1、The SKEW-T DiagramSKEWT DiagramThe most common diagram used to plot rawinsonde soundings and to analyze stability is called the SKEWT-logP diagram because it is constructed with T+klnP on the vertical axis(SkewT)and -RdlnP on the horizontal axis(logP).The operational version of the diagram is rotate
2、d so that pressure decreases toward the top like it does in the atmosphere.Variables on the SKEWTFive different variables are plotted on the basic SKEWT.1.Isobars2.Isotherms3.Dry adiabats4.Pseudoadiabats5.Mixing ratiosIsobars1.Isobars are roughly horizontal lines that represent the pressures on the
3、SKEWT.They are plotted every 10 mb and are labeled every 50 mb1000 mb950 mbIsotherms2.Isotherms are nearly straight lines running from the lower left to the upper right that represent the temperatures.Both temperatures and dew point temperatures are plotted using the isotherms.Isotherms are plotted
4、every 1C and are labeled every 5C.1000 mb950 mb1520Dry Adiabats3.Dry adiabats are nearly straight curves running from the lower right to the upper left that represent the Dry Adiabatic Lapse Rate.Unsaturated air parcels move vertical along the adiabats as they are lifted or sink.The temperature of a
5、n unsaturated air parcel moving vertically during and adiabatic process can be determined by following the adiabat1000 mb950 mb1520AdiabatsPseudoadiabats4.Pseudoadiabats are curves running from the bottom of the diagram and gradually curving toward the upper left that ultimately become nearly parall
6、el to the dry adiabats.They represent the Saturated Adiabatic Lapse Rate.The change in temperature of saturated air parcels moving vertically occurs along the pseudoadiabats.1000 mb950 mb1520AdiabatsPseudoadiabatsMixing ratios5.The mixing ratios are also plotted as straight dashed lines running from
7、 the lower left to upper right more steeply than the isotherms.The mixing ratios can be used to find the height at which condensation would occur under various conditions if unsaturated parcels rise.1000 mb950 mb1520AdiabatsPseudoadiabatsMixing ratiosSoundings on the SKEWTOne of the most basic uses
8、of a SKEWT diagram is to plot temperatures and dew point temperatures observed by radiosondes in order to display and analyze the vertical structure of the atmosphere.Sample Soundingpressure(mb)T(C)Td(C)1000 26 20 950 23 19 900 20 17 850 14 11 800 11 8 750 7 4 700 5 1 650 3 0 600 0 -6 550 -4-10 500
9、-8-15 450-10-18 400-14-22(Lifting Condensation Level,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.Lifting Condensation LevelThe Lifting Condensation Level(LCL)represents the level to which an unsatur
10、ated air parcel must be lifted in order for it to become saturated and for condensation to begin.The LCL would be the cloud base on days where lifting of air is the mechanism producing the clouds.Determining the LCL on the SKEWT1.Plot a point where the isotherm for the air temperature intersects the
11、 isobar for the air pressure.Finding the LCL(Cont.)2.Plot a point where the isotherm for the dew point temperature intersects the isobar for the air pressure.Finding the LCL(Cont.)3.Draw a line along the dry adiabat from the point plotted where the air temperature intersected the air pressure.Findin
12、g the LCL(Cont.)4.Draw a line along the mixing ratio from the point plotted where the dew point temperature intersected the air pressure.Finding the LCL5.The Lifting Condensation Level is the point at which the two lines drawn in steps 3 and 4 intersect.LCLSample Soundingpressure(mb)T(C)Td(C)1000 26
13、 20 950 23 19 900 20 17 850 14 11 800 11 8 750 7 4 700 5 1 650 3 0 600 0 -6 550 -4-10 500 -8-15 450-10-18 400-14-22LCL=918 mbAbove the LCLIf the parcel is lifted above the Lifting Condensation Level it will be saturated and it will cool at the Saturated Adiabatic Lapse Rate.Above the LCL,the parcel
14、should be lifted along the curved pseudoadiabats.对流凝结面(Convective Condensation Level,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(Level of
15、 Free Convection,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(Mixing Condensation Level,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
16、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)(Positive Area,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.
17、)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)(Negative Area,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
18、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Stability IndicesThere are many indices of the stability or potential instability of air that are based on the radiosonde sound
19、ings and that can be plotted on SKEWT diagrams.Stability Indices(Cont.)There are many ways to compute stability indices.A stability index may be computed for a given pressure level(e.g.the surface).Stability Indices(Cont.)In some applications the parcels air temperature is computed as the average of
20、 the temperatures within 50 mb of the Earths surface,and the dew point temperature is taken from the level where the pressure is 25 mb lower than the pressure from the Earths surface.Lifted IndexThe Lifted Index(LI)is computed as the temperature of the air in the environment outside a rising air par
21、cel at 500 mb(usually measured by a radiosonde)minus the temperature a parcel would have if it were lifted to the 500 mb level.LI=Tobserved at 500 mb Tparcel lifted to 500 mb Lifted Index(Cont.)The Lifted Index indicates the stability of an air parcel if it were to be lifted to the 500 mb level.When
22、LI 0,then the parcel would be stable.Lifted Index(Cont.)When the Lifted Index is negative or is slightly positive(i.e.LI 24,then the potential for convection exists;K 30,then the potential for strong thunderstorms exists.K Index(Cont.)What is the K Index of our sample sounding?Sample Soundingpressur
23、e(mb)T(C)Td(C)1000 26 20 950 23 19 900 20 17 850 14 11 800 11 8 750 7 4 700 5 1 650 3 0 600 0 -6 550 -4-10 500 -8-15 450-10-18 400-14-22Sample Soundingpressure(mb)T(C)Td(C)1000 26 20 950 23 19 900 20 17 850 14 11 800 11 8 750 7 4 700 5 1 650 3 0 600 0 -6 550 -4-10 500 -8-15 450-10-18 400-14-22K Index(Cont.)K=14C (-8C)+11C (5C-1C)K=29CLimitations of LI and KLI only provides information about the possible stability/instability of a parcel that is lifted to 500 mb.K only provides information about the environment at 850,700 and 500 mb.
