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Variations of the Surface Wettability Index over the Tibetan Plateau During 1971-2005

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Variations of the Surface Wettability Index over the Tibetan Plateau During 1971-2005of,the,Index,over,The,index

    Variations of the Surface Wettability Index over the Tibetan Plateau During 1971-2005 V0L.24NO.2ACTAMETEOROLOGICASINICA

    VariationsoftheSurfaceWettabilityIndexovertheTibetan

    PlateauDuring19712005

    2010

    DUJun1,2(杜军),LIChun.(李春),LABa.(拉巴),Luobuciren.(罗布次

    ),andLIAOJian.(廖健)

    1InstituteofPlateauMeteorology,ChinaMeteorologicalAdministration,Chengdu610071 2ClimateCenteroftheTibetanAutonomousRegion,Lhasa850001

    3MeteorologicalBureauofTibetanAutonomousRegion,Lhasa850001

    (ReceivedMarch24,2009)

    ABSTRACI'

    Basedon1971

    2005monthlymeanmaximum/minimumtemperature,windspeed,relativehumidity, sunshineduration,andprecipitationdataat25stationsovertheTibetanPlateau,astudyofthelargest

    potentialevapotranspiration(LPE)isperformedbyusingthePenman

    Monteithmode1.Thesurfacewetta-

    bilityindexfSWI1iscalculatedandexamined,togetherwithitsspacedistribution,interannua1andseasonaI

    variations.aswel1asassociatedcauses.Theresultssuggestthattheannua1arearainfallexhibitsapro-

    nouncedincreasingtrendat15.0mmperdecade;theannualLPEshowsadifferent

    degreedecreaseat

    4.6一一

    71.6ram/10yr.InthesouthwesternNgariprefectureandNyalamcounty,theannualswIdispla

ys

    insignificantdeclinetrendscomparedtoincreasingtrendsinotherareasofTibet(0.02

    0.09perdecade).

    ForTibet.onaverage,theswIexperiencesanoticeableriseat0.04/10yr,particularlyin19812005.Ona

    seasonalbasis.theSWIshowsincreasingtrends,especiallyinsummer.Inthe1970s

    1980s,theinterannual

    variationischaracterizedmainlybylowertemperatureandlowerhumidity.Fromthe1990s,airtemperature

    keepsonrising,leadingtoanappreciableincreaseinSWI,displayedasatypeofwarmandhumidclimate.

    Thesalientincreases(decrease)ofprecipitationandrelativehumidity(meantemperaturedailyrange)are

    theprincipalcausesofthegreatlyenhancedSWIintheregion.Thepronounceddecreaseinmeanwindand

    sunshinedurationalsoplaysanactiverole.

    Keywords:largestpotentialevapotranspiration,surfacewettabilityindex,affectingfactors,trend,Tibetan

    Plateau

    Citation:DuJun,LiChun,LaBa,eta1.,2010:variationsofthesurfacewettabilityindexovertheTibetan

    Plateauduring19712005.AetaMeteor.Sinica,24(2),251258.

    1.Introduction

    Theglobalclimatewarminghasbeenundis

    putable.AspresentedintheFourthAssessmentRe

    portofIPCC.globaltemperaturehasrisenby0.74.C.

    onaverage,forthelastcentury(19062005)and11of

    the12yearssufferingthewarmesttemperatureoc

    curredin19952006since1850.IPCC(2007)claimed

    thatthetemperatureriserateinthepast50yearsis

    almosttwotimesasmuchasthatofthelast100years. Therefore,studyoftemperaturechangebecomesthe leadingtopic.Ontheotherhand.moreandmoresci

    entistsaredevotedtostudyingchangesinthewater regime,thatis,theregionalhumiditypattern(Ciet a1.,2002;Shi,2003;Lieta1.,2003).Studyofwettabil

    itydistributionaimstodeterminetheregionallargest possibleevapotranspirationandthewettabilityindex (ChenandZhang,1996;ZhangandLi,1999).Inre

    centyears,manyChineseresearchershavemadein

    vestigationsonaridityandwetnessinChinafMaand Fu,2001;Jineta1.,2004;Ma,2005;SunandYuan, 2006;Liueta1.,20061.Theydiscoveredthatfrom the1960sto1970stheretookplaceanabruptshift fromawettertoadrierclimate,butwithdifierent intensitiesfromoneregiontoanotherintheCOUntry. Thefluctuationoftheclimateandtheconversionbe

    tweendryandmoistelimatetypesdisplayamarked interdecadalfeature(seeYangeta1.,2003).During 19612000.overeastof100.EinnorthernChina.the borderlinebetweensemi..moistandsemi..aridclimates SupportedbytheNationalNaturalScienceFoundationofChinaunderGrantNo.40865008a

    ndtheCMAChengduInstitute

    ofPlateauMeteorologyunderGrantNo.LPM2008010. tCorrespondingauthor:dujun0891@163.com. (ChineseversionpublishedinVo1.67,No.1,2009) 252ACTAMETEoROLoGICASINICA

    experiencedconstantfluctuationswhilemarchingeast

    ward.Thedivisionlinemadeafarthereastwardand southwardshiftinthe1990scomparedtothatofthe

    1960s,resultingintheexpanded(diminished)semi- aridl(semi-wet)area.Thisimpliesthattheclimateis goingtowardsaridity.Overwestof100.E,theareaof ext'remedrynesshasbeenbeingdiminishingandthe humityindexisontheincrease(Wangeta1.,20o4). LoOkingattheTibetanclimateinthepast30years, wefoundthattemperatureandprecipitationareon theincreasea8opposedtothedecreaseofthelargest potentialevapotranspiration(LPE)overtheplateau, ofllichmostregionsshowthetrendfromadrytoa wetclimate(Wueta1.,2005).Overthemiddleval- leyoftheYarlungZangboRiver,rainfall(LPE)from themid-late1980sto1990sincreased(significantlyde- creased),thusresultinginmoistenedsurfacecondition (Dueta1.,2006).

    Analysisofthe19712005trendsofchangeinsur-

    facehumidityandassociatedcausesovertheplateau allows118todifferentiateecologicalenvironmental propertiesbetweenthepastandthepresentandtoin- vestigatethecausesofchangesintheTibetanecology, therebyprovidingscientificbasisforstudiesonfuture climatechangeandfortheprotectionandrestoration oftheecologythere.

    2.DataandtheSWIdefinition

    Alimitednumberofmeteorologicalstationswith nottoolongobservationhistoryaredistributedover VoL.24

    thevaststretchofTibet.Forobjectiveanalysi~ofthe surfacewettability,weusedthe1971-2005monthly maximum/minimumtemperature,relativehumidity,

    precipitation,windvelocityandsunshinehoursm 25stationsfseeFig.1).

    Theexpressionforthesurfacewettabilityindex (SWI)hastheform

    W=P/ETo,(1)

    whereWistheSWI,Pismonthlyrainfall(ram),and EToismonthlyLPE(ram).FortheLPEcalcula- tiontheFAO(FoodandAgricultureOrganization)- recommendedandcorrectedPenman-Monteithmodel To)Wasutilized,whichisoftheform,

    E=

    900

    ,

    =

    o.77×(0.248+0-752)

    )(0.56-0.08)

    ?

    (0.1+0.9n/N),

    G=0.14(T~一一1),

    (3)

    (4)

    inwhichRnisnetradiationinMJm-2day-1(Zuoet a1.,1993),Gissoilheatflux(MJm.day),,yisa

    humidityconstant.?istheslopeofcurveofsaturation vaporpressure,isthewindat2-mlevel(ms-1), enisactualvaporpressure(hPa),esissaturation vaporpressure(hPa),istheStefen-Boltzmann Fig.1.ThedistributionofselectedstationsovertheTibetanregion

    NO.2DUJun,LIChun,LABa,etal

    constant(4.903x10-9MJK-4mday),,

    (Tmin,)isthemaximum(minimum)temperatureat theabsolutetemperaturescale(K),(正一1)isthe

    meantemperatureforthecurrent(previous)month (.C),n(N)ismeasuredsunshine(possibleinsola- tion)hours(h),andRs.isradiationinclearsky(MJ m2day1.

    Byarithmeticaveraging,theseasonalfeatures oftherelatedmeteorologicalelementsareobtained fromtheseriesofannualrainfall,meantemperature, LPEandSWI,withDecemberFebruarydenotingwin

    ter,MarchMayspring,JuneAugustsummer,and

    September——Novemberautumn.Theclimaticcharac

    teristicsofTibetanSWIinthestudyperiodareex

    amined.

    Thetrendofmeteorologica1variablesisestimated by

    Y=aoait

    253

    whereYstandsforameteoro1ogica1factor,tfortime, a0aconstantterm,andalthelineartrend,with al×10denotingtheclimatictendencyrateperdecade. 3.Analysisofresults

    3.1Precipitationtendencyrate

    Table1showsthatonaverage,the1971-2005 (19812005)rainfallexhibitsapronouncedincreasing trendat15.0f28.8)mmperdecadefortheTibetan region,andthe19712005seasonalprecipitationison theincrease,too,withthesummerincreasereaching 8.8mmperdecade.suchbeingthecasefor19812005

    seasonalrainfallexceptwinter.Andthetrendsseem

    tofurtherintensifvinfuture.especiallyinsummer whichiscrucialtoplantgrowth.AsshowninT1a. ble1.thesummertendencyratereaches21.9mmper decadeinthisseason.

    Table1.Tendencyratesofannualandseasonalrainfall,LPE,andSWIover1971

    2005and19812005

    Note:superscripts?,?and?

    denote,respectively,thesignificanceatthe10%,5%,and1%levels

    3.2Thelargestpotentialevapotranspiration AccordingtotheclimaticLPEtendencyratesin 19712005forallthestationsexceptPurang(Fig.2a), adeclineat4.671.6mmperdecadeisshown.with theresultsfor76%ofthestationsbeingsignificantat the<10%levels.andthemaximaldeclineobservedat Tesdangstation(71.6mmperdecade),nextbeingat

    Zayul(52.9mmperdecade).

    FromthespacedistributionofLPEtrendsona seasonalbasis,itisfoundthatinsignificantincreases (1.63.8mmperdecade)appearinNgariandNyingri prefecturesaswellasChalicountyinspring,incon

    trasttothedecreasingtrendsof1.3一一23.2mmper

    decadeformostoftheotherpartsoftheTibetanre

    gion,withthebiggestdropatTesdang(significantat the1%leve1).Insummer(Fig.2b),allthestations showreducingLPEtrends(1.917.2mmperdecade)

    exceptthePurangandTsonacountiesthatwitnessthe unchangedLPE,withthebiggestdropof17.2mnlper decadeatTesdang.Inautumn,littlechangeinLPE isseeninthesouthernpartoftheShannanprefecture

andBomecounty,buttheLPEdecreasesby1.2

    

    18.4mmperdecadefortheotherareas.withthe maximumdropatTesdang(18.4mmperdecade)

    thatissignificantatthe1%leve1.Inwinter.theLPE trendsincreaseby1.5mmperdecadeintheNyin

    griprefectureandPurangcounty,andformostofthe otherpartsdecliningtrendsoccurby2.2一一14.2mm

    perdecade,with70%ofthestationspassingthesig- nificancetestsatthe10%levelandbeyond. Table1showsthatforthewholeregion,theyearly ACTAMETEOROLoGICASINICAVOL.24

    F.g.2?Yearly(a)andsummer(b)meandistributionsofthe1971

    2005LPEclimatictendencies(mmperdecade)over theTibet.

    averagedLPEin1971-2005isdecreasedby-24.0mm perdecadeandtheseasonaltrendsareallnegativefde

    creasing),withthelargestdropinwinterat-7.5mm perdecade.Alltheseasonsbutwinterin1981-2005 showthattheLPEdecreaseisenhancedby-5.9

    -

    13.3mmperdecade,withthemaximuminsummer, andtheannualLPEisdecreasingby34.3mmper

    decade,suggestingthattheLPEdecliningtrendsin theTibetanPlateauarequitestrong.

    3.sVariationofthesurfacewettabilityindex Figure3depictsthechangeofSWIover1971

    2005,indicatingthatinsouthwesternNgariprefecture andNyalamcountytheSWIexhibitsmarginallyde-

creasingtrendsat-0.01一—0.02perdecadeincon-

    trasttotheincreasingtrends(O.02_0.09perdecade) forotherzones(thefiguresfrom13stationsaresiguif- icantatthe10%levelandbeyond).Thepronounced increaseof>0.06perdecadeinSWIisfoundover theNyingchi,northernQamdo,an

    .

    ,

    deasternNakchu

    prefecture

    Accordingtotheseasonalmeantendencyratesof SWI,Puranginspringshowsadecreaseat-0.05per decadefsignificantatthe10%leve1);inmuchofthe Ngariprefecture,westernNakchuprefectureandSW Shigatse,theSWIchangeslittleincontrastto0.02_ 0.12perdecadeforotherzonesoftheTibetanregion (fourstationswithfiguressignificantatthe<10% No.2DUJun,LIChun,LABa,etal

    Fig.3.Thedistributionof19712005climatictendenciesofSWIovertheTibet leve1),with0.12perdecadeforBomicounty.Insum- mer,theSWIofsouthwestNgari,NyalamandTsona showsdecreasingtrendsf0.0l一一0.03perdecade),

    withthemaximumdropatPurang,whileothersta- tionsdisplayincreasingtrendsrangingover0.020.26 perdecadefthemaximumriseissignificantatthe1% leve1atNyingristation).Inautumn,nogreatchange ofSWIoccursinNgariandBomebutaninsignificant drophappensinNgari.with0.18perdecadeasthe

    biggestdropinNyalam.theincreasesof0.020.21ev

    crydecadeatotherstations,andthelargestincrease

    atthe1%significancelevelatTengchen.Inwinter, forNgariandtheYarlungZangboRiver,LhasaRiver, andNianchuRiverbasinaswellasinNyingri,the SWIchangeswithin0.01bycontrasttotheincreas

    ingtrendsinotherzonesat0.020.19perdecade,the largestfigurebeingforthePagristation. TheaveragedannualSWIin1971——2005displays

    anoticeableincreasingtrend(Table1)of0.04per decade.Particularlyin19812005,theincreaseis

    0.06perdecade.TheseasonalSWIshowsincreasing trends,too,especiallyinsummer(0.07perdecade). TheseasonalSWI(withexclusionofwinter)in1981 2005increasetodifierentdegrees,with0.14perdecade insummer.

    Inaddition,throughtheanalysisofdepartures (Fig.4)foundbyan11yrmovingmethodforsum

    255

    met,winterandannualSwI(denotingthedrynessor wetness1andalsotemperature(forcoldorwarmair), wecometothefollowingconclusions:

    f11Inthesummersofthe1970s,theTibetanSWI islowerandtemperatureiscooler,displayingthein- terannualvariationsfeaturedmainlybyacoldanddry climate.Intheearlyandmid1980s,temperatureis normalandSWIisinnegativedepartures.Fromthe latemid19908tothefirst5yrofthe21stcentury,both temperatureandSWIarehigherthanthemean,ex

    hibitinganinterannualfeaturedominatedbyawarm andmoistclimate.

    f2)Inthe1970-1974winters,thereoccurred

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