwith
subtropicalairoverthestillwarmMediterraneanSea.Thiscausedseverethunderstormsandextreme
rainfallalongthecoastsofMoroccoandAlgeriaandlaterontheBalearicIslands.Associatedwiththe
meridionaltransportanintrusionofstratosphericairbelow3kmabovesealevelwasobserved.The
maximumpotentialvorticity(PV)derivedfromUKMeteorologicalOfficeanalysisdatawasabout9.3
potentialvorticityunits(pvu)at330Kattheequatorwardpositionof35◦N.Theseveryhighvalueswent
alongwithremarkablyenhancedtotalozonelevelsobtainedfromGOMEbackscattermeasurements
ofcollocatedGOME/ERS-2overpasses.FurtherinvestigationofGOMEdatashowedunusuallyhigh
levelsofnitrogendioxide(NO2)abovethewesternMediterranean.Wepresentanewmethodtoderive
thetroposphericcontentofnitrogendioxide(NO2)fromacombinationofsatellitemeasurementsand
resultsofachemicaltransportmodel.Weshowthatabouttwo-thirdofthetotalatmosphericcontentof
nitrogendioxideintheobservedplumeisfoundinthetroposphere,duetolightningactivity,advection
andverticaltransportinthethunderstormsfromtheplanetaryboundarylayer(PBL)toatmospheric
levelsaboveclouds.
1. Introduction andnitrogendioxideduringthisweatherevent,both
derivedfromGOMEnadirbackscattermeasurements,
From9to11November2001,aseriesofheavythun- drewourattentiontoitandweanalyseditstemporal
derstormswasobservedoverMorocco,Algeriaandthe andspatialevolutioninthestratosphereandthetro-
BalearicIslandsthatcausedseveredamagetothein- posphere.Wefurtherusedthethree-dimensional(3D)
frastructure,mainlyduetoflooding.Althoughheavy chemical-dynamicaltransportmodelROSE(Roseand
rain and cyclones are typical events in the western Brasseur,1989)tosimulatetransportandthenitrogen
Mediterraneaninwinter,morethan750peopledied dioxide background distribution in the stratosphere
intheflood(“elhemla”),whichwasthesecondworst andtocomparethelatterwithGOMEmeasurements.
naturaldisasterinAlgeriaduringthelast40years.The Anewmethodwasdevelopedthatcombinessatellite
unusualpatternofthespatialdistributionoftotalozone observationsandROSEresultsinordertoderivethe
troposphericnitrogendioxidecontent.
∗Correspondingauthor. This severe weather event originated from an
e-mail:[email protected] arctic cold front along which air masses moved
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
994 W.THOMASETAL.
within two days over several thousand kilometres 2. Data
southward from Scandinavia over the Iberian penin-
sulatoNorthAfrica,wherecyclogenesisstarted.Cy- 2.1. Tracegasmeasurements
clogenesis is a well known process for cut-off lows
Theimpactofasevereweathereventonthechem-
whenairmassesofrelativelyhighpotentialvorticity
ical composition of the atmosphere is analysed us-
(PV) rapidly push down southwards from polar re-
ingbackscattermeasurementsfromtheGlobalOzone
gions.ApproachingtheaxisofthetroughinaRossby
MonitoringExperiment(GOME)onboardtheERS-
waveaconversionofstaticstabilityintoabsolutevor-
2 satellite (see e.g. Burrows et al., 1999 and refer-
ticitytakesplacebystretchingoftheverticalaircol-
ences therein). An enhanced version of the opera-
umn.Theformationofacut-offlowstartsandcoin-
tionalGOMEDataProcessor(GDP)asdescribedin
cidesinthiscasewiththedryintrusionofstratospheric
Loyolaetal.(1997)wasusedtoderivetotalcolumnre-
air,whichoccursfrequently(Kentarchosetal.,1998).
sultsofozoneandnitrogendioxide.TheGOMEspec-
TheseprocessesaretriggeredbyPVanomalies,asre-
trometerisabletomeasuretheatmosphericcolumnar
centlydescribedbyBluestein(1993).Duetothelarge-
contentofanumberofminortraceconstituents(e.g.
scaleRossbywavepatternozone-richairinthelower
ozone, nitrogen dioxide, bromine monoxide, sulfur
stratosphereistransportedsouthward.
dioxide,formaldehyde,chlorinedioxide)between240
Nitrogen oxides play a key role in the formation
and793nm,withspectralresolutionsvaryingfrom0.2
of tropospheric ozone, a photochemical process that
to0.4nm.GOMEisanadir-lookingacross-trackscan-
remarkably influences the chemical composition of
ninginstrumentwithatypicalfootprintsizeofabout
theatmosphere.Thequantitativecontributionsofboth
320×40km2.
naturalandanthropogenictroposphericsourcesofni-
Since GOME pixels cover a relatively large area,
trogen dioxide to the total NO budget are not very
2 the footprints will often be partially or totally cov-
well known, due to sparse ground-based measure-
ered by clouds. Clouds are opaque in the UV/VIS
ments.Here,satellite-basedmeasurementsmaycon-
spectral range (except optically thin cirrus clouds),
tributesignificantlytoprovidetroposphericnitrogen
whichneedtobetakenintoaccountiftracegastotal
dioxide levels on a global scale (Lauer et al., 2002;
columnsareretrieved.ThecloudcoverageofaGOME
Velders et al., 2001; Martin et al., 2002). Recent
footprintisdeterminedbyalinearleast-squaresfitof
airbornemeasurementssubstantiatedthesupposition
GOMEspectratosimulatedspectrainandaroundthe
that considerable contributions originate from light-
oxygenA-bandbetween758and778nm.Weusea
ning in thunderstorms and vertical transport of pol-
relationship between the measured absorption depth
lutedairmasses(i.e.NO -richair)fromtheplanetary
2 in the oxygen A-band, the cloud optical thickness,
boundary layer (PBL) to higher atmospheric levels
the cloud-top height and the fractional cloud cover
(Huntrieseretal.,2002;Brunneretal.,2001).Atthe
ofaGOMEpixelasdescribedindetailbyKuzeand
tropopause level further contributions from air traf-
Chance(1994).CloudcoverageresultsfromGOME
fic have also to be taken into account (Schumann,
can be lower than corresponding results from other
1994).IntheupperstratosphereNO ismainlypro-
2 satellitesensors(KoelemeijerandStammes,1999)but
duced via atomic oxygen from nitrous oxide (N O)
2 comparereasonablywellifopticallythickcloudsare
and is subsequently transported poleward into the
present,asitismainlythecaseinourstudy(seee.g.
lower stratosphere. There, denitrification and trans-
Fig.2).
portintothetropospherecontributetothemainloss
ThecoreelementoftheretrievalpackageisaDOAS
processes.ThedailyaveragedstratosphericNO col-
2 (DifferentialOpticalAbsorptionSpectroscopy)fitting
umn is dominated by available solar insolation and
technique that involves a multi-linear regression of
showsapronouncedlatitudinaldependency,whiletro-
GOME-measured optical densities against a number
posphericNO levelsarehighlyvariableintimeand
2 of reference spectra. It provides trace gas column
space.
amounts along the viewing path of the instrument
The trace gas retrieval technique and the ROSE
(Burrowsetal.,1999).Thedifferentialabsorptionof
model are described in section 2; in section 3 we
trace species along the absorption path is modelled
present the synoptic analysis of the troposphere and
accordingtoBeer’slaw
stratosphere,the3DresultsofROSE,andthederiva-
tionofthetroposphericnitrogendioxidecontent.Our
conclusionisgiveninsection4. dI(λ)=−I(λ)σ(λ)C(s)ds (1)
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
ALGERIANSEVEREWEATHEREVENT 995
where the incremental change of intensity dI(λ) at Acommonwayofcalculatingtheverticalcolumn
wavelengthλthroughaslantpathdistancedsispro- density(VCD)ofaspecies(herenitrogendioxide)is
portional to the absorption cross-section σ(λ) times tocalculatefirstarepresentativeslantcolumnSC
total
the intensity I(λ) and the absorber column amount byaddingthefittedslantcolumnSCandaclimato-
C(s).Whenthereareseveralweakabsorbers,thecon- logicalslantcolumncontentbelowclouds(ifpresent),
tributionsareadditive,providedthatthetotaloptical followedbyadivisionthroughthetotalairmassfac-
density remains small and therefore saturation does torAMF whichistheweightedaverageofclearsky
total
notoccur.Theozoneretrievalwasperformedbetween andcloudyAMFs.Aso-calledghostverticalcolumn
325and335nm,whiletheNO retrievalusedafitting (GVC) below clouds is derived from climatological
2
windowbetween425and450nm.Detailsaboutthe trace gas profiles. The vertical column density now
spectralfittingcanbefoundinBurrowsetal.(1999) reads:
andThomasetal.(1998). SC SC+ f ×GVC×AMF
The resulting trace gas slant columns are then VCD= AMFtotal = f ×AMFc+(1− f )×AMc F
converted to geometry-independent vertical column total c c c g
(3)
amountsthroughdivisionbyappropriateairmassfac-
tors (AMF). Air mass factors describe the enhance- wheref isthecloudcoveragebetween0and1,SCthe
c
mentoftheabsorptionofagiventracegasduetoslant fitted slant column, GVC the ghost vertical column,
pathsofincidentlightintheatmosphere: and AMF and AMF the air mass factors down to
g c
τ groundandtocloud-top.
AMF= slant (2) Recent studies (Lambert et al., 1999; 2000) com-
τ
vert paringGOMEandTOMSdataagainstground-based
whereτ ,τ aretheslantandtheverticaloptical measurementsindicatethattheagreementofGOME
slant vert
densities,respectively.Theslantopticaldensities,i.e. totalozonecolumnsiswithin2–4%forsolarzenith
theAMFs,arederivedfromradiativetransfercalcula- anglesbelow70◦,whichisthecaseinourstudy.Total
tionsanddonotrequireinputfromGOMEmeasure- nitrogendioxidecolumnsderivedfromGOME,other
ments other than viewing geometry and geolocation satellite instruments (HALOE, POAM) and ground-
information. However, AMFs depend further on the basedmeasurementsagreeingeneralwithin5–20%,
albedo of the underlying surface and the altitude of but within ±5×1014 molecules cm−2 in areas of
thereflectingsurface,whichcanbeeithertheground lowNO intheplanetaryboundarylayerandwithin
2
orthecloud-top,andatmosphericprofilesoftemper- ±8×1014 moleculescm−2 inareasofverylowslant
ature,pressureandtracegasconcentration.Radiation columnsofnitrogendioxide.Recentstudies(Richter
propagationthroughcloudsisnotconsidered.Instead, andBurrows,2002;Martinetal.,2002)showedthat
clouds are treated as reflecting boundaries in the at- GOMEnitrogendioxidemeasurementspartiallysuf-
mosphere. ferfrominstrumentalartefactsthatmaycauseatem-
Ozone AMFs were calculated on the basis of the porallyvaryingoffsetintheslantcolumnsupto2×
TOMS V7 ozone profile climatology (Klenk et al., 1015 molecules cm−2 depending on the time of the
1983;Chuetal.,1989)usingLIDORT(Spurretal., year.Thesestructures,however,appeartobesimilar
2001),andareobtainedfromlook-uptablesbyneural fromyeartoyear.ForNovember2001,aslightunder-
networktechniques.Thetotalozonecontentisderived estimationofretrievedNO slantcolumnsaround4×
2
fromaniterativeprocedurethatsearchesforthebest 1014moleculescm−2canbeestimatedusingtheoffset
suitedozoneprofile,i.e.theAMFasfunctionofthe determinedfor1998/1999byMartinetal.(2002).
integratedprofilecontent(andanumberofothergeo-
physical parameters, as mentioned above). A differ-
2.2. ThechemicaltransportmodelROSE
entapproachisfollowedfornitrogendioxideAMFs,
which are calculated in two steps. A fast radiative The spatial background distribution and the
transfermodelcalculatesAMFson-line,considering temporal evolution of stratospheric ozone and ni-
onlysinglescatteringeffects.Themultiplescattering trogen dioxide are simulated using the 3D global
isthenaccountedforbymultiplicativecorrectionfac- chemical-transportmodelROSEasdescribedindetail
torsthatwerederivedfromradiativetransfersimula- in Rose and Brasseur (1989), Granier and Brasseur
tions(Rozanovetal.,1997),restoredfromalook-up (1991)andRieseetal.(1999).Themodelcoversthe
table. relevant gas-phase stratospheric chemical processes.
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
996 W.THOMASETAL.
Heterogeneous processes on polar-stratospheric Thisdatasetdescribesclimatologicalzonalmean
clouds and on sulfuric aerosols are also included distributionsofasub-setofspeciesrelevantforstrato-
in the model. It accounts for about 100 reactions, sphericconditions.Themodel’stropopauseisdefined
including oxygen, hydrogen, carbon, nitrogen, by a potential vorticity of 1.6 pvu (see section 3.2)
chlorine and bromine species. The chemical rate or a potential temperature of 380 K for the tropics.
constants and cross-sections are taken from Sander In order to investigate the impact of the tropopause
et al. (2000). Photolysis rates are derived from a heightmodelrunswerealsoperformedforadifferent
look-uptabledependingonsolarzenithangle,ozone tropopauseheightcorrespondingto3pvu(seesection
columnandaltitude.Thechemicalrateequationsare 3.4).Themodelrunscoveringtheperiod6–14Novem-
solved by considering a chemical equilibrium state ber2001wereinitialisedwithdatafromtheDFDnear-
fortheshort-livedspecies(e.g.ClO,NO,HO,BrO). real-time(NRT)service(http://auc.dfd.dlr.de/ROSE).
Asemi-implicitschemeisusedfortheintegrationof ThisNRTservicedeliversverticalozoneprofilesusing
the more stable reactants (e.g. HNO , N O, NH ). GOMEtotalozonecolumndensitiesthatareassimi-
3 2 4
All short-lived species are grouped and integrated latedintoROSEonanoperationalbasis.
using families, e.g. ClO =Cl+ClO, NO =NO+
x x
NO . All long-lived species are transported using
2 3. Results
the semi-Lagrangian scheme of Smolarkiewicz and
Rasch(1991).Windandtemperaturefieldsarederived
3.1. Synopticanalysis
from24-hanalysesoftheUKMeteorologicalOffice
(UKMO) following Swinbank and O’Neill (1994), The meteorological situation during the weather
whichareavailableforpressurelevelsfromgroundup event is characterised by strong surface winds and
to0.3hPa.Thisdatasetdefinesaconsistentsynoptic heavyrainfall.Figure1(leftpanel)showsanintense
state using satellite-based temperature soundings upperleveltroughthatpushesfartotheSouthwestof
and radiosonde observations assimilated in a global EuropeandmeetstheAlgeriancoastfromweston9
circulationmodel(GCM). November2001.TheUKMOanalysisofthetempera-
ForthisstudyweuseaROSEversionwitha5.6◦× tureandthegeopotentialheightisgivenatthe464hPa
5◦longitude–latitudesphericalgridon43log-pressure level(∼6km).TheMETEOSAT-7IR-imagefromthe
levelsbetween0and60kmaltitude,resultinginaver- sameday(Fig.2,leftpanel)showsthatthecloudsys-
ticalstepsizeof1.3km.Inthemodel’stroposphere tems of the frontal zone extend from Finland across
all chemical species are relaxed to 2D model data CentralEuropetoSpain.Thecoldfrontmadethesur-
(Brasseuretal.,1995)withatimescaleof10d. facetemperaturesinAndalusia(Spain)dropatmidday
Fig.1. UKMOanalysisofthetemperatureinK(greyscale)andthegeopotentialheightingpm(isolines)at464hPa(∼6km)
at12:00UTCfor9November2001(leftpanel)and10November2001(rightpanel).
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
ALGERIANSEVEREWEATHEREVENT 997
Fig.2. METEOSAT-7IRdataat12:00UTCfor9November2001(leftpanel)and10November2001(rightpanel);A,
Algiers.
from25to18◦Candtofrostatnight.Heavyrainfall overpass time over the area is around 10:20 h local
beginsattheAlgeriancoastonlate9November2001 time(herecloseto10:20UTC)oneachdayduetothe
and ends at about noon on 10 November 2001. Cy- sun-synchronous polar orbit of ERS-2. High gradi-
cloniccurvatureandshearvorticitydefinethepositive entsintotalozonecolumnfrompixeltopixel(several
vorticityadvection(PVA)attheAlgeriancoast.This 10DU)indicatestrongdynamicactivityoverEurope.
PVAimpliesadeepeningofthelow-pressuresystem Asearlyasin1950Reedshowedthatthetotalozone
and a cut-off low developed with maximum poten- columnisanexcellenttracerforupperlevelweather
tialvorticityinthatarea(Fig.1,rightpanel).Onits systems modulated by tropopause and lower strato-
east side very warm lower air from the tropics was sphere variability. A well suited synoptic diagnosis
mixed with higher cold air of arctic origin, leading quantityisthecorrespondingisentropicPVat330K
to a highly unstable air mass and initiating a cyclo- (Hoskinsetal.,1985)whichisdepictedforeachday
genesis with a tropical warm core which is unusual on the right panels of Fig. 3. PV is a conservative
attheselatitudes.Twoprocessesintensifiedthecon- quantityinfrictionlessandadiabaticflowandtheval-
vectivedevelopment:(1)thecoldmaritimearcticair uesonthe330Kisentropecorrelatewellwiththeto-
crosses over the still warm Mediterranean, where it talozonecolumnatmidlatitudes(Hoodetal.,1999).
picksupmoistureandmeetsmaritimesubtropicalair Theessentialphysicsoftherelationshipbetweentotal
and(2)thestrongsurfacewindsfromnorth-westto- ozoneandvorticityisdescribedinVaughanandPrice
wardsthemountainousAfricancoastcausedintense (1991).
orographicrainfall.Figure2showsthecloudsrotating Atthebeginningoftheinvestigatedperiod,weob-
aroundAlgiersduetothecut-offlowprocess.Onboth servelowozonelevelsbelow200DUoverNorthern
days,9and10November2001,extraordinaryrainfall Europeon9and10November2001forminganozone
ratesof126Lm−2in12hand136Lm−2in6hwere mini-hole(Figs.3aandc).Itisaccompaniedbyahigh-
reported which led to the flooding disaster in Alge- pressuresystemwhichischaracterisedbyanelevated
ria.Theregionalmaximumrainratein6hinAlgeria tropopause and a negative PV anomaly and governs
wasmorethanthe30yearsmonthlymeanNovember the large-scale motion field (Fig. 3b). Hence, on its
rainfallrateof129Lm−2. frontside(east)anupperleveltroughisformingover
Scandinavia and the Baltic. It moves eastwards and
rapidlyexpandsinasouth-westwarddirectionleading
3.2. OzoneandPVanalysis
toenhancedozonelevelsoverSpain(Fig.3c).While
ThetotalozonedistributionasderivedfromGOME the upper-level trough deepens, the tongue of high
backscattermeasurementsbetween9and11Novem- PV elongates and thins with a pronounced NE–SW
ber2001isshowninFig.3,leftpanels.TheGOME axis (Fig. 3d). The formation is due to equatorward
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
998 W.THOMASETAL.
Fig.3. GOME-measuredozonedistributionon9November2001(a),10November2001(c),11November2001(e)and
corresponding isentropic potential vorticity on the 330 K surface derived from UK Meteorological Office stratospheric
analysesforthesamedaysat12:00UTC(b,d,f).
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
ALGERIANSEVEREWEATHEREVENT 999
Rossbywavebreaking(McIntyreandPalmer,1985). mosphere. The dynamic tropopause can be defined
From10to11November2001,thestructureformed asanisosurfaceofpotentialvorticity.Hoerlingetal.
byhighPVlevelsbreaksupandthetipofthetongue (1991)discusseddifferentthresholdvaluesfrom1to
separates, rolling up cyclonically, forming a cut-off 3.5pvuforthedefinitionofthedynamictropopause
lowcentredoverMorocco(Fig.3f).Thecoreshows whiletheWMOrecommendedthe1.6pvuisolineat
high PV values of about 9.3 pvu (1 pvu = 10−6 K midlatitudes (WMO, 1985). According to Price and
m2 kg−1 s−2), indicating a reservoir of polar strato- Vaughan(1993),thisisreasonablewhenstudyingcut-
sphericairmassesataverypronouncedsouthernloca- offlows,asitisthecasehere.However,outsidethe
tionof35◦N.AssociatedwiththeequatorwardRossby cut-offlowthe3pvuisolinemaybeamorerepresen-
wavebreakingeventweobservehightotalozonecol- tativevalue.Inourcasestudythe3pvuisolineisabout
umn values over Southern Spain, Morocco, Algeria 100hPahigherthanthe1.6pvuisolineintheareawith
andtheMediterraneanSea(Figs.3cande).Theto- thecut-offlow,whiletheeffectislesspronouncedout-
tal ozone content at these low latitudes increases by sidewherechangesarebelow25hPa.Theimpactof
about 100 DU, from 280 DU on 8 November 2001 differenttropopauseheightsisdiscussedinmorede-
to380DUon10November2001.On11November tailsinsections3.4and3.5.Thetropopauselowering
2001themaximumtotalozonehasdecreasedagainto withintheupperleveltroughleadstoanincreaseof
355DU. thestratosphericairreservoirandcontributestoanin-
Inordertoinvestigatetheequatorwardlocationof creaseofthetotalozonecolumnamounts.Bymeans
thePVmaximumfromaclimatologicalandstatisti- ofthePVasastratospherictracer,thefoldingshows
cal point of view, 11 years of UKMO analysis data isentropicflowtoitswesternflank,indicatingstrato-
wereexamined.Potentialvorticitywasdeterminedon sphericintrusion.AsdepictedinFig.3dthetipofthe
adailybasisfrom1992to2002at35◦Nandforalllon- equatorwardpointingPVstructurebrokeawayon10
gitudesonthe330Kisentrope.DuringthisperiodPV November2001,formingacut-offlow.Forthatday,
valuesreachedasimilarlevelonlyonfourdays,which Fig.4(leftpanel)showsanorth–southcross-section
underlinesthestrongmeridionaltransportduringthis ofPVandpotentialtemperaturealongtheGreenwich
outstandingweatherevent. meridianfrom20◦ to60◦N,basedonanalysesofthe
Besideshorizontaltransportphenomenawealsoex- EuropeanCentreofMediumRangeWeatherForecast
amined changes in the vertical structure of the at- (ECMWF).
Fig.4. PV north–south cross-sections for 10 November 2001 at 0◦W (left panel) along the Greenwich meridian and
METEOSAT-7 water vapour image (right panel) of 10 November 2001 11:30 UTC. Potential vorticity isocontours are
inwhitewhileisentropesareinblack.The1.6pvuisoline(thickblackline,leftpanel)denotesthedynamicaltropopause.The
nextisoline(white)correspondsto3pvuandthedifferenceto1.6pvuisshadedinbrightgrey.Thegeographicalposition
ofAlgiersismarkedbytheAat36◦Nintheleftpanel.TheGreenwichmeridianat0◦Wismarkedbytheblacklineinthe
METEOSAT-7image(Source:UKMetOffice).
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
1000 W.THOMASETAL.
The cyclogenesis around Algiers (36◦N) is asso- 3.4. ROSEsimulationsofnitrogendioxide
ciatedwithasignificantenfoldingofthetropopause
ROSE simulations starting on 6 November 2001
wherestratosphericozone-richairintrudesto700hPa
were performed to compare simulated stratospheric
on10November2001andsplitsupintotwotongues
nitrogendioxidedistributionswithcoincidentGOME
inthecross-section(Fig.4,leftpanel).Thisisdueto
observationsduringtheperiodofinterest.Themodels
thecyclonicspinofthefoldaroundtheformingcut-off
tropopauseisdefinedasdescribedinsection2.2using
low,whichisconfirmedbytheMETEOSAT-7water
the1.6pvuorthe380Kpotentialtemperatureisoline.
vapourimagefrom10November2001at11:30UTC
ThedifferencesbetweenGOMENO totalcolumnre-
(Fig.4,rightpanel).Thedryintrusionofstratospheric 2
sults and modelled stratospheric results can then be
airintothecycloneisapparent:itformsastreamerin-
usedtoestimatethetroposphericNO contentwhich
dicatedbyadarknarrowbandreachingstraightfrom 2
isfurtherdiscussedinsection3.5.
RussiatoMoroccoandthenspinningcyclonically.
Toseparatetheeffectoftransportfromlocalchem-
ical transformations we analysed the temporal de-
velopment of total ozone and a tracer (chemically
3.3. Nitrogendioxideobservations
passive ozone, initialised with ozone values from 5
TheNO distributionderivedfromGOMEmeasure- November 2001) for a fixed model location at 0◦E,
2
ments does not show a similar temporal and spatial 40◦N.Airmasseswithhighozoneloadingweretrans-
pattern as ozone and PV in the same period. How- ported southward, as confirmed by PV analysis (see
ever,on10November2001,enhancedNO levelsare section 3.1). We observe a maximum increase of
2
observedalongtheCantabriccoast(Spain)andover the simulated total ozone column during 8 to 10
Morocco(alsooverMacedonia),whichcoincideswith November 2001 and these results compare qualita-
night-timelightningactivity(Fig.5,upperpanels).En- tively well with coincident GOME observations. By
hancedNO concentrationsintheuppertroposphere comparingthetemporalbehaviourofozonewiththat
2
causedbylightningactivityhaverecentlybeenmea- ofthetracerweestimatethatchemicaltransformations
suredbyHuntrieseretal.(2002);theseplumesmay locallyonlycontribute5%tothesimulatedchangeof
persistforseveralhours.Anotherareawithhigherni- thestratosphericozonecolumn.
trogendioxidelevelsisvisibleoverFrancewhichcan Recent comparisons of ROSE results with obser-
beattributedtotroposphericpollution.Here,GOME vations showed that the NO temporal behaviour is
2
cloud coverage results indicate a low cloudiness of reasonably well described by the model (Sen et al.,
below20%providingsatellitemeasurementsdownto 1998;Toon,1991).MaximumdailyNO columnsare
2
surfacelevels.On11November2001,duringandafter simulatedaftersunset(atsurfacelevels)beforeNO
2
heavythunderstorms,anNO2plumeisobservedover isconvertedmainlyintoitsnight-timereservoirN2O5.
the western Mediterranean Sea around the Balearic MinimumNO columnsarepresentaftersunrisewhen
2
IslandsandtowardstheNorth-Africancoast(Fig.5, mostoftheNO isquicklyphotolyzedintoNO,thus
2
lowerleft). loweringthetotalcontentbyoneorderofmagnitude.
Beforethecoldfrontreachedtheareaon8Novem- StratospherictotalNO levelsslightlyrecoverduring
2
ber2001thetypicalNO verticalcolumnwasaround day time, typically by about 1×1014 molecules
2
(2.5–3.5)×1015 ±3.5×1014 moleculescm−2,but cm−2 h−1. To evaluate the model performance we
higheroverFrance,whileanunusuallyhighmaximum compare the simulated minimum NO values with
2
totalcontentof7.27×1015 ±4.4×1014 molecules available HALOE November observations in the
cm−2wasobservedon11November2001.Anegative midlatitudes (Gordley et al., 1996) for sunrise con-
biasofabout−1.4×1014moleculescm−2needtobe ditions (http://haloedata.larc.nasa.gov/Haloe/home.
consideredtocorrectforpossibleartefactsinGOME html).AdirectcomparisonofthespatialNO distri-
2
backscatterdata(Martinetal.,2002).Thegivenerror butionswithindependentspace-bornemeasurements
levelsarederivedfromtheslantcolumnfittingpreci- iscompromisedduetomissingdataduringtheperiod
sion(i.e.standarddeviation),thecorrespondingcloud ofinterest.ROSENO minimumtotalcolumnvalues
2
fittingprecisionandanassumedprecisionofAMFsof above 20 km altitude compare well with HALOE
1%whichisduetoradiativetransfermodelling.Errors measurements with model values being slightly
ofreferencespectrawerenotconsidered.Theorigin higher. Model results are in excellent agreement
oftheNO plumeisfurtheranalysedinsection3.5. with HALOE observations near the stratospheric
2
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
ALGERIANSEVEREWEATHEREVENT 1001
Fig.5. GOME-measurednitrogendioxidedistributionon10November2001(upperleft)and11November2001(lowerleft),
andlightningstatisticsforbothonedayearlier,the9November2001(upperright)and10November2001(lowerright).
SlightlyincreasedcolumndensitiesareobservedoverthestraitofGibraltarandalongtheCantabriccoaston10November
2001(upperleft),wherebothnight-timelightningactivitywasreported.On11November2001,unusuallyhighNO2column
densitiesofabout7.2×1015moleculescm−2weremeasuredovertheMediterraneanSea(lowerleft).Lightningstatistics
weretakenfromhttp://www.wetterzentrale.de.
NO maximum around 30 km altitude. Modelled At the GOME overpass time during the period of
2
NO columns near the 100 hPa level (∼16 km) interest the temporal changes of NO due to chem-
2 2
tend to be higher than corresponding HALOE ical transformation are typically low. ROSE strato-
values, but deviations stay within the known bias spheric NO columns calculated at 10:00 UTC de-
2
of HALOE. The simulated average zonal variability creasesmoothlyfrom2.1×1015moleculescm−2on
of 10% for the NO column corresponds well with 6November2001to1.6×1015moleculescm−2on11
2
HALOEobservations. November2001at40◦N0◦E,whereGOME-derived
Tellus55B(2003),5
teb301533/teb078 Tellus.cls October1,2003 14:29
1002 W.THOMASETAL.
Fig.6. Totalstratosphericnitrogendioxidecontentfor10November2001(leftpanel)and11November2001(rightpanel)
ascalculatedbyROSEatGOMElocaloverpasstimes(10:20LT).Themaximumstratosphericcontentisabout2.6×1015
moleculescm−2overtheSaharandesert,whileitisaround1.8×1015moleculescm−2intheareasouthwardoftheBalearic
IslandswheremeasuredNO2valueswerehighest.Theday-to-dayvariationinthelatterarearemainssmall.
NO levelswerehighest.Therefore,thetemporalres- November2001(seeFig.5,lowerleft)doesnotap-
2
olutionofUKMOanalysisdataofabout24hissuf- pearinthesimulations.Note,however,thatROSEdoes
ficientlyhightodescribetheimpactofstratospheric notconsidertroposphericemissions(section2.2).The
transport.Thevariationfrom10to12November2001 troposphericNO contentisfurtherdiscussedinthe
2
islowerthan5×1014moleculescm−2.ROSENO col- followingsection.
2
umnresultsatGOMEoverpasstime(10:20LT)on10
and11November2001areshowninFig.6.
3.5. Derivationandanalysisoftropospheric
TheROSE-modelledspatialdistributionofNO is
2 nitrogendioxide
dominated by the bulk of NO in the stratosphere
2
around30km.TherelativelylowNO columnsover Severalmethodstoderivethenitrogendioxidecon-
2
NorthernandCentralEuropeandevendowntoSpain tent in the troposphere are discussed in the litera-
on10November2001canbeattributedtowave-like ture. Velders et al. (2001) apply both the so-called
transport in stratospheric levels at higher latitudes. tropospheric excess method (Richter and Burrows
Measured ozone fields and PV analysis indicate a 2002) and image processing technique to derive the
morepronouncedmeridionaltransportinloweratmo- troposphericNO columnarcontent.Thetropospheric
2
sphericlevelsespeciallyon9November2001(Figs.3a excessmethodallowsonetoseparatetroposphericand
andb).ThesimulatedNO columncontentduringthe stratospheric contributions to the total NO content
2 2
GOMEoverpasstimevariesbetween(0.9–2.6)×1015 under the assumption of a relatively stable (both in
moleculescm−2fromNorthtoSouth,whichislower time and space) NO distribution over the free Pa-
2
thancorrespondingGOMEmeasurementsoutsidethe cificOcean.Laueretal.(2002)comparetropospheric
NO plume over water surfaces and high latitudes. NO columnsderivedfromGOMEmeasurementsand
2 2
The strong increase of measured NO levels on 11 results of a coupled chemistry–climate model. The
2
Tellus55B(2003),5