lecture07.ppt

Stability CloudDevelopment Thischapterdiscusses DefinitionsandcausesofstableandunstableatmosphericairProcessesthatcauseinstabilityandclouddevelopment Somequestionswewilltrytoask Howandwhydocloudsformonsomedaysandnotonothers Whydoestheatmospheresometimesproducestratusclouds thinlayered whileothertimeswegetcumulus orcumulonimbuscloudstoform Theanswerislargelyrelatedtotheconceptofatmosphericstability Stability Movement Arock likeaparcelofair thatisinstableequilibriumwillreturntoitsoriginalpositionwhenpushed Iftherockinsteaddepartsinthedirectionofthepush itwasinunstableequilibrium StableExample CloudDevelopment unstableenvironment Ifthemarbleisonthetopofthebowlandyougiveitalittlepush itrollsoffthebowl doesNOTcomebacktoit soriginalposition Thisisanunstablesituation Unstableair parcel verticalmotionoccurs commonlyproducescumulus cumulonimbuscloudsSo thequestionbecomes howdoesonedeterminethestabilityoftheatmosphere Howdoyouknowifairisstable Todeterminewhetherornotaparcelwillriseorsinkintheatmosphere onemustcomparetheparcelstemperature Tp withthatoftheenvironment Te atsomealtitude ifTp Tewhatwilltheparceldo ifTp Tewhatwilltheparceldo ifTp Tewhatwilltheparceldo StabilitySummary ifTp Te parcelrises ifTp Teparceldoesnotmoveupordown ifTp TeparcelsinksSo toassessstability whattwopiecesofinformationdoweneed Verticalprofilesofatmospherictemperaturearecollectedat12 00UTCeverydayatselectNWSofficesbylaunching Movement Temperature Risingairexpands usingenergytopushout whichslowsandadiabaticallycoolstheair Aparcelofairmaybeforcedtoriseorsink andchangetemperaturerelativetoenvironmentalair whichissampledusingradiosondeballoons Figure7 2 Dry MoistAdiabaticRates Figure7 3A Figure7 3B Aparcelofrisingdryaircoolsatapproximately10 C km whilelatentheatreleasedintotheparcelbycondensingvaporkeepsthemoistadiabaticlapseratenearerto6 C km Becausetheenvironmentalaircoolsmoreslowly theparcelofairheldbythehelicopterisalwayscoolerandsinksbacktoitsoriginalposition RisingandCooling considerarisingparcelofair Astheparcelrises itwilladiabaticallyexpandandcool adiabatic aprocesswheretheparceltemperaturechangesduetoanexpansionorcompression noheatisaddedortakenawayfromtheparcel theparcelexpandssincethelowerpressureoutsideallowstheairmoleculestopushoutontheparcelwalls包裹扩展自低压力允许空气分子外推墙的包裹 sinceittakesenergyfortheparcelmoleculesto pushout ontheparcelwalls theyuseupsomeoftheirinternalenergyintheprocess 因为它需要的能量包裹分子 推出 包裹的墙壁 他们使用了一些他们的内部能量的过程 therefore theparcelalsocoolssincetemperatureisproportionaltomolecularinternalenergy因此 包裹也冷却因为温度正比于分子内部的能量 SinkingandWarming considerasinkingparcelofair Astheparcelsinks itwilladiabaticallycompressandwarm随着包裹下沉 它将绝热地压缩和温暖 adiabatic aprocesswheretheparceltemperaturechangesduetoanexpansionorcompression again noheatisaddedortakenawayfromtheparcel theparcelcompressessinceitismovingintoaregionofhigherpressure因为它是包裹压缩进入一个地区的高压力 duetotheparcelcompression theairmoleculesgaininternalenergy由于包裹压缩 空气分子获得内部能量 hence themeantemperatureoftheparcelincreases butnoheatisadded 因此 平均温度的包裹增加 Sonowlet srevisitmoistanddrylapserates 所以现在 让我们重温潮湿和干燥的气温递减率 Dryadiabaticlapserate Asaparcelofairrises itcools butatwhatrate 作为一个包裹的空气上升 它冷却 但以何种速度 rateoftemperaturechangewithheightiscalledthelapserate温度变化的速度与高度称为递减率 unitsoflapserateare Ckm 1 Let sfirstconsideranunsaturatedparcelofair greenistheenvlapserate unsaturatedparcelscoolatarateof10 Ckm 1 thisiscalledthedry adiabaticlapserate Whatwillbetheparcel stemperaturebeat1km Dryadiabaticlapserate continued Whatwillbetheparcel stemperaturebeat2km MoistAdiabaticLapseRate Forasaturatedparcelofair i e whenit sT Td thenitcoolsatthemoistadiabaticlapserate 6 Ckm 1 Whatwillbetheparcel stemperaturebeat3km Supposetheairpressureoutsideaconventionaljetairlinerflyingatanaltitudeof10kmis250mb Further supposetheairinsidetheaircraftispressurizedto1000mb Iftheoutsideairtemperatureis 50 C whatwouldbethetemperatureofthisairifbroughtinsidetheaircraftandcompressedatthedryadiabaticratetoapressureof1000mb assumethatapressureof1000mbisequivalenttoanaltitudeof0m 假设空气压力外传统喷气式飞机的飞行高度是250mb的10公里 进一步 假设飞机里的空气加压到1000mb 如果外面的空气温度是 50 C 是温度的空气如果带在飞机和压缩在一个压力干绝热率1000mb 假设一个1000mb的压力相当于海拔0m MoistAdiabaticLapseRate Whatwillbetheparcel stemperaturebeat4km MoistAdiabaticLapseRate Whydoestheparcelcoolataslowerrate 6 Ckm 1 whenitissaturatedthanat10 Ckm 1whenitisunsaturated 为什么包裹冷却较慢的速率 6 C公里1 当它是比在10 C的饱和非饱和时1公里 StableAtmosphericConditions Coldairbelowwarmairisconsideredstablebecausecoldairhasahigherdensity andwhendisplacedwillgravitatebackbelowthewarm lighter air 冷空气低于热空气被认为是稳定的 因为冷空气有更高的密度 当流离失所的将会被跌破 温暖的空气Amorningatmospherehasthisstableverticalprofile keepingfogunmixed 一个上午的气氛已经这个稳定的垂直剖面 使雾纯粹的 Figure7 4 SubsidenceInversion Figure7 5 Coolersurfaceairiscalledatemperatureinversion causedbyradiationalcooling coldadvection orsubsidingairthatshrinksintotalthickness 冷却器表面空气称为逆温现象 造成冷却 冷平流换能器辐射 或下沉空气收缩在总厚度 Asitsinksandshrinks itstoplowersagreaterdistanceandwarmsmorethantheairrestingatearth ssurface 因为它下沉 收缩 其最高降低更大距离和变暖超过空气停留在地球表面 ThreekindofStability AbsolutelyStableAbsolutelyUnstableConditionallyUnstableTheconditionforabsolutestabilityis Gd Gm Ge Gdisthedryadiabaticlapserate 10 Ckm 1 Gmisthemoistadiabaticlapserate 6 Ckm 1 Geistheenvironmentallapserate variable 0 Ckm 1inthiscase Q Howwouldyoucharacterizethestabilityofaninversionlayer Inversionsareabsolutelystable notethattheabsolutestabilitycriteria Ge Gm Gdanunsaturatedorsaturatedparcelwillalwaysbecoolerthantheenvironmentandwillsinkbackdowntotheground一个不饱和或饱和包裹将永远是温度比环境 会沉降回地面Q Howdoyouformstablelayersintheatmosphere AbsolutelyUnstableAtmosphere Figure7 6A Absolutelyunstableconditionsindicatethataliftedparcelofair whetherdryormoist willbewarmerthanthesurroundingenvironmentalair andhencecontinuetorise 绝对不稳定条件表明解除包裹空气 无论是干燥或潮湿 将温暖的比周围环境空气 因此继续上升 AbsolutelyUnstableTheconditionforabsoluteinstabilityis Ge Gd GmGeistheenvironmentallapserate 30 Ckm 1 Gdisthedryadiabaticlapserate 10 Ckm 1 Gmisthemoistadiabaticlapserate 6 Ckm 1 anunsaturatedorsaturatedparcelwillalwaysbewarmerthantheenvironmentandwillcontinuetoascend一个不饱和或饱和包裹将永远温暖的比环境 将继续提升 ConditionallyUnstable Conditionsareinitiallystablewhentheenvironmentallapserate LR isbetweenthedryandmoistadiabaticLR 条件稳定时的最初环境递减率 LR 在干燥和潮湿的绝热LR Airthatisdryisalwayscoolerandsinks butwhentheparcel sdewpointisreacheditcoolsmoreslowly andmaybecomewarmerthanthesurroundingair creatinginstability 空气 是干总是冷却器和下沉 但是当包裹到达的露点冷却较慢 而且可能成为温暖的比周围的空气 创造不稳定 Figure7 7A ConditionallyUnstableWhatisthecondition Theconditionforconditionalinstabilityis Gd Ge GmGdisthedryadiabaticlapserate 10 Ckm 1 Geistheenvironmentallapserate 7 8 Ckm 1 Gmisthemoistadiabaticlapserate 6 Ckm 1 StabililtySummary Environmentaltemperaturesdeterminestabilityforrisingparcelsofair 环境温度稳定上升的确定包裹空气 Theatmosphereisabsolutelyunstablewhentheenvironmentallapserate ELR issteeperthanthedryLR absolutelystablewhentheELRislesssteepthanthemoistLR andconditionallyunstableotherwise 大气是绝对不稳定当环境温度梯度 ELR 是比干LR陡峭 绝对稳定当ELR是少比湿润LR 陡峭和条件不稳定的否则 Figure7 8 TRIGGERSFORINSTABILITYHeatMixingConvection HeatasInstabilityTrigger Figure7 9 Astheenvironmentallapserate ELR steepens itbecomesmoreunstable 随着环境温度梯度 ELR 将趋于陡峭 它变得更加不稳定 Heatingairbelow throughcontactwithahotsurfaceorfire willcausetheELRtosteepen 加热空气下 通过接触热表面或火 将导致ELR曲线变得陡峭起来 Coolingairaloftbycloudsradiatingenergyorcoldadvectionwillalsoincreaseinstability 冷却空气在空中云层辐射能量或冷平流也将增加不稳定性 MixingasInstabilityTrigger Windsthatmixupperandloweratmosphericairwillsteepentheenvironmentallapserate 上部和下部的风混合会使大气环境温度梯度 Windforcedconvectionwillwarmdescendingandlowerairwhilecoolingrisingandupperair 风强制对流将温暖的下行和较低的空气在冷却上升和高空 Figure7 10 LayerofAirInstability Figure7 11 Liftingastablelayerofaircausesittostretchoutvertically withthetopsectionexperiencingagreaterelevationchangeandmorecoolingthanthelowersection evenwhenbothcoolatthedryadiabaticrate 提升一个稳定层的空气使其伸展垂直 顶尖的部分经历更大的高程变化和更多的冷却比低剖面 即使都在干绝热冷却速率 Thisisanexampleof ConvectiveInstability Storms Figure7 12 Liftingastablelayerofairthatissaturatedbelowandunsaturatedaloftwillcausethemoistairtocoolmoreslowly andcreateaverysteepandunstableenvironmentallapserate 提升一个稳定层空气的饱和和不饱和高空以下将导致湿空气冷却较慢 并创建一个非常陡峭的和不稳定的环境温度梯度 Severethunderstormsareassociatedwithsuchprocesses 严重的雷暴相关流程等 CloudDevelopmentTriggers Figure7 13A Convection Clouds Pocketsofwarmairriseasthermalswithinvisiblewatervapor andatthedewpointtemperaturecondensationcreatesthecloudbase 口袋里的热空气上升与看不见的水蒸气上升暖气流 露点温度冷凝创建云基地 Risingairfrombelowisreplacedbysinkingairfromabove creatingareasofbluesky 不断上升的空气从下面是下沉的空气从上面所取代 创建区域的蓝色的天空 Figure7 14 CumulusClouds ClearSky DevelopmentofCumulusCloud Thepanelsrevealverticaltrendsfortemperatureandmoistureduringclouddevelopment 面板显示温度和水分垂直趋势在云的发展Theleveloffreeconvectioniswhererisingairbecomeswarmerthanenvironmentalair 自由对流的水平是上升的空气变得温暖比空气环境 Figure7 16 Stability CloudHeight Figure7 17A Environmentallapserates ELR determinethestability andasthetimeofdaychangestheELR thecloudprofilecanchange 环境气温递减率 ELR 确定稳定 随着一天时间的变化 云的ELR概要可以改变 CumulustoCumulonimbus Figure7 18 Topography Clouds Windsblowingmoistairtowardamountainwillexperienceorographicuplifttoanelevationwheredewpointisreachedandcloudsareformed 风吹向山上潮湿的空气会经验到一个高度地形隆起露点是达到和云彩形成Whenthecondensedmoisturefallsasrainfall theleewardsideofthemountainiskeptinarainshadow 当冷凝水分下降雨量 背风一侧的山是保存在一个雨影 Figure7 19 MountainWaveClouds Figure7 20 Asmoistaircondensesintocloudsonthewindwardsideofthemountain sinkingandswirlingwindsontheleewardsideshapealenticularmountainwavecloud 像潮湿的空气凝结成云在迎风一面山下沉和旋风在背风侧塑造一个透镜状山波云 Figure7 21 ChangingCloudForms Figure7 22 Uniformwindsmayalignalargeregionofcirrocumulusandstratocumuluscloudsintorowsofcloudstreets 统一的风可能对齐一个大型区域和层积云卷积云成一排排云街 Billow CastellanusClouds Figure7 23 Rapidchangesinwindspeedwithelevationcancreatewavelikebillowsatthecloudtop whileconditionallyunstableairaboveacloudmaycauseliftingandcondensationintheshapeofcastles 在风速快速变化与高程可以创建波浪般的巨浪在云顶 虽然条件不稳定上方空气云可能会引起起重和凝结在城堡的形状 Figure7 24 Mixing Stratocumulus Whenairisstableandclosetosaturation thenforcedwindmixingoftheatmosphere 当空气稳定 接近饱和 然后迫使风混合的氛围 a steepenstheenvironmentallapserate将趋于陡峭环境递减率b movesmoistlow levelairaloft潮湿的空气在空中移动低级Thecoolerandmoisterairaloftthencondensesintoadeckofclouds 冷却器及潮湿的空气在空中然后凝结成一副云 Figure7 25 ConditionalInstability example consideraparcelwithasurfacetemperatureanddewpointof30 Cand 10 C respectively theparcelisinitiallyforcedtoriseinanenvironmentwheretheenvironmentallapserate Ge is8 Ckm 1upto8km let sfollowtheparcelupward ConditionalInstability 1km theparcelisrisingdryadiabitically 10 Ckm 1 asitisunsaturated noteTp Tesosomethingisforcingtheparcelupward onwardto2km ConditionalInstability 2km theparcelhasjustbecomesaturated noteTp Tesosomethingisstillforcingtheparcelupward onwardto3km ConditionalInstability 3km theparcelisnowrisingmoistadiabatically 6 Ckm 1 noteTp Teso somethingisstillforcingitupward onwardto4km ConditionalInstability 4km theparcelisstillrisingmoistadiabatically 6 Ckm 1 notethatnowTp Te whathappensiftheparcelispushedupwardjustalittle noteTp Te A itwillriseonit sownsincenowitislessdensethanthesurroundingenvironmentalair它将随着它自己的因为现在是密度小于周围环境空气 TheheightwhereTpbecomesequaltoandthenlargerthanTeiscalledtheleveloffreeconvection高度 Tp变得等于然后比Te称为水平的自由对流 theparcelisstillrisingmoistadiabatically 6 Ckm 1 theparcelwillcontinuetoriseuntil Tp Te abovethatpoint TpTe theatmosphereisunstabletoparcelmovement thisisanexampleofaconditionallyunstableatmosphere theconditionisliftingtheparcelabove4kmwhereitcanthenriseonit sown