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龍卷、雷達探測和預(yù)警南京信息工程大學(xué)大氣物理學(xué)院什么是龍卷一個自地面向上伸展的劇烈旋轉(zhuǎn)氣柱Aviolentlyrotatingcolumnofair,usuallypendanttoacumulonimbus,withcirculationreachingtheground.通常由管狀云生成,伴隨著巨大的風(fēng)聲
Itnearlyalwaysstartsasafunnelcloudandmaybeaccompaniedbyaloudroaringnoise.是破壞力最強的大氣現(xiàn)象,但影響范圍很有限
Onalocalscale,itisthemostdestructiveofallatmosphericphenomena.龍卷的風(fēng)速和破壞力我國2004-2008期間的龍卷情況龍卷的破壞力等級—Fujita分級龍卷風(fēng)的形成—水平風(fēng)隨高度的切變水平風(fēng)隨高度的切變、觸發(fā)氣流的旋轉(zhuǎn),形成龍卷超級單體的結(jié)構(gòu)—包含龍卷風(fēng)雷暴云的圖像雷暴云的圖像龍卷風(fēng)的圖像龍卷風(fēng)的圖像美國Ohio州1974-4-3的一次龍卷風(fēng)Bluestein:June102004Nebraska龍卷風(fēng)美國奧克拉荷馬的一次多龍卷情況(May111982Friendship,OK)1999年5月3日德克薩斯龍卷風(fēng)的剖面結(jié)構(gòu)龍卷風(fēng)場特性的簡單模式RankineVortex模式:旋轉(zhuǎn)平衡:中心區(qū)域切向風(fēng)速:外圍切向風(fēng)速:中心氣壓差:實驗室模擬VortexChambers蘊含龍卷的雷暴單體模型蘊含龍卷的雷暴單體模型超級單體(龍卷?)的雷達回波情況Markowskietal.MWR20022003年7月8~9日安徽無為廬江一帶的龍卷個例03-07-0810:31AnhuiHubeiJiangsuHenan03-07-0813:58HenanHubei安徽Jiangsu03-07-0820:00HubeiJiangsuHenanAnhui03-07-0820:3403-07-0821:1203-07-0821:2403-07-0821:5703-07-0822:3222:38LujingWuwei22:49LujingWuwei22:55廬江無為SupercellConceptualModel(LemonandDoswell1979)23:01廬江無為23:0723:12LujingWuwei23:1223:1223:18LujingWuwei23:23LujingWuwei23:2323:2323:29LujingWuwei23:29SRR0.503-07-0823:2923:35Wuwei23:45Wuwei23:57Wuwei00:16WuweiHanshan00:27WuweiHanshanLujing00:27HanshanWuweiLujiang2015-6-1翻船事故、雷達回波12015-6-1翻船事故、雷達回波22015-6-1翻船事故、雷達回波32015-6-1翻船事故、雷達回波42015-6-1翻船事故、雷達回波52015-6-1翻船事故、雷達回波62015-6-1翻船事故、雷達回波62015-6-1翻船事故、雷達回波62015-6-1翻船事故、雷達回波72015-6-1翻船事故、雷達回波8雙線偏振雷達探測龍卷ElRenoF5May242011強龍卷的風(fēng)速情況Whatisasquallline?Whatisasquallline?Asqualllineisasystemofthunderstormsthathaveformedintoaline.Thisoftenoccursaheadofacoldfront,wherewindshearcombinedwithunusuallywidespreadliftingoftheloweratmospherecausesconvectiontobecomearrangedinabandedstructure.Thelineitselfwillberelativelynarrow,anditisusuallyfollowedbyamuchlargerareaoflighraintrailingbehindit.Whileasqualllinecanhaveheavyrain,highwinds,andsmallhail,tornadoesandlargehailaremorecommonlyassociatedwithisolatedthunderstorms,notsqualllines.Whatisasquallline?Whatcausesthunderstorms?Thunderstormsformwhenanairmassbecomessounstablethatitoverturns(convects)violently."Unstable"meansthattheairinthelowestlayersisunusuallywarmandhumid,orthattheupperlayersareunusuallycool,oroftentimes,both.
Pocketsofrisingnear-surfaceairinanunstableairmassexpandandcool,andassomeofthewatervaporpresentcondensesintoaclouditreleasesheat,whichthenmakestheairparcelevenwarmer,forcingittorisestillhigherintheatmosphere.
Ifthelowerlevelairissufficientlywarmandhumid,andthehigheraltitudeairissufficientlycool,thisprocesscontinuesuntilatallconvectivecloud--thethunderstorm--isformed.Theresultcanbeastormextendingashighas40,000to60,000feet(8to12miles).Theupperportionsofthestorm--eveninthewarmtropics--aremadeofice:icecrystals,graupel,snow,andsometimeshail.About50%oftherainreachingthesurfaceinathunderstormoriginatedasiceintheupperreachesofthestorm.
Theupdraftsinthunderstormscanbeverystrong--50knotsormore--whichcanhelpsupporttheweightofhailstonesastheygrow.Suchupdraftscauseextremeturbulenceforaircraft,whichwillonlyflythroughthestrongestportionsofthunderstormsifthepilotshavenootherchoice.Despitethelargestressesthisputsonplanes(andtheirpassengers),modernjetaircraftaredesignedtowithstandthosestresses.
Thefollowingenhancedphotographshowstheclassicsupercellthunderstorm,aparticularlylarge,intense,anddestructivestormthatcanproducelargehailandtornadoes:
Thunderstormsaremostcommonintheafternoonoverland,whendaytimeheatingofthelandbythesuncausesthelowerpartofthetropospheretobecomeunstablefromhighertemperaturesandmorewatervaporintheair.
Or,somethunderstormscanformasresultoftheupperatmospherebecomingunusuallycool,duetotheapproachofanupperairdisturbance.Inthiscasestormscanformatanytimeofday,evenwhentherehasn'tbeendaytimeheatingoftheloweratmosphereoverland.
Theremustbesufficientwatervaporinorderforthestormtoform,sincecloudandprecipitationoriginatesaswatervapor.Thisisthefuelforthethunderstorm.Asthestormusesthisfuel,itisconvertedtorainfall.Eventually,thestormstabilizestheatmospherebyusinguptheexcesswatervaporandcoolingtheloweratmosphere,andwarmingtheupperatmosphere.
Technically,lightningmustbeproducedinorderfortheresultingcloudsystemtobecalledathunderstorm.Adiscussionofwhatcauseslightningcanbefoundhere.Interestingfacts:NATURE'SAWESOMEPOWERItisestimatedthattherearearound44,000thunderstormsthatoccuraroundtheEartheveryday.Theaveragethunderstormreleasestheenergyequivalentofa20kilotonnuclearweapon,orasmallnuclearpowerplant.TornadoWhatcausestornadoes?Tornadoesforminunusuallyviolentthunderstormswhenthereissufficient(1)instabilityand(2)windshearpresentintheloweratmosphere.
Instabilityreferstounusuallywarmandhumidconditionsintheloweratmosphere,andpossiblycoolerthanusualconditionsintheupperatmosphere.Windshearinthiscasereferstothewinddirectionchanging,andthewindspeedincreasing,withheight.Anexamplewouldbeasoutherlywindof15mphatthesurface,changingtoasouthwesterlyorwesterlywindof50mphat5,000feetaltitude.
Thiskindofwindshearandinstabilityusuallyexistsonlyaheadofacoldfrontandlowpressuresystem.Theintensespinningofatornadoispartlytheresultoftheupdraftsanddowndraftsinthethunderstorm(causedbytheunstableair)interactingwiththewindshear,resultinginatiltingofthewindsheartoformanuprighttornadovortex.Helpingtheprocessalong,cyclonicallyflowingairaroundthecyclone,alreadyslowlyspinninginacounter-clockwisedirection(intheNorthernHemisphere),convergesinwardtowardthethunderstorm,causingittospinfaster.Thisisthesameprocessthatcausesaniceskatertospinfasterwhenshepullsherarmsintowardherbody.
Otherprocessescanenhancethechancesfortornadoformation.Forinstance,dryairinthemiddleatmospherecanberapidlycooledbyraininthethunderstorm,strengtheningthedowndraftsthatasistintornadoformation.Noticethatinvirtuallyeverypictureyouseeofatornadothetornadohasformedontheboundarybetweendarkclouds(thestormupdraftregion)andbrightclouds(thestormdowndraftregion),evidenceoftheimportanceofupdraftsanddowndraftstotornadoformation.
Also,anisolatedstrongthunderstormjustaheadofasqualllinethatthenmergeswiththesqualllineoftenbecomestornadic;isolatedstormsaremorelikelytoformtornadoesthansqualllines,sinceanisolatedstormcanformamoresymmetricflowpatternaroundit,andtheisolatedstormalsohaslesscompetitionfortheunstableairwhichfuelsthestormthanifitwerepartofasolidline(squallline)ofstorms.
Becausebothinstabilityandwindsheararenecessaryfortornadoformation,sometimesweaktornadoescanoccurwhenthewindshearconditionsarestrong,buttheatmosphereisnotveryunstable.Forinstance,thissometimeshappensinCaliforniainthewinterwhenastronglowpressuresystemcomesashore.Similarly,weaktornadoescanoccurwhentheairmassisveryunstable,buthaslittlewindshear.Forinstance,Florida--whichreportsmoretornadoesthananyotherstateintheU.S.--hasmanyweakertornadoesofthisvariety.Ofcourse,themostviolenttornadoesoccurwhenbothstronginstabilityandstrongwindsheararepresent,whichintheU.S.occursinthemiddlepartofthecountryduringthespring,andtoalesserextentduringfall.Becausebothinstabilityandwindsheararenecessaryfortornadoformation,sometimesweaktornadoescanoccurwhenthewindshearconditionsarestrong,buttheatmosphereisnotveryunstable.Forinstance,thissometimeshappensinCaliforniainthewinterwhenastronglowpressuresystemcomesashore.
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