【作者】 寿亦萱；

【导师】 许健民；

【作者基本信息】 南京信息工程大学 ， 大气遥感科学与技术， 2007， 博士

【摘要】 高空急流—锋是对流层上部最突出的天气系统之一，它对中尺度灾害天气发生发展过程具有十分重要的作用。本文以我国上空的副热带高空西风急流为研究对象，通过常规观测、卫星、雷达资料分析以及新一代高分辨率中尺度数值模式（WRF）模拟，对2005年6月、2006年5月和6月发生在我国东部的三次高空急流相关的天气过程中高空急流-锋云系与急流-锋暴雨中尺度系统的特征、三维结构以及发生发展和演变过程的动力学机制进行了研究，得到的主要结果如下：1．典型的高空急流—锋系统在卫星云图上具有四个基本特征：叶状云、涡旋逗点云系、暗带以及急流出口区右侧排列有序的对流云团波串或横向云带。它们所出现的位置、微物理结构都具有明显特征。由中高云组成的叶状云带主要集中在急流轴的南侧，呈西南—东北走向，在红外和水汽图像上几乎为不透明。从其微物理结构来看，这条叶状云带的高层主要由密实的冰晶、雪粒子构成，而位于零度线以下则主要由云水粒子构成，它们大量集中在云带的西南端，这里对应南方强降水区。水汽图像上，“S”水汽型北边界以北的暗区位于急流的气旋性弯曲一侧。这条狭窄暗带沿着叶状云北侧边缘分布，随着气流向下游平流，卷入下游的云团，形成逗点云。在急流气旋性切变一侧产生的一些云带在形成逗点云之前常会表现为发展中的对流云团波串或者横向云带形式。这些对流云团波串或横向云带常常是随着急流靠近，急流核的中心风速加大而产生的，且它们通常相对固定地出现在急流出口区的左侧附近，随环境风方向向下游方向排列。就其微物理结构来看，急流气旋性切变一侧的云系内的微物理垂直结构与急流反气旋性切变一侧基本相同，但是急流气旋性切变一侧云系的云冰、云水含量相对较低。2．急流轴南北两侧的云系都具有锋面云系的特征，但具有不同的发生发展机制。从气流轨迹分析来看，急流轴以南的叶状云带的形成主要受西南、东南和西北气流等三股气流的影响。云带主要由西南和东南暖湿空气辐合抬升形成。而由西北干冷空气形成的干带，一部分从高层向下侵入到叶状云尾部的层状降水区内，与这里由降水拖带产生的下沉空气相遇，形成强而一致的下沉气流致使云带后部云区逐渐变窄云顶降低直至最后消散，而另一部分气流到达对流层中低层后又转为上升运动，叠置在低层的暖湿气流之上，造成大气不稳定度增大，因此整个急流反气旋性切变一侧的云带可以被看作是干带与暖输送带相互作用的结果。急流北侧的云系也沿锋面发展，但云系高度较低。此处的云带主要是由槽后西北气流在下沉辐散过程中与地面低压相遇，受地面摩擦辐合作用而被强迫抬升，在抬升过程中又将低空西南暖湿空气输送到高空而形成的。3．水汽图像上在高空急流气旋性弯曲一侧的狭长的暗带对应于低湿、高位涡区。位涡场和湿度场的特征表明高空急流气旋性弯曲一侧的暗区是高空干空气侵入的源头。干侵入机制对促进急流两侧云系发展都具重要作用，一方面干侵入过程中干冷空气使对流层中低层的湿度减小，造成大气上干冷、下暖湿的不稳定结构，促进急流云系内深对流发展。另一方面，干侵入过程引起位涡下传，导致低空正位涡和风场异常，诱生地面气旋发展，因此它还可能是导致急流轴气旋性切变一侧的云团无论在产生时具有何种形态最终都表现为涡度逗点状形态特征的关键的动力机制。湿位涡的分布特征表明急流轴附近的大气具有较大的湿斜压性。从稳定性角度看，急流轴两侧特别是急流轴反气旋性切变一侧叶状云系产生和发展除了与对流不稳定机制有关，还可能受条件性对称不稳定机制的影响。条件性对称不稳定有利于倾斜对流的发生发展，这种倾斜对流对处于强风速切变环境中的急流云系长时间维持和发展是至关重要的。4．急流云系上的不稳定能量具有不均匀分布的特征。从能量分布特征来看，有两个对流有效位能的高值中心，分别位于急流轴反气旋性切变一侧的叶状云系的后部，以及急流轴气旋性切变一侧的逗点云系的凹部到头部的一个相对较小的区域内。而下沉对流有效位能的大值中心分别对应于急流核入口区的左侧和出口区的右侧，它们在云图上与无云区相对应。这进一步证实了云图上位于急流气旋性切变一侧的暗区是高空干冷空气下沉侵入的表征。5．急流的运动学结构分析表明，在急流核两端（入口区和出口区）非地转风具有明显的气旋性切变特征，而急流核南北两侧非地转风则具有反气旋性切变的特点。在涡度场上，急流核的入口区和出口区两端都为非地转风正涡度区，而在急流轴两侧则为非地转风负涡度区，其结构与Cunningham和Keyser（2000）提出的急流非地转风涡度“四象限”分布特征基本一致。6．应用罗斯贝数（Ro）和非线性平衡方程的残差（ΔNBE）诊断发现，高空急流气旋性一侧靠近出口区的地方为Ro和ΔNBE大值区，ΔNBE的量级达到10^-8s^-1。从ΔNBE的含义可知，高空急流气旋性一侧靠近出口区的地方具有明显的非地转性。而且随着急流的曲率加大，这种非地转性进一步增强。另外，从Richardson数（Ri_g）分析来看，高空急流出口区也是切变不稳定区，这里正对应于急流出口区附近的中尺度重力波发生区和强降水区。7．根据小波分析的多分辨率频域滤波特性，构造了基于小波变换的带通滤波器以分析高空急流出口区引发的重力波的特征。分析结果表明，位于我国上空的副热带高空西风急流出口区附近的重力波的水平波长一般在100-500km，振幅约为1—10hPa，垂直速度场上的波动略落后于位温场上波动，位相差约1／4个水平波长。具有明显的中尺度重力波的特征，从云系分布特征看，云图上急流出口区左侧出现的对流云团波串和横向云带与该处的中尺度重力波关系密切。8．就地转适应与切变不稳定在高空急流出口区附近所产生的中尺度重力波的发生发展中的作用来说，高空急流出口区附近中尺度重力波是在地转调整加快以及切变不稳定快速增大的过程中产生发展起来的，且产生位置位于非地转平衡能量的下游频散区与切变不稳定区的叠合区内。从地转调整和切变不稳定的产生顺序来看，地转调整提前于切变不稳定产生。高空急流气旋性切变一侧的高位涡区与该处的△NBE正值区具有很好的对应关系。根据ΔNBE与位涡（PV）的含义可知，急流出口区附近的中尺度重力波可能是由高空锋系统内的一股具有强非地转平衡性的气流所引发的。9．对2005年6月10日在我国东北地区发生的一次急流锋暴雨（以下简称“05.06”暴雨）发生机制的分析结果表明，此次暴雨是发生在高空槽东移加深过程中的一次中尺度对流天气过程。从大尺度特征来看，中尺度对流系统处于前倾疏散的高空槽槽前，高空辐散，低空辐合，为MCS发生提供了有利的大尺度动力条件；暴雨发生前对流层低层有西南—东北走向的湿舌，为暴雨提供了有利的水汽条件；高空干冷平流与低空的暖湿平流形成的差动平流，则为此次暴雨提供了不稳定条件。此外，从地面接收到的太阳辐射能量分布情况来看，下垫面不均匀加热引起的热力环流是这次暴雨过程中尺度对流系统发生发展的一个重要的触发机制。从中尺度对流系统特征看，影响沙兰河上游的中尺度对流系统具有多单体风暴结构特征的孤立对流系统的特征，而且系统中气流具有后方入流前方出流的特点。从对流系统的移动规律来看，导致沙兰河上游暴雨的雷暴云为左移风暴。10．就高空急流的作用而言，“05.06”暴雨的雨带以及卫星与雷达回波图上显示出对流云团与由高空急流出口区上激发的中尺度重力波时空分布特征相吻合，高空急流出口区所激发的中尺度重力波对触发和维持此次暴雨过程具有重要作用。高空急流出口区引起的高层的重力波把能量向下输送，使低层波动加强，在上下两列波动垂直方向同相叠加的地区产生出大振幅的中尺度重力波，引发强降水。此外，在研究中发现，“05.06”暴雨过程中强对流云团分布与地面切变线走向具有很好的一致性，但是在这条切变线上对流强弱分布却是不均匀的，其中在弧形切变线转折处对流最强。分析认为，造成这种现象的一种可能解释是，这是由切变线走向与盛行环境风向的配置关系引起的；而从波动能量传播与不稳定能量关系来看，这种现象又可能和高空急流激发的中尺度重力波的传播方向与湿舌的走向有关。除了非地转平衡性以外，高空急流所引起的中高层大气切变不稳定也是造成沙兰河上游中尺度对流系统左移的一个重要动力条件。更多还原

【Abstract】 Upper-level jet-front system is one of the most outstanding weather systems inthe upper troposphere. It plays an important role to the development of themesoscale disaster weather systems. By comprehensive inspection using thesounding, satellite and radar observations, theory analysis and modeling simulationsby WRF model, a high resolution model for meso-scale and micro-scale weatherresearch developed by NCAR U.S.A, this paper is aimed at exploring the structuresand dynamic mechanisms of the subtropical upper-level jet-frontal zone cloudsystems related with three heavy rain processes over China occurred on 9-11 June2005, 24-26 May 2006 and 7-10 June 2006 respectively. The major results made inthe course of this study are listed as follows:1. A typical upper-level jet-front cloud system may have four characteristicfeatures: a baroclinic leaf cloud, a dark area, vortex comma clouds andasequential convective cloud clusters or transversal cloud lines. A baroclinic leafis often observed on the cyclonic side vicinity of an upper level jet orientedsouthwest-northeast. Composed of high-or medium-level clouds, a typical cloudleaf is normally opaque in IR and VIS images. With respect to its microphysicalstructure, the upper-level part of the cloud leaf is mainly composed of dense iceand snow crystals, while the lower-level part plentiful liquid water. Those liquidcloud droplets involved in the cloud leaf are seem to be concentrated within thesouth-west part of the cloud corresponding with the rainfall on the anti-cyclonicside of jets. A dark area is observed on the cyclonic side of the jet distributingalong the jet edge in WV （water vapor） images. This dark area is then advecteddownstream with environment flow and wrapped into the cloud head andgradually evolves the downstream cloud into a comma cloud. Some commacloud on the cyclonic side of jetstream sometimes display as a sequence ofconvective cloud clusters or some transversal cloud lines at its nascent stage.These kinds of convective cloud clusters or some transversal cloud lines areoften seen formed within the left-exit zone of upper-level jet and developedwhen the upper-level jet is closed to or the jet core is strengthened. The microphysical structure of the cloud within this area is generally same as that ofthe cloud leaf on the anti-cyclonic side of the jet, except that the ice and liquidwater content of the former are some lower than the latter.2. The cloud leafs in both south and north sides of the jet stream can be identifiedas a frontal cloud, while with different development machanisms. According tothe trajectory analysis, the cloud leaf is affected by three branches of flowduring its development: southwesterlies, southeasterlies and northwesterlies.Both the south-westerlies and south-easterlies are from the mid-lower level andare responsible for the cloud leaf genesis. A part of cold and dry air parcels onthe dry belt within north-westerlies spreads downward from upper-level andintrudes into the stratified rainfall area at the end of the cloud leafs. This airflowencountered with the dowdraft air by rainfall makes the total dowdraftstrengthening in such a manner as to cause the rear of the cloud dissipating;while the other part turn to an updraft at the middle level after a short termdescending. This branch of airflow superposites on the warm moist air from thelower which cause the local atmosphere instability increase rapidly. Thereforethe whole cloud band on the anti-cyclonic side of jet can be manifested as theoutcome of the interplay of the dry and warm conveyor belts. The cloud band onthe cyclonic side is remained as a frontal cloud. Comparing the cloud betweenthe two sides of the jet, the frontal cloud on the cyclonic side is relativelythinner with lower height. With respect to the genesis of the cloud on thecyclonic side, it is speculated to be generated in the course of the upstreamnorthwest flow to the west of the pressure trough being abruptly changed fromdescending into ascending by encountering a surface low. During the clouddevelopment, the updraft is included with a part of continuous warm and moistairflow.3. The dark area on the cyclonic side of the upper-level jet in WV imageriescorresponds to a low moist and high PV region. In the PV perspective, thisphenomenon signifies that the dark area on the cyclonic side of the upper-leveljet is the source of the dry intrusion from the upper-level. Dry intrusion acts as apromoter to the development of the clouds on both side of upper-level jet. Onthe one hand, it decreases the moisture at the middle level of the tropospheremaking the instability increase which is good for the deep convectiondeveloping in the cloud cluster. On the other hand, dry intrusion can cause the high-value PV at the upper level of the troposphere glide down to thelower-level to trigger a surface cyclone. According to the trajectory analysis, dryintrusion may be a key mechanism for the cloud clusters on the cyclonic side ofthe upper-level jet taking on a comma-shape finally. The moist potentialvorticity distribution shows characteristics of moist baroclinicity along the jetaxes. With respect to the stability, it seems that the conditional instability andthe instability-related tilt updraft are the possible dynamic mechanisms fortriggering and maintaining the cloud leaf on the anti-cyclonic side of theupper-level jet.4. From the characteristics of the energy distribution, it is seen that the instableenergy is not symmetrically distributed in a jet-front cloud system. There aretwo CAPE centers. One is located at the rear of the cloud band on theanti-cyclonic side of the jet; the other is within a small area between the head ofthe comma cloud and its maximum inflection area on the cyclonic side of the jet.While the maximum on the DCAPE distribution are located on the left side ofthe enter area and right side of the exit area of the jet respectively, whichcorrespond to the dark areas in satellite images. This fact seems to further verifythat the dark area on the cyclonic side of the jet can be identified as the token ofthe dry intrusion from the upper level.5. With respect to the kinematic structure of an upper-level jet, ageostrophic windson the two ends of a jet streak （enter area and exit area） have evident cyclonicshear, while ageostrophic winds on the south and north sides of the jet streakanti-cyclonic shear. On the vorticity field of ageostrophic wind, the two endsalong the jet streak correspond to positive vorticity areas, while the south andnorth sides of the jet steak correspond to negtive vorticity areas. Such a structurein ageostrophic vorticity field generally coincides with the "four quadrant"model of ageostrophic vorticity for a straight jet streak suggested byCunningham and Keyser in 2000.6. Diagnosis study based on the Lagrangian Rossby number （Ro） and the residualof nonlinear balance equation （ANBE） shows that the cyclonic side of theupper-level jet is occupied by both the high value areas of Ro and ANBE. Thescale of the△NBE around this area is over 10^-8s^-1. According to the concept of△NBE, it is manifested that there is an evident ageostrophic on the cyclonic sideof the upper-level jet and with the increasing curvature of the jet, the ageostrophic enhances. Besides, according to the diagnosis study of shearinstability around the jet based on Richardson number （Ri）, it is noticed that theexit area of the upper-level jet is a shear instability area where mesoscale gravitywave and heavy rainfall may occur.7. In the light of the advantage of wavelet analysis in the multi-resolutionfrequency filtering, a band pass filter is designed based on wavelet transform foranalyzing the characteristics of the gravity wave around the exit area of anupper-level jet. The analysis results show that the horizontal wavelength of thegravity wave around the exit area of the subtropical upper-level jet in thevicinity area of China is about 100-500km, and the amplitude 1-10hPa. Besides,the wave in the vertical velocity field is about 1/4 wavelength behind that in thepotential temperature field. Such facts display the common characteristics of atypical mesoscale gravity wave. Considering the cloud distribution, it seems thatthe sequential convective cloud clusters or transversal cloud lines on the left sideof the jet exit area are close related with this kind of meso-scale gravity wave.8. With respect to the influence of the geostrophic adjustment and the shearinstability on the mesoscle gravity wave on the left side of the jet exit area, itseems that the mesoscale gravity wave is generated through the quickening ofthe geostrophic adjustment and increase of the shear instability. And the locationof the mesoscale gravity wave is just within the superposition of the downstreamunbalanced energy radiation area and the shear instability area. As for the timesequence of the geostrophic adjustment and the shear instability, the formerseems occur ahead of the latter. Superposing the PV and△NBE on the cyclonicside of the upper-level jet shows the maxima of PV correspond closely to thosepositive△NBE maxima in this region. According to the definition of PV and△NBE, it is speculated that the mesoscale gravity wave within the exit area ofthe upper-level jet is possible induced by a strong imbalance flow generatedfrom the upper-level jet-front system.9. An elementary diagnosis of the dynamic mechanism of the jet-front systemrainfall in the eastem central portion of Heilongjiang province on 10th June 2005（to be called as "05.06" northeastern rainstorm hereafter） is performed. Theresults show that this rainstorm occurred in the process of a forward tiltingupper-level trough with a diverging dispersive structure, moving eastward anddeepening. The rain-producing MCSs （Meso-scale Convective Systems） systems are in the foreside area of this upper trough, and the large-scale kinetic energysuitable for system development is supplied by the low level convergence-upper level divergence mechanism; a SW-NE oriented moist tongue located atthe lower level of troposphere was obviously seen before the rainstorm occurs,which feeds the rainfall area with favorable moisture condition; and thedifferential advection induced by dry and cold air superposing the warm andmoist air results in the increase in local instability. Furthermore, the distributionof incoming solar radiance at underlying surface shows differential heatingwhich is an important trigger to the MCSs of this rainstorm. As for thecharacteristics of the mesoscale convective system （MCS） during this rainstorm,the MCS is an isolated convective system with a multi-cell storm structure; andthe system is characterized with a backward input and forward output structure.As for the movement direction, this MCS belongs to a left-moving storm.10. As far as the influence of the upper-level jet on the "2005.06" rainstorm isconcerned, the space-temporal characteristics of the rain band and convectivecloud clusters displayed in the satellite and radar imageries finely agree with thatof the mesoscale gravity wave triggered by the upper-level jet on the left side ofits exit area. Such facts imply that the mesoscale gravity wave triggered by theupper-level jet on the left side of its exit area may therefore have played animportant role in initiating and maintaining this rainstorm. It is the gravity waveinduced by the jet that transports the energy at the upper-level downward tostrengthen the wave at the lower-level. The rainfall maxima are located at thearea where the waves at upper and lower levels happen to have the samespace-temporal phase. Furthermore, with respect to the relationship betweenmeso-scale shear line and the MCSs, it is found that over the shear line, theconvective cells located around the bend part of the shear line are the mostintense. After a comprehensive analysis, two possible explanations to thisphenomenon are considered. One is related to the orientations between thesurface meso-scale shear line and the ambient wind fields and the other is tied tothe relationship between the radiation direction of the meso-scale gravity waveand the orientation of the moist tongue. As for the formation mechanism of theleft-moving MCS, the shear instability induced by the upper-level jet may haveacted as a key role.更多还原