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The CD-ROM can be ordered through any rnember ofthe Board of Publishers or through NATO-PCO, Overijse, Belgiurn. Series I: Global Environmental Change, Vol. 50 Springer Berlin Heidelberg New York Barcelona Budapest HongKong London Milan Paris Santa Clara Singapore Tokyo Gravity Wave Processes Their Parameterization in Global Climate Models Edited by Kevin Hamilton Princeton University Geophysical Fluid Dynamics Laboratory Princeton, NJ 08542, USA With 189 Figures (2 Colour Plates) and 15 Tables Springer Published in cooperation with NATO Scientific Affairs Division Proceedings of the NATO Advanced Research Workshop "Gravity Wave Processes and Their Parameterization in Global Climate Models", held in santa Fe, USA, April 1-5, 1996 LIbrary of Congress Cataloging-in-PublicatIon Data Gravity wave processes their parauterlzation in global cllrute models I edIted by KevIn Hamilton. p. cm. -- <NATO ASI series. Series I. Global envlron.ent change : voI. 50) "Published In cooperatIon wIth NATO SClent1flc Affairs Division." "Proceedings of the NATO Advanced Research ~orkshop "Gravity ~ave Processes and Their ParaJleterl;tatlon in Global Cllrute Models". held In Santa Fe. USA. AprIl 1-5, 1996"--T.p. verso. InclUdes bibllographlcal references and index. ISBN.I): 978-)-642-64495-5 c·ISBN-13: 978-)-642-60654-0 001: 10.1007/978-3-642-60654-0 1. Cllrutology--MatheJiatical lIodels--Congresses. 2. GravIty waves--Congresses. 3. Atmosphere, Upper--Mathematlcal lIodels- -Congresses. I. Hnllton. KeVin, 1956- II. North AtlantIc Tr~aty Organization. ScIentIfic Affairs DivIsiOn. III. NATO A.. anced ResearCh rlorkshop "GraVity rlave Processes ahd Their Parneterization in Global Cliute Models' 11996 Sarna Fe. [New MexiCO)) IV. Serles. OC851.G64 1997 551.51·5--dc21 96-37587 CIP ThiswO<1<issubjecttocopyright.Allrightsarereserved.whelherthewholeorparl ofthemalerial is concerned, specifically the righfs of translation. reprinting, reuse of illustrations, rec~ation, broadcasting,reproductiorlonmicfOfilmorInanyotherway,andsloragein<!alabanks.Duplication ofthispublicationorparlsthefeofis perm~ted orYyundertheprovisionsoftheGermanCopyright lawofSeplember9. 1965.in~scurrentversion,andpermissionforusemustalwaysbeoblained from SprinQef-Ver1a9.VIOlations are liable for prosecution l.J"lder theGerman Copyright Law. C Springer·Venag Berlin Heidelberg 1997 SolleO"crn:printoftilehanJe<....:rlSIedition1997 TypeseUing: Camera ready by aulhors/ed,tor Printed on acid·free paper SPIN: 10499104 31/3137·543210 PREFACE Gravitywavescanacttotransfermeanhorizontalmomentumfromthegroundtolevels aloft in the atmosphere or from one layer of the atmosphere to another. Flow over topography can generate stationary gravity waves that break nonlinearly in the troposphereandlowerstratosphere. Suchwavestransfermomentumfromthebreaking region to the earth's surface, and thisprocessis thought to actas a significantdrag on the eastward mean winds in the midlatitude troposphere. It is known that numerical simulationmodels ofthe global atmosphererun withoutany attempt to impose such a drag tend to produce results characterized by unrealistically intense eastward surface windsinmidlatitudes. Otherprocesses(suchasconvection,jetstreaminstabilitiesetc.) can produce gravity waves with nonzero horizontal phase speeds and which act to transfer mean momentum between the troposphere and the middle atmosphere. Comprehensive numerical models of the atmosphere generally produce unrealistic simulations of the extratropical stratospheric/mesospheric circulation unless some accountistakenoftheeffectsofthesegravitywaves. Ifcrediblesimulationsofclimate (andpredictionsofclimateresponsetoanthropogenicforcing) aretobeobtained,some physicallyjustifiableparameterizationofthemomentum transport due tounresolvable gravity waves needs tobeformulated. Thisissueisnowrecognizedasoneofthemost importantchallengesindynamicalmeteorology. Inordertobeginaddressing thisproblemaNATO AdvancedResearchWorkshopwas heldinSantaFe,U.S.A.,fromApril 1-5, 1996. Themeetingwasattendedbyatotalof 36scientistsfrom 10countries. Theworkshopwasnotableinbringingtogetherexperts inobservationsandtheoryofgravitywavesalongwithrepresentativesofseveralclimate modellinggroups. This volumeincludes26papersrepresentingcontributionsfrom 30 ofthe meeting participants and serves as anexcellentsummary ofthe scientificissues discussed. IwouldliketothanktheScientificAffairsDivisionofNATOforprovidingthebulkof the financial supportfor the meeting. Additional support was provided by the World Climate Research Programme as part ofits initiative on Stratospheric Processes and theirRoleinClimate(SPARC) andbytheSolar-TerrestrialEnergyProgram. FinallyI wish to thank the attendees for their enthusiastic participation at the workshop (even through some ofthe very long sessions that were scheduled!) and for submitting the excellentpapersthatmakeupthisvolume. KevinHamilton CONTENTS SOMEPROBLEMSRELATINGTOTHEOBSERVEDCHARACTERISTICSOF GRAVITYWAVES INTHEMIDDLEATMOSPHERE I.Hirota GRAVITY-WAVEPARAMETERSINTHELOWERSTRATOSPHERE R.A. Vincent,S.I.AllenandS.D.Eckermann 7 MEASUREMENTSOFINTERMITTENCYINTHEUPPERSTRATOSPHERE ANDMESOSPHERE R.I.SicaandM.D.Thorsley 27 CLIMATOLOGYANDHYDRODYNAMICSOURCESOFINTERNAL GRAVITYWAVESINTHEMIDDLEANDUPPERATMOSPHERE N.M.Gavrilov 45 OBSERVATIONALSTUDIESOFGRAVITYWAVESASSOCIATEDWITH CONVECTION KS~ ~ EXPERIMENTALCONSTRAINTSONGRAVITYWAVEPARAMETERIZATION FROMINSITUMEASUREMENTSOFTEMPERATUREANDTURBULENCE F.-1Ltibken 69 ANALYSISOFINTERMITTENCYINAIRCRAFTMEASUREMENTSOF VELOCITY,TEMPERATUREANDATMOSPHERICTRACERSUSING WAVELETTRANSFORMS 1T.Bacmeister,S.D.Eckermann,L.Sparling,K.RChan, M.LoewensteinandM.H.Proffitt 85 OBSERVATIONSOFGRAVITYWAVESWITHTHEUARSMICROWAVE LIMBSOUNDER D.L.WuandJ.W.Waters 103 MAINTENANCEOFTHETROPICALOSCILLATIONSOFZONAL-MEAN WINDS INTHEMIDDLEATMOSPHEREBYCONVECITVELY-FORCED GRAVITYWAVES F.SassiandR.RGarcia 121 NUMERICALMODELLINGOFINERTIA-GRAVITYWAVEEMISSIONBY FRONTSANDJETS M.J.ReederandM.Griffiths 137 AMODELOFNON-STATIONARYGRAVITYWAVESINTHESTRATOSPHERE ANDCOMPARISONTOOBSERVATIONS M.I.Alexander 153 THEPROPAGATIONANDBREAKINGOFGRAVITYWAYESEXCITEDBY FORCINGINTHETROPOSPHERE RR.Garciaand1M.Prusa 169 ESTIMATEOFATMOSPHERICDISSIPATIONDERIVEDFROMUARSIHRDI MEASUREMENTS V.A. Yudin,M.A.GellerandB.V.Khattatov 187 VIII ONTHENONLINEARINTERACTIONSBETWEENGRAVITYWAVESIN SHEARFLOWS J. VannesteandF.Vial 199 GRAVITYWAVESPECfRALMODELSANDTHESHAPESOFGRAVITY WAVESPECfRAATLOWVERTICALWAVENUMBERS C.D.WarnerandM.E.McIntyre 217 NONLINEARITYOFTHESATURATIONSPECfRUM C.O.flines 227 SENSITIVITYSTUDIESUSINGTHEIDNESANDFRITTSGRAVITYWAVE DRAGPARAMETERIZATIONS C.McLandress 245 MODELINGTHESEASONALCYCLESANDEQUATORIALOSCILLATIONS WITHAPARAMETERIZATIONOFTHEDOPPLERSPREADGRAVITY WAVETHEORY H.G.Mayr,J.G.Mengel,C.O.flines, K.L. ChanandN.F. Arnold 257 THEREPRESENTATIONOFSUB-GRIDSCALEOROGRAPHYINGCMs F.LottandM.Miller 275 THEEFFECTOFPARAMETERIZEDGRAVITYWAVEDRAGON SIMULATIONSOFTHEMIDDLEATMOSPHEREDURINGNORTHERN WINTER1991/1992-GENERALEVOLUTION B.N.Lawrence 291 THEPARAMETERIZATIONOFGRAVITYWAVEDRAGBASEDONTHE NONLINEARDIFFUSIONOFWAVBSPECfRA A.S.Medvedev,G.P. KlaassenandB.A.Boville 309 EFFECfSOFGRAVITYWAVEDRAGINTHEBERLINTROPOSPHERE STRATOSPHERE-MESOSPHEREGCM S.Pawson 327 THEROLEOFPARAMETERIZEDDRAGINATROPOSPHERE-STRATOSPHERE MESOSPHEREGENERALCIRCULATIONMODEL K.Hamilton 337 SEASONALSIMULATIONSWITHTHECANADIANMIDDLEATMOSPHERE MODEL:SENSITIVITYTOACOMBINATIONOFOROGRAPlflCAND DOPPLERSPREADPARAMETERlZATIONSOFGRAVITYWAVBDRAG N.A.McFarlane,C.~&Landress andS.Beagley 351 MIDDLEATMOSPHERESIMULATIONSWITHTHEECHAM4MODEL: SENSITIVITYTOTHEDOPPLERSPREADGRAVITYWAVB PARAMETERIZATION E.Manzini,N.A.McFarlaneandC.McLandress 367 THEMESOSPHEREINTHEEXTENDEDUGAMPGCM W.A.NortonandJ.Thubum 383 SomeProblemsRelatingto theObserved Characteristics ofGravity Wavesinthe MiddleAtmosphere Isamu Hirota DepartmentofGeophysics Kyoto University Sakyoku, Kyoto606 Japan ABSTRACT. Acritical review is presentedofthe understandingofgravity wave character isticsbasedonearlierobservationalstudiessincethe 1980's. Followingcommentsonthebasic idea ofthe gravity wave dynamics and the capability ofobservational techniques, results of rocketsonde, balloonsondeand MU radardataanalyses are brieflyrecalled. Itis stressedthat the"climatologyofgravity wavesources"is stillopento question,eventhough we havehad somecasestudiesofvarious processesforthe generationofgravity waves inthe loweratmo sphere. Anotherimportantquestionistheverticaldistributionofgravitywaveactivitiesrelating towavebreaking(and/ordissipation)processesinthestratosphereandmesosphere,inconnec tion withseasonal variationsofthe backgroundfield. Finallycritical commentsare added on thecurrentattemptofthegravitywaveparameterizationinGCMsbyemphasizingtheneedfor many moredetailedobservationsinthefuture. 1. Introduction Through the progress made in observing the middle atmosphere in the last three decades, theclimatologyoftheglobalcirculationhas beenestablished(e.g.,CIRAmodel),andourun derstandingofthemechanismforthegenerationandmaintenanceofthemeanzonalwindand temperaturefield hasbeensubstantiallydevelopedin termsoflarge-scaledynamicsas wellas radiativeandphoto-chemicalprocesses. Inadditiontothis,around 1980,itbecamewidelyrecognizedthatverticallypropagating,in ternalgravitywavesplayanimportantroleindeterminingthelarge-scalewindfieldinthemid dle atmospherethrough their momentum transport and deposition. In their pioneering works Lindzen(1981),Matsuno(1982)andHolton(1982)showedtheeffectofthebreakingofgravity wavesonthemeanzonalflow,andinthelastdecadetherehavebeenmanyattemptstoparame terizethegravitywaveeffectinthenumerical modellingofthemiddleatmosphereonaglobal basis. Itshouldbenoted,however,thatthesenumericalmodelsassumeaprioriphysicalparameters ofgravitywavessuchaswaveamplitudesandfrequencies withouthavingsubstantialevidence forthese in the actualatmosphere. Inthe presentpaper,therefore, Ishouldlike toreview our observationalstudiestoclarifywhatpropertiesofgravitywavesareknownandwhatisneeded forfuturestudies. 2. Difficultiesinobservationsofgravity waves Sinceinternalgravity waves have largeramplitudes inassociation withtheir vertical prop- agation in such a way as Ivl '" POI/2 '" exp(z/2H), it is easier to separate them from the NATOAS)Series.Vol.I50 GravityWaveProcesses TheirParameterizationinGlobalClimateModels EditedbyKevinHamilton ©Springer-VerlagBerlinHeidelberg1997 2 backgroundflowandlarge-scalewavesinthemiddleatmospherethaninthetroposphere. MST radars, lidars,rocketsandballoonshavebeenusedasobservationaltoolsforthispurpose. However,thesetechniquesareessentiallybasedonsingle-stationobservationsto obtainthe physicalquantityofgravitywavesasafunctionofheightandtimeatafixed location(theonly exception is the recent satellite observation under development). Because ofthis limitation, observableparametersateachstationareverticalwavelengthandDoppler-shiftedfrequency,in additiontothemeanfield. Horizontalwavelengthandintrinsicfrequency, forexample,haveto beestimatedindirectly with theaid oftheoretical considerations (e.g., dispersion relation) by assumingthedominanceofmonochromaticwaves. Moreover,inviewoftheinhomogeneousdistributionofwavesourcesandintermittentoccur renceofgenerationprocesses, wehavetobecarefulindetectingrepresentativecharacteristics ofgravitywavesinbothcasestudiesandlong-termstatistics. 3. Earlystatisticsformorphologyandclimatology Inordertohaveanoverviewofthegravitywaveactivityinthemiddleatmosphereonaglobal basis,thoughinarathercrudemanner,Hirota(1984)firstpresentedtheclimatologyofgravity waveswiththeaidofmeteorologicalrocketobservationsgatheredatmanystationsoverseveral years. By applying a high-pass filter to daily rocket data in the height range of20 - 65km, windandtemperaturefluctuationswithverticalscale:S lOkmareextracted,andtheirvariances (r.m.s.) wereusedas aroughmeasureofthegravity waveactivity. Alargeseasonal variation witha maximuminwinterathighlatitudesas wellasevidencefor asemiannualcycleatlow latitudeswerefoundfromthesestatistics. Thissortofclimatologyhasbeenrecentlyrevisedby Eckermann,HirotaandHocking(1994)byexpandingthedataperiodto 1977-1987. Hirotaand Niki (1985) madea st.atisticalstudy basedon thesamerocketdatato showthat theshortverticalscalewindfluctuations aredue mainlytoupwardpropagatinginertia-gravity waves, as judged from the rotation ofhorizontal wind vectors in hodographs. Note that the degreeofellipticpolarizationofhodographsgivesusinformationabouttheintrinsicfrequency was wellas thedirectionofhorizontal propagation ofgravity waves. Results ofthe statistics indicatethatthedominantvaluesof f/w (j : Coriolisparameter)fall intotherangeof 0.2'" 0.4inspiteoftheseasonalchangeofthemeanflow. Thissuggestsawidevarietyofhorizontal phasevelocities. Itshouldbereminded, ofcourse,thatrocketsoundingsaretooinfrequenttoresolvethebe haviourofgravity wavesintimeandaretoosparseingeographicaldistributionto resolvethe continent-oceancontrastandthedifferencebetweenthenorthernandsouthernhemispheres. 4. Wavesourcesin theloweratmosphere Ourgoalintheclimatological studyofgravity waves is togivefull informationabout their effectsonthe globalcirculationofthe middleatmospherein terms ofgravity wave forcing of themeanflow throughmomentumdeposition. Thewave varianceshowninearlyclimatology is not necessarilythe forcing itselfbutmerely arough measureofthe dominant structureand behaviourofgravitywavespropagatingfrombelow. Henceournextstepshouldbetoobserve wavegenerationprocessesintheloweratmosphere. Ithas beenassumedthatgravitywavesshouldbegeneratedinthetropospherebytheeffect oftopography,synopticdisturbances,convectiveactivity,geostrophicadjustmentaroundthejet streamandshearinstabilities,butinthesecasesdetailsarepoorlyknownandrelyonafewcase studies. 3 As regards thetopographiceffecton gravity wave generation, Sato (1990) madeadetailed analysisofverticalwinddisturbancesinthetroposphereandlowerstratosphereasobservedby theMUradaratShigaraki,Japaninwintertimewhenthetroposphericwesterliesaredominant. Inastatisticalapproach,sheshowedthattheverticalwinddisturbancesareduemainlytogravity waves generated by the effectofmountain trains located to the westofthe MU radarsite. It is quite interesting to note that, among 8 observation periods within 1985-1987, the gravity waveintensityisnotuniformbutratherintermittent,i.e.,activeperiodsandquietperiodsexist, suggestingthatthegenerationofgravitywavesbytopographyisnotsimplyafunctionofsurface winds. Concerningwavegenerationfromthemid-latitudejetstream,HirotaandNiki(1986)madea casestudyofinertia-gravitywavesaroundtheintensewesterliesoverJapaninwinterusingthe MUradar. Fromtheirhodographanalysis,theyfoundthatthedirectionofhorizontalwindvector rotation,namelythedirectionofverticalpropagation,isoppositeaboveandbelowtheheightof thejetcore(~ llkm). Thisfactstronglysupportsthetheoreticalideaofthejetstreamasawave source. Thisanalysis wasextendedby Kitamuraand Hirota(1989)overtheJapaneseislands (27°-45°N),withtheaidofoperationalrawinsoundeobservationsat18stationsfortheyearof 1986. Resultsofthestatisticalanalysisrevealedthatthegravitywaveswithverticalwavelengths of2~Skmaredominantinthelowerstratosphereandtheirdistributioninameridionalplane along 1400E iscloselyrelatedtothesubtropicaljetoverJapan. Itisalso noteworthythatthis work proves for the first time the usefulnessofroutine balloon observations for the study of gravitywaves. However, recentstudy by Sato (1994) based on 3-yeardata ofthe MU radar claimed that thegeostrophicadjustmentatthejetaxisisnotnecessarilythemaingeneration mechanismof gravitywaves,sincemostoftheseinthelowerstratospherepropagatemeridionallytowardthe jet. Therefore, wehavetosaythattherestillremainsaproblemwiththegenerationprocessof geostrophicadjustmentaroundthejetstream. Anotherimportantcandidategravitywavesourceisconvectionassociatedwithsynoptic-scale disturbances. ByusingtheMUradar,Sato(1993)madeadetailedanalysisofsmall-scaledistur bancesduringthepassageofatyphoon(tropicalcyclone)overJapanandshowedclearevidence ofverticallypropagatinggravitywavesinthelowerstratospherewithacharacteristicperiodof severalhours whichappearedinassociation withthe intense convective motions(rainbands) inthetyphoon. Itisalsonotedthatherresultrepresentsquitetypicalbehaviourbuttheintense convection, observedinthetyphoon, is aratherfragmentary phenomenonin theextratropics. (GravitywavegenerationfromconvectionsinthetropicswillbediscussedbySatointhisvol ume.) AstatisticalstudyontroposphericsourcesofgravitywaveexcitationwasalsomadebyFritts andNastrom(1992)basedondataobtainedaboardcommercialaircraftflyingoverthewestern partofUSAandeasternPacific. Theirstudyshowedtherelativecontributionofvarioussources suchastopography,jetstream,frontandconvection. As was mentionedearlier, the global distribution ofwave sources is, however, quite inho mogeneous inspace and time, and henceourknowledge ofthe "climatologyofgravity wave sources"isstillverypoor. Althoughrecentprogressinsatelliteobservations(cf. WuandWaters, 1996)wouldbepromisingforthispurpose,itisemphasizedthatmanymoredetailedobserva tions,usingvarioustechniquesandcoveringwideregionsoflatitudesandlongitudesandacross long timespan is ofgreatimportance. In this regards, the use ofa global network ofroutine balloonobservationsisrecommended.