By employing the 6.7μm satellite vapor cloud images and NCEP/NCAR 1°×1° reanalysis datasets, the characteristics and mechanism of the dry intrusion, as well as its impacts on the low vortex precipita- ...By employing the 6.7μm satellite vapor cloud images and NCEP/NCAR 1°×1° reanalysis datasets, the characteristics and mechanism of the dry intrusion, as well as its impacts on the low vortex precipita- tion at the Meiyu front are explored in this paper. It is found that the formation,development and main- tenance of the low vortex precipitation at the Meiyu front are closely related to the evolution of the dry intrusion. The dry intrusion is characterized by high potential vorticity (PV), low humidity and cold air. The dry intrusion exhibits as an obvious dark zone on vapor cloud images, an area in which atmos- pheric relative humidity is lower than 60%. However, the features of the dry intrusion on the vapor im- ages are clearer than that of the humidity field, for the former is the digital vapor cloud images with high temporal and spatial resolution, and it can be used to explore the finer characteristics of the develop- ment, evolution and supplement of the intrusion during the development of the low vortex. The dry intrusion impacts accompanying the low vortex precipitation at the Meiyu front come from all levels of the troposphere, with the strongest intrusion located at the upper troposphere. The dry and cold air intrudes the vicinity of the low vortex from the upper isentropic surface to the lower one, slanting eastward from lower to higher level. The low vortex precipitation region is usually situated in front of the dry intrusion where the relative humidity gradient is higher. The research also reveals that the mechanism of the dry intrusion is that the high potential vorticity descends from the upper troposphere to the lower level, therefore, the dry intrusion can be used as an important index of the high PV forcing. To the west of the low vortex precipitation, the upper level northerlies descend across the isentropic surface, then the dry cold advection can trigger the instable development in the mid-low troposphere. The dry intrusion enhances the low vortex precipitation. Meanwhile, because of the 展开更多
NCEP GFS(Global Forecast System)analytical data(available 4 times per day),satellite cloud image data and real-time observations of path and intensity of Typhoon Morakot are employed to investigate the variation of sy...NCEP GFS(Global Forecast System)analytical data(available 4 times per day),satellite cloud image data and real-time observations of path and intensity of Typhoon Morakot are employed to investigate the variation of synoptic dynamics in its intensity and structure before and after the landing.This study intends to offer some hints for the forecast of intensity and structure of typhoons.Results show that in the tangential direction,the averaged asymmetry amplitude of wind on the radius of a large-value center of the low-level wind can be used as an important parameter for diagnosing the intensity of typhoons.Besides,the maximum of the upper dry potential vorticity in Morakot’s center tends to extend downward along the intensive gradient of tangential wind situated on the inner side of a large-value center of the low-level tangential wind.Additionally,the radial advection of the tangential wind determines the variation of tangential wind in conjunction with the vertical transmission of the tangential wind,the inertial centrifugal force and the Coriolis force.These four items are dominant in the motion equation of tangential wind based on a cylindrical coordinate without the effects of friction and turbulence.Moreover,the low-level convergence center of the typhoon has a tendency of shifting and developing along the intensive gradient of the tangential wind in the tangential direction.展开更多
基金Supported by the National Natural Science Foundation of China (Grant No. 40205008)Heavy Rain Opening Foundation (Grant No. IHR2005K04)the National Basic Research Program of China (Grant No.2006CB403601)
文摘By employing the 6.7μm satellite vapor cloud images and NCEP/NCAR 1°×1° reanalysis datasets, the characteristics and mechanism of the dry intrusion, as well as its impacts on the low vortex precipita- tion at the Meiyu front are explored in this paper. It is found that the formation,development and main- tenance of the low vortex precipitation at the Meiyu front are closely related to the evolution of the dry intrusion. The dry intrusion is characterized by high potential vorticity (PV), low humidity and cold air. The dry intrusion exhibits as an obvious dark zone on vapor cloud images, an area in which atmos- pheric relative humidity is lower than 60%. However, the features of the dry intrusion on the vapor im- ages are clearer than that of the humidity field, for the former is the digital vapor cloud images with high temporal and spatial resolution, and it can be used to explore the finer characteristics of the develop- ment, evolution and supplement of the intrusion during the development of the low vortex. The dry intrusion impacts accompanying the low vortex precipitation at the Meiyu front come from all levels of the troposphere, with the strongest intrusion located at the upper troposphere. The dry and cold air intrudes the vicinity of the low vortex from the upper isentropic surface to the lower one, slanting eastward from lower to higher level. The low vortex precipitation region is usually situated in front of the dry intrusion where the relative humidity gradient is higher. The research also reveals that the mechanism of the dry intrusion is that the high potential vorticity descends from the upper troposphere to the lower level, therefore, the dry intrusion can be used as an important index of the high PV forcing. To the west of the low vortex precipitation, the upper level northerlies descend across the isentropic surface, then the dry cold advection can trigger the instable development in the mid-low troposphere. The dry intrusion enhances the low vortex precipitation. Meanwhile, because of the
基金Natural Fundamental Research and Development Project"973"Program(2013CB430103)National Natural Science Foundation of China(41375058)Foundation of Science&Technology Innovation Team in Jiangsu Province
文摘NCEP GFS(Global Forecast System)analytical data(available 4 times per day),satellite cloud image data and real-time observations of path and intensity of Typhoon Morakot are employed to investigate the variation of synoptic dynamics in its intensity and structure before and after the landing.This study intends to offer some hints for the forecast of intensity and structure of typhoons.Results show that in the tangential direction,the averaged asymmetry amplitude of wind on the radius of a large-value center of the low-level wind can be used as an important parameter for diagnosing the intensity of typhoons.Besides,the maximum of the upper dry potential vorticity in Morakot’s center tends to extend downward along the intensive gradient of tangential wind situated on the inner side of a large-value center of the low-level tangential wind.Additionally,the radial advection of the tangential wind determines the variation of tangential wind in conjunction with the vertical transmission of the tangential wind,the inertial centrifugal force and the Coriolis force.These four items are dominant in the motion equation of tangential wind based on a cylindrical coordinate without the effects of friction and turbulence.Moreover,the low-level convergence center of the typhoon has a tendency of shifting and developing along the intensive gradient of the tangential wind in the tangential direction.
文摘利用NCEP 1°×1°资料、加密自动站降水资料及FY2C红外云图资料对2012年7月6-9日及2011年7月4—6日期间发生在四川盆地并造成强降水的4次相似MCC过程的卫星云图特征进行了对比分析,探讨了MCC生成及发展旺盛期的环流场特征,进一步诊断了MCC发展过程的水汽、热力以及动力机制(垂直散度通量、干位涡).研究表明:2例过程中4次MCC的第1、3个是在进入盆地的卷云羽上的对流云团中生成和发展,2、4个MCC是在残留云系上的对流云团中生成、发展和合并,形成过程中均表现为亮温降低、面积增大,川西高原及MCS周围云系逐渐减弱的特点.在强且持续的孟湾水汽输送和不稳定层结条件下,对流层低层西南涡区辐合抬升造成强烈的散度通量辐合向上输送与高层辐散耦合是MCC发展持续的动力机制,且850 h Pa垂直散度通量辐合区与MCC持续区域及观测的地面强降水落区具有良好的时空对应关系.中高纬的高值位涡具有向南向下延伸,低纬低层的低值位涡具有向上向北传播的特点.当干冷空气与较强暖湿气流相交汇时,850h Pa急流左前侧的西南涡区强烈的风场切变与辐合导致强上升运动,触发对流不稳定能量释放,激发MCC云团,造成强降水.
