Stationary wave dynamics.pdf

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1、9StationaryplanetarywavedynamicsThischapterisconcernedwiththelineardynamicsofstationaryplanetarywaves.WebeginbybrieflyshowingsomeobservedkeycharacteristicsofsuchawavefieldinSection9.1.Section9.2discussestheconceptualissues.Wedelineatethedynamicalimpactsofazonalmea

2、nflowonthepropagationofstationaryplanetarywavesinSection9.3.TheexcitationofastationarywavefieldbyalocalizedthermalforcingorbyalocalizedorographyisanalyzedseparatelyinSections9.4and9.5withasimplestpossiblemodel.Ineachcase,wedelineatetheimpactsoftheindividualdynamic

3、alfactorssuchasthebeta-effect,thetypesofshearandthedirectionofthebasiczonalflowontheforcedwavefieldbeforeweexaminethecombinedinfluenceofalloftheminthevarioussubsections.TherelevanceofthismodelisfinallyillustratedwithasimulationstudyofthesummermeanAsianmonsoonalcircu

4、lationinSection9.6.9.1ObservedcharacteristicsofstationaryplanetarywavesTheannual,seasonalorevenmonthlymeanweathermapsatdifferentlevelshaveaverybroadwavystructure.Suchaflowisstationarybydefinition,@=@t¼0,withrespecttoanannual,seasonalormonthlytimescalerespectively.

5、Stationaryplanetarywavesareconsiderablystrongerinwinterthaninsummersimplybecausetheforcingsthatgiverisetothemaredistinctlystronger.WeillustratethewinterplanetarywavefieldwiththeJanuarymeanflowoverthenorthernhemispherein2003.TheplotsareconstructedusingtheNCEP/NCARR

6、eanalysisDataset.Thehorizontalstructureofthe250mbgeopotentialheightfield,Z250,andthecorrespondingdeparturefieldfromthezonalmean,Z250,areshowninFig.9.1a,b.ThetotalflowisparticularlystronginmiddlelatitudesasevidencedbythetwopronouncedtroughsinFig.9.1a.Associatedwith

7、thesetroughsarethetwomajorlocalizedwintermeanjets.WeseeinFig.9.1bthattheJanuaryZ250valuesaregreatestaround50N,varyingbetween350mand350m.Thevaluessouthof~20Narequitesmall.ItisinstructivetodecomposetheZ250fieldateachlatitudeintoFouriercomponents,ZðÞm;,wheremis

8、thezonalwavenumberandthelatitude.Thedistributionoftheamplitudeofthesecomponents,jjZmðÞ;,asafunctionofmandisshowninFig.9.2.Weseethatthequasi-stationarywavesaremuchs