天气学分析诊断-T-lnp-图-详解

TheSKEW-TDiagramSKEWTDiagramThemostcommondiagramusedtoplotrawinsondesoundingsandtoanalyzestabilityiscalledtheSKEWT-logPdiagrambecauseitisconstructedwithT+klnPontheverticalaxis(SkewT)and-RdlnPonthehorizontalaxis(logP).Theoperationalversionofthediagramisrotatedsothatpressuredecreasestowardthetoplikeitdoesintheatmosphere.VariablesontheSKEWTFivedifferentvariablesareplottedonthebasicSKEWT.IsobarsIsothermsDryadiabatsPseudoadiabatsMixingratiosIsobarsIsobarsareroughlyhorizontallinesthatrepresentthepressuresontheSKEWT.Theyareplottedevery10mbandarelabeledevery50mb1000mb950mbIsothermsIsothermsarenearlystraightlinesrunningfromthelowerlefttotheupperrightthatrepresentthetemperatures.Bothtemperaturesanddewpointtemperaturesareplottedusingtheisotherms.Isothermsareplottedevery1°Candarelabeledevery5°C.1000mb950mb1520DryAdiabatsDryadiabatsarenearlystraightcurvesrunningfromthelowerrighttotheupperleftthatrepresenttheDryAdiabaticLapseRate.Unsaturatedairparcelsmoveverticalalongtheadiabatsastheyareliftedorsink.Thetemperatureofanunsaturatedairparcelmovingverticallyduringandadiabaticprocesscanbedeterminedbyfollowingtheadiabat1000mb950mb1520AdiabatsPseudoadiabatsPseudoadiabatsarecurvesrunningfromthebottomofthediagramandgraduallycurvingtowardtheupperleftthatultimatelybecomenearlyparalleltothedryadiabats.TheyrepresenttheSaturatedAdiabaticLapseRate.Thechangeintemperatureofsaturatedairparcelsmovingverticallyoccursalongthepseudoadiabats.1000mb950mb1520AdiabatsPseudoadiabatsMixingratiosThemixingratiosarealsoplottedasstraightdashedlinesrunningfromthelowerlefttoupperrightmoresteeplythantheisotherms.Themixingratioscanbeusedtofindtheheightatwhichcondensationwouldoccurundervariousconditionsifunsaturatedparcelsrise.1000mb950mb1520AdiabatsPseudoadiabatsMixingratiosSoundingsontheSKEWTOneofthemostbasicusesofaSKEWTdiagramistoplottemperaturesanddewpointtemperaturesobservedbyradiosondesinordertodisplayandanalyzetheverticalstructureoftheatmosphere.SampleSoundingpressure(mb) T(°C) Td(°C)1000 26 20950 23 19900 20 17850 14 11800 11 8750 7 4700 5 1650 3 0600 0 -6550 -4 -10500 -8 -15450 -10 -18400 -14 -22(LiftingCondensationLevel,简称:LCL)

为当空气块自地面受外举升并沿乾绝热线上升而开始产生凝结的高,因此可知其为动作用所导致之凝结面.一般而言,此凝结面为层云之云底高所在位置.【求法】(1)由气压(P)与点(Td)之交点沿饱和混合比线上升.(2)由气压(P)与温(T)之交点沿乾绝热线上升.者之交点即为举升凝结面.LiftingCondensationLevelTheLiftingCondensationLevel(LCL)representstheleveltowhichanunsaturatedairparcelmustbeliftedinorderforittobecomesaturatedandforcondensationtobegin.TheLCLwouldbethecloudbaseondayswhereliftingofairisthemechanismproducingtheclouds.DeterminingtheLCLontheSKEWTPlotapointwheretheisothermfortheairtemperatureintersectstheisobarfortheairpressure.FindingtheLCL(Cont.)Plotapointwheretheisothermforthedewpointtemperatureintersectstheisobarfortheairpressure.FindingtheLCL(Cont.)Drawalinealongthedryadiabatfromthepointplottedwheretheairtemperatureintersectedtheairpressure.FindingtheLCL(Cont.)Drawalinealongthemixingratiofromthepointplottedwherethedewpointtemperatureintersectedtheairpressure.FindingtheLCLTheLiftingCondensationLevelisthepointatwhichthetwolinesdrawninsteps3and4intersect.LCLSampleSoundingpressure(mb) T(°C) Td(°C)1000 26 20950 23 19900 20 17850 14 11800 11 8750 7 4700 5 1650 3 0600 0 -6550 -4 -10500 -8 -15450 -10 -18400 -14 -22LCL=918mbAbovetheLCLIftheparcelisliftedabovetheLiftingCondensationLevelitwillbesaturatedanditwillcoolattheSaturatedAdiabaticLapseRate.AbovetheLCL,theparcelshouldbeliftedalongthecurvedpseudoadiabats.对流凝结面(ConvectiveCondensationLevel,简称CCL)

当地球表面受太阳射加热作用,而使空气块产生上升运动,并造成凝结之高即称之为『对凝结面』,因此可知其为热作用所导致之凝结面.一般而言,此凝结面为积云之云底所在高.【求法】由Td与1000hPa之交点沿饱和混合比线上升或下至与探空温曲线相交所在之高即称为CCL.对流温度(Tc):

当地球表面受太阳辐射加热作用后开始形成对流时之地面温度。求法：由CCL沿干绝热线向下至地面气压时所具有之温度。自由对流面(LevelofFreeConvection,简称:LFC)

当空气块经干绝热举升达饱和,而在随后的饱和绝热过程中当空气块变得比周围环境空气暖时之高即称为『自由对高』.此时由于空气块本身所具有之浮较环境场大,因此空气块可自由上升，直到温较周围环境温低时才停止.【求法】由LCL/CCL沿湿绝热线上升至与探空温曲线相交所的高度.混合凝结层(MixingCondensationLevel,简称:MCL)近地层有风扰动而使气层完全混合,则当其发生饱和时之最低高即称为混合凝结层.求法：经过平均之后的饱和混合比线与平均干绝热线之交点即为混和凝结面.可用面积法加以判断.平衡面(简称:EL)：当一原本温度较周围环境高的浮升空气块，上升过程中降温，其温度再与周围环境温相等时之高即称为『平衡面』,此时空气块继续上升,则空气块之温会低於环境大气.求法：探空温曲线与对凝结面后或抬升凝结高度后的湿绝热线再相交点之位置即为平衡面之所在高.正面积(正能区)(PositiveArea,简称:PA)【定义】由于空气块于气层内所沿升之绝热线温度均大于四周环境之温,因此空气块能自由上升,此时湿绝热线与探空温度曲线所夹之面积与空气块所释放出的动总合成正比,此面积即称之为『正能区』.正面积的大小代表著空气块在上升过程中所释放出的能大小,也就表示大气的对流有效位能【求法】1.因加热作用所形成:(如:太阳射)由对凝结高(C.C.L.)沿湿绝热线上升至与温探空曲线相交为止,此探空曲线与湿绝热线所围之区域即为因加热作用所导致之正能区.2.因举升作用所形成:(如:地形,锋面)由自由举升凝结面(L.F.C.)沿湿绝热线上升至与温探空曲线相交,此温探空曲线与湿绝热线所围成的区域即为因举升作用导致之正能区负面积(负能区)(NegativeArea,简称:NA)【定义】当空气块于某气层内上升的绝热线温度均小於其四周环境之温度时,此空气块于该气层内

需要依赖外界环境能量的输入才可运动,此时湿绝热线与探空温曲线所围之面积与空气块获自四周环境之动能成正比,此面积即称为『负能区』.所以负面积的大小代表著外届对上升空气块作功使其达到自由上升之作功.

【求法】1.因加热作用所形成:由对凝结高(LCL)沿干绝热线下至地面为止,此温度探空曲线与干绝热线所围成之区域即为因加热作用所导致的负能区.2.因举升作用所形成:由地面沿干绝热线上升至L.C.L.,再从L.C.L.沿湿绝热线至与探空温曲线相交为止(此点即为举升凝结面(L.F.C.)),此所围成的区域即为因举升作用所导致的负能区StabilityIndicesTherearemanyindicesofthestabilityorpotentialinstabilityofairthatarebasedontheradiosondesoundingsandthatcanbeplottedonSKEWTdiagrams.StabilityIndices(Cont.)Therearemanywaystocomputestabilityindices.Astabilityindexmaybecomputedforagivenpressurelevel(e.g.thesurface).

StabilityIndices(Cont.)Insomeapplicationstheparcel’sairtemperatureiscomputedastheaverageofthetemperatureswithin50mboftheEarth’ssurface,andthedewpointtemperatureistakenfromthelevelwherethepressureis25mblowerthanthepressurefromtheEarth’ssurface.LiftedIndexTheLiftedIndex(LI)

iscomputedasthetemperatureoftheairintheenvironmentoutsidearisingairparcelat500mb(usuallymeasuredbyaradiosonde)minusthetemperatureaparcelwouldhaveifitwereliftedtothe500mblevel.LI=Tobservedat500mb–Tparcelliftedto500mb

LiftedIndex(Cont.)TheLiftedIndexindicatesthestabilityofanairparcelifitweretobeliftedtothe500mblevel.WhenLI<0,thentheparcelwouldbeunstable.LI>0,thentheparcelwouldbestable.

LiftedIndex(Cont.)WhentheLiftedIndexisnegativeorisslightlypositive(i.e.LI<4),itindicatesagreaterchanceofthunderstorms.LiftedIndex(Cont.)WhatwastheLIofoursamplesounding?LI(Cont.)LI=-8°C-(-6.5°C)=-1.5°CKIndexTheKIndex(K)combinestheeffectsofatmosphericstabilityandmoistureintoasingleindex.Kisbasedentirelyondatafromanatmosphericsoundinganditdoesnotconsidertheparcel.KIndex(Cont.)K=T850mb–T500mb+Td850mb–(T–Td)700mbwhereT850mb–T500mbincorporatestheeffectsoftheenvironmentallapserate,andTd850mb-(T–Td)700mbincoporatestheeffectsofthemoistureinthelowertroposphere.KIndex(Cont.)TheKIndexwillbelargestwhenthelowertroposphereisunstableandmoist.WhenK>24,thenthepotentialforconvectionexists;K>30,thenthepotentialforstrongthunderstormsexists.KIndex(Cont.)WhatistheKIndexofoursamplesounding?SampleSoundingpressure