TheGeneralCirculation成都信息工程學院大氣科學系陳科藝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:氣流沿緯圈方向運動稱為緯圈（向）環流meriodionalcirculation:沿經圈方向移動稱為經圈（向）環流。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‘absolutevelocity’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