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TheGeneralCirculation成都信息工程學(xué)院大氣科學(xué)系陳科藝Reference:/group/dave/at605.htmlTheGeneralCirculationIntroductionAnoverviewoftheobservationsMomentumandEnergyThegeneralcirculationasturbulenceTropicalatmosphere-oceaninteractionsNumericalmodel§1Introduction§1Introduction1.1WhatistheGAC?§1Introduction1.1WhatistheGAC?GACisunderstoodtobeasetofdifferentsynopticsituationswhichareconstantly(ornearlyconstantly),suchasactioncenters,streamcurrents,monsooncirculationetc.)GACoftheatmosphereisusuallyconsideredasanaveraged-on-timestate
oftheatmosphere.GACisalsounderstoodtomeanthecombinationofsomelarge-scalestatisticalcharacteristicsoftheatmosphere.§1Introduction1.1WhatistheGAC?Thecirculationisglobalinextent.Thecirculationtransportsdryair,energy,threephasesofwater,momentum,andtoomanyadditionalquantitiestolisthere.IntheEarth’satmosphere,thecirculationsofenergyandwaterarecloselylinked.Thecirculationisconstantlyfightingagainstfriction,butitissustainedbythermalforcing.Ingeneral,atmosphericcirculationisthelarge-scalemovementofair,andthemeans(togetherwiththeoceancirculation)bywhichheatisdistributedonthesurfaceoftheEarth.§1Introduction1.1WhatistheGAC?Thelarge-scalestructureoftheatmosphericcirculationvariesfromyeartoyear,butthebasicstructureremainsfairlyconstant.However,individualweathersystems–mid-latitudedepressions,ortropicalconvectivecells-occur"randomly“.ZonalandMeridionalFlowZonalflow–normalflowofwinds–flattenedjetstreamMeridionalflow–disturbanceintheflowofwinds–curvyjetstreamMeridionalZonalZonalcirculation:氣流沿緯圈方向運(yùn)動(dòng)稱為緯圈(向)環(huán)流meriodionalcirculation:沿經(jīng)圈方向移動(dòng)稱為經(jīng)圈(向)環(huán)流。Zonalwinds–bloweasttowestorwesttoeastMeridionalwinds–blowsouthtonorthornorthtosouthVideo1Video2fundamentaldrivingforce
TheEarth'satmosphereisputintomotionbecauseofthedifferentialheatingoftheEarthsurface.
TropicsreceivemoreheatthanthepolesEquatorNSHADLEYCELLISATHERMALLY-DIRECTCIRCULATIONSurfacecirculationpublishedbyHalley(1686)EdmondHalley(November8,1656–January14,1742)wasanEnglish
astronomer,geophysicist,mathematician,meteorologist,andphysicist.In1686Halleypublishedthesecondpartofhisexpedition,beingapaperandchartontradewindsandmonsoons.Inthisheidentifiedsolarheatingasthecauseofatmosphericmotions.Healsoestablishedtherelationshipbetweenbarometricpressureandheightabovesealevel.SinglecellmodelGeorgeHadley(1735)Theearthiswarmedattheequator.AirexpandsupwardanddivergestowardthepoleAtthepoleaircoolsandflowsbacktowardtheequatorSinglecellmodelBecausetheearthisrotatingthewindisshiftedtotheright(leftinSH)NotaveryrealisticmodelGeorgeHadley(February12,1685–June28,1768)wasanEnglishlawyerandamateurmeteorologistwhoproposedtheatmosphericmechanismbywhichtheTradeWindsaresustained.HerelatedthedirectionoftradewindstotherotationoftheEarthinhisfamouspaper"ConcerningtheCauseoftheGeneralTradeWinds,"publishedin1735.coolingheating
Westerlymustbesouthwardforthecontinuityofmass.But,observedwindinmid-latitudeisSouthwesterly!(IsHadley‘smodelwrong?)Hadley’sworkwasnotnoticedfor50yearsuntilmentionedbyDalton(1793).coolingheatingSolarheatingistheforcingConservationof‘a(chǎn)bsolutevelocity’driftingwestwardwhenmovingsouthwardFrictionwillslowdownrotationoftheEarth.TheremustbewesterlyinthehighlatitudetomaintaintheEarth’srotation.TheHadleyCell3-cellmodelEachhemisphereisdividedinto3distinctcirculationcells3-cellmodelPolarcellFerrelcellHadleycellEachcellhasassociatedpressureandwindpatterns3-cellmodelITCZ=IntertropicalconvergencezoneCreatestheequatoriallow,rainiestregionsonearthLittlewind–doldrums3-cellmodel20-300SubtropicalHigh–verydryLocationofmanyoftheearth’sdesertsLittlewind–horselatitudesBetween0-200NEtradewinds3-cellmodelSubpolarlow–plentyofrainMidlatitudewesterliesPolarhigh–prettydryPolareasterliesGlobalatmosphericcirculation030306060NSLatitudeHighHighLowLowSurfacePressureSystemsHADLEYCELLSARETHERMALLYDIRECTCIRCULATIONSFERRELCELLSARETHERMALLY-INDIRECTCIRCULATIONSAsare(weak)polarcellsLow030306060NSLowLatitudeHighHighLowLowAtmosphericVerticalCirculationSurfacePressureCenters030306060NSWesterlyEasterlySurfaceWinds§1Introduction1.2Whatmakesitgo?BasicsofAtmosphericCirculationTwoprinciplesHotairrisesAirflowsfromhightolowpressure(gradients)+CoriolisEffect§1Introduction1.2Whatmakesitgo?1.TheEarth’sradiationbudget:An“upperboundarycondition”onthegeneralcirculation.RadiativeBudgetEarthwithoutrotationorcontinentsEquatorheatsupHotairrisesLPatgroundHPaloftPolescoldColdairsinksHPatgroundLPaloft§1Introduction1.2Whatmakesitgo?EquatorheatsupAirrisesLPatgroundHPaloftPolescoldAirsinksHPatgroundLPaloftLPHPHPLPLPLPHPHP§1Introduction1.2Whatmakesitgo?LPLPHPHPLPLargecirculationcellsonsurfaceoftheEarth-airflowsfrompolestowardequatoratgroundlevelEarthwithoutrotationorcontinents§1Introduction1.2Whatmakesitgo?AtmosphericCirculationEarthwithoutrotationorcontinentsThenetradiationatthetopoftheatmosphere,whichisthedifferencebetweentheabsorbedsolarradiationandtheOLR,ispositiveinthetropicsandnegativeinhigherlatitudes.Thisimpliesthatenergyistransportedpolewardsomehow,insidethesystem.Aportionofthisenergyistransportedbytheatmosphere,andtherestistransportedbytheoceans.§1Introduction1.2Whatmakesitgo?-radiationbudgetAddrotation(CoriolisEffect)Earthwithrotation,nocontinentsNetresultCoriolisdeflectsfreelymovingobjectsto:TherightoftheirdirectionofmotionintheNH(clockwise)TheleftiftheirdirectionofmotionintheSH(counterclockwise)§1Introduction1.2Whatmakesitgo?-earthrotation§1Introduction1.2Whatmakesitgo?3.Meridionalenergytransportsbyatmos-oceansystemThecirculationoftheatmosphereandoceanshasamoderatingeffectontheglobaldistributionoftemperature,tendingtowarmthehigherlatitudesandcoolthetropics.However,thesesamethermalcontrastsrepresentasourceofenergy(called“availablepotentialenergy”)thatmakestheglobalcirculationsoftheatmosphereandoceanspossible.§1Introduction1.2Whatmakesitgo?-radiationbudget4.SurfaceboundaryconditionsThepropertiesoftheEarth’ssurface,andtheirgeographicalvariations,stronglyaffecttheGAC.Themostimportantpropertiesarefollows:TemperatureWetnessElevationVegetationSeaiceLandice
§1Introduction1.2Whatmakesitgo?4.SurfaceboundaryconditionsSomeobservationfactsSea/landbreezesSeabreezeLandwarmsfasterinthedaythenwaterCausesairtoexpand,rise,divergeandthuscreatelowpressureAirmovesfromtheseatobalancethepressureLandbreezeAtnightthelandcoolsfasterthanthewaterandthesituationisreversedLand-Sea
BreezesFigure6.19Moonsoons,acontinentalscaleland-seabreezeMountain-Valley
breezesFigure6.20Mountain/valleybreezesValleybreezeDaytimeheatingofthemountaincausesairtowarm,riseandbereplacedbyairfromthevalleyMountainbreezeAtnightthemountaincoolsanairsinksdowntothevalleyFoehnWindsthatflowdowntheleesideofmountainrangesMountainsblockthewind;thiscanbecalledamechanicalforcing.Itwillinfluencethegeographicaldistributionofprecipitation.Mountainscanalsoexertathermalforcing
ontheatmosphere.4.Sur
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