天氣學分析診斷-T-lnp-圖-詳解

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,簡稱:LCL)

為當空氣塊自地面受外舉升并沿乾絕熱線上升而開始產生凝結的高,因此可知其為動作用所導致之凝結面.一般而言,此凝結面為層云之云底高所在位置.【求法】(1)由氣壓(P)與點(Td)之交點沿飽和混合比線上升.(2)由氣壓(P)與溫(T)之交點沿乾絕熱線上升.者之交點即為舉升凝結面.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.對流凝結面(ConvectiveCondensationLevel,簡稱CCL)

當?shù)厍虮砻媸芴柹浼訜嶙饔?而使空氣塊產生上升運動,并造成凝結之高即稱之為『對凝結面』,因此可知其為熱作用所導致之凝結面.一般而言,此凝結面為積云之云底所在高.【求法】由Td與1000hPa之交點沿飽和混合比線上升或下至與探空溫曲線相交所在之高即稱為CCL.對流溫度(Tc):

當?shù)厍虮砻媸芴栞椛浼訜嶙饔煤箝_始形成對流時之地面溫度。求法：由CCL沿干絕熱線向下至地面氣壓時所具有之溫度。自由對流面(LevelofFreeConvection,簡稱:LFC)

當空氣塊經干絕熱舉升達飽和,而在隨后的飽和絕熱過程中當空氣塊變得比周圍環(huán)境空氣暖時之高即稱為『自由對高』.此時由于空氣塊本身所具有之浮較環(huán)境場大,因此空氣塊可自由上升，直到溫較周圍環(huán)境溫低時才停止.【求法】由LCL/CCL沿濕絕熱線上升至與探空溫曲線相交所的高度.混合凝結層(MixingCondensationLevel,簡稱:MCL)近地層有風擾動而使氣層完全混合,則當其發(fā)生飽和時之最低高即稱為混合凝結層.求法：經過平均之后的飽和混合比線與平均干絕熱線之交點即為混和凝結面.可用面積法加以判斷.平衡面(簡稱:EL)：當一原本溫度較周圍環(huán)境高的浮升空氣塊，上升過程中降溫，其溫度再與周圍環(huán)境溫相等時之高即稱為『平衡面』,此時空氣塊繼續(xù)上升,則空氣塊之溫會低於環(huán)境大氣.求法：探空溫曲線與對凝結面后或抬升凝結高度后的濕絕熱線再相交點之位置即為平衡面之所在高.正面積(正能區(qū))(PositiveArea,簡稱:PA)【定義】由于空氣塊于氣層內所沿升之絕熱線溫度均大于四周環(huán)境之溫,因此空氣塊能自由上升,此時濕絕熱線與探空溫度曲線所夾之面積與空氣塊所釋放出的動總合成正比,此面積即稱之為『正能區(qū)』.正面積的大小代表著空氣塊在上升過程中所釋放出的能大小,也就表示大氣的對流有效位能【求法】1.因加熱作用所形成:(如:太陽射)由對凝結高(C.C.L.)沿濕絕熱線上升至與溫探空曲線相交為止,此探空曲線與濕絕熱線所圍之區(qū)域即為因加熱作用所導致之正能區(qū).2.因舉升作用所形成:(如:地形,鋒面)由自由舉升凝結面(L.F.C.)沿濕絕熱線上升至與溫探空曲線相交,此溫探空曲線與濕絕熱線所圍成的區(qū)域即為因舉升作用導致之正能區(qū)負面積(負能區(qū))(NegativeArea,簡稱:NA)【定義】當空氣塊于某氣層內上升的絕熱線溫度均小於其四周環(huán)境之溫度時,此空氣塊于該氣層內

需要依賴外界環(huán)境能量的輸入才可運動,此時濕絕熱線與探空溫曲線所圍之面積與空氣塊獲自四周環(huán)境之動能成正比,此面積即稱為『負能區(qū)』.所以負面積的大小代表著外屆對上升空氣塊作功使其達到自由上升之作功.

【求法】1.因加熱作用所形成:由對凝結高(LCL)沿干絕熱線下至地面為止,此溫度探空曲線與干絕熱線所圍成之區(qū)域即為因加熱作用所導致的負能區(qū).2.因舉升作用所形成:由地面沿干絕熱線上升至L.C.L.,再從L.C.L.沿濕絕熱線至與探空溫曲線相交為止(此點即為舉升凝結面(L.F.C.)),此所圍成的區(qū)域即為因舉升作用所導致的負能區(qū)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