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1.
东亚高空纬向风的季节内振荡对登陆中国大陆热带气旋活动的影响 总被引：1，自引：2，他引：1
使用1979—2015年欧洲长期天气预报中心所提供的ERA-Interim再分析资料和中国气象局上海台风研究所整编的西北太平洋热带气旋（TC）最佳路径资料,分析了7—8月东亚高空纬向风的季节内振荡（ISO）信号特征及其与登陆中国大陆热带气旋（TC）的关系。结果表明:（1）200 hPa纬向风在副热带、中纬度地区季节内振荡显著,尤其是在纬向西风带中,有两个南北分布的大值中心,方差贡献均超过50%。（2）基于东亚高空纬向风的ISO和EOF典型空间模定义的西风指数(EAWI),可以用来描述东亚高空纬向西风在ISO尺度上的经向移动。（3）在西风指数的ISO负位相期间,登陆中国东南沿岸22 °N以北的TC增多;反之减少。在西风急流出口南侧的副热带区域,200 hPa ISO纬向风向北移动,使纬向西风位置偏北,出现东风异常,从而使西风减弱;TC引导气流为向西的异常,有利于TC登陆中国大陆偏北沿岸;同时有异常的ISO纬向异常东风切变,有利于TC登陆过程的维持。（4）在西风指数的ISO负位相期间,在对流层高层西风急流出口区向南输送的天气尺度的E矢量,在TC登陆地区,出现异常扰动涡度通量的辐合,引起了该区域的西风减弱。 相似文献
2.
The extratropical transitions(ETs)of tropical cyclones(TCs)over China and the ocean east to 150°E are investigated by the use of best-track data and JRA-25 reanalysis spanning 1979-2008.The ET events occurring north of 25°N and in the warm season(from May to October)are extracted from the reanalysis to emphasize the interaction between TC and midlatitude circulation.Statistical analysis shows that 18.5%of the warm-season TCs go through land ETs north of 25°N in the western North Pacific.And 20.5%of the ET events occur over the ocean east of 150°E.Most(62.2%)ET TCs over China gradually die out after ET,but more(70.7%)ocean ET cases have post-ET reintensification.The evolutions in cyclone phase space and the composite fields for land and ocean ETs,as well as the ET cases with and without post-ET reintensification,are further analyzed.It is found that most TCs with ET over China and those without post-ET reintensification evolve along the typical ET phase path as follows:emergence of thermal asymmetry→losing upper-level warm core→losing lower-level cold core→evolving as extratropical cyclone.The TCs undergoing ETs over ocean and those with post-ET reintensification form a high-level cold core before the ET onset.The TCs with land ET have long distance between the landing TC and a high-level trough.That makes the TC maintain more tropical features and isolates the TC flow from the upstream and downstream jets of the midlatitude trough.The structure of circulation leads to weak development of baroclinicity in land ET.On the contrary,shorter distance between ocean TC and high-level trough makes the high-level trough absorb the TC absolutely.Under that baroclinicity-favorable environment,strong cold advection makes the TC lose its high-level warm core before ET onset.The composite fields confirm that the TC with ocean ET has stronger baroclinic features.Generally,the TC at land ET onset is located to the south of the ridge of the subtropical high,which tends to prevent the TCs from interacting with midlatitude circulation.But for the ocean ET,the situation is just the opposite.Similar analyses are also carried out for the TCs with and without post-ET reintensification over both land and ocean east of 150°E.The results further prove that the TC with stronger baroclinic characteristics,especially in the circumstance favorable to its interaction with high-level midlatitude systems,has more opportunity to reintensify as an extratropical cyclone after ET. 相似文献
3.
This study examines the features and dynamical processes of subseasonal zonal oscillation of the western Pacific subtropical high (WPSH) during early summer, by performing a multivariate empirical orthogonal function (MVEOF) analysis on daily winds and a diagnosis on potential vorticity (PV) at 500 hPa for the period 1979–2016. The first MV-EOF mode is characterized by an anticyclonic anomaly occupying southeastern China to subtropical western North Pacific regions. It has a period of 10–25 days and represents zonal shift of the WPSH. When the WPSH stretches more westward, the South Asian high (SAH) extends more eastward. Above-normal precipitation is observed over the Yangtze–Huaihe River (YHR) basin. Suppressed convection with anomalous descending motion is located over the subtropical western North Pacific. The relative zonal movement of the SAH and the WPSH helps to establish an anomalous local vertical circulation of ascending motion with upper-level divergence over the YHR basin and descending motion with upper-level convergence over the subtropical western Pacific. The above local vertical circulation provides a dynamic condition for persistent rainfall over the YHR basin. An enhanced southwest flow over the WPSH’s western edge transports more moisture to eastern China, providing a necessary water vapor condition for the persistent rainfall over the YHR basin. A potential vorticity diagnosis reveals that anomalous diabatic heating is a main source for PV generation. The anomalous cooling over the subtropical western Pacific produces a local negative PV center at 500 hPa. The anomalous heating over the YHR basin generates a local positive PV center. The above south–north dipolar structure of PV anomaly along with the climatological southerly flow leads to northward advection of negative PV. These two processes are conducive to the WPSH’s westward extension. The vertical advection process is unfavorable to the westward extension but contributes to the eastward retreat of the WPSH. 相似文献
4.
The present study identifies a significant influence of the sea surface temperature gradient(SSTG) between the tropical Indian Ocean(TIO; 15°S-15°N, 40°-90°E) and the western Pacific warm pool(WWP; 0°-15°N, 125°-155°E) in boreal spring on tropical cyclone(TC) landfall frequency in mainland China in boreal summer. During the period 1979-2015, a positive spring SSTG induces a zonal inter-basin circulation anomaly with lower-level convergence, mid-tropospheric ascendance and upper-level divergence over the west-central TIO, and the opposite situation over the WWP, which produces lower-level anomalous easterlies and upper-level anomalous westerlies between the TIO and WWP. This zonal circulation anomaly further warms the west-central TIO by driving warm water westward and cools the WWP by inducing local upwelling, which facilitates the persistence of the anomaly until the summer. Consequently, lower-level negative vorticity, strong vertical wind shear and lower-level anticyclonic anomalies prevail over most of the western North Pacific(WNP), which decreases the TC genesis frequency. Meanwhile, there is an anomalous mid-tropospheric anticyclone over the main WNP TC genesis region,meaning a westerly anomaly dominates over coastal regions of mainland China, which is unfavorable for steering TCs to make landfall in mainland China during summer. This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in mainland China. 相似文献
5.
Tropical cyclone potential hazard in Southeast China and its linkage with the East Asian westerly jet 总被引：1，自引：0，他引：1
A new synthesized index for estimating the hazard of both accumulated strong winds and heavy rainfall from a tropical cyclone (TC) is presented and applied to represent TC potential hazard over Southeast China. Its relationship with the East Asian westerly jet in the upper troposphere is also investigated. The results show that the new TC potential hazard index (PHI) is good at reflecting individual TC hazard and has significantly higher correlation with economic losses. Seasonal variation of TC-PHI shows that the largest TC-PHI on average occurs in July-August, the months when most TCs make landfall over mainland China. The spatial distribution of PHI at site shows that high PHI associated with major landfall TCs occurs along the southeast coast of China. An East Asian westerly jet index (EAWJI), which represents the meridional migration of the westerly jet, is defined based on two regions where significant correlations exist between TC landfall frequency and zonal wind at 200 hPa. Further analyses show that an anomalous easterly steering flow occurred above the tracks of TCs, and favored TCs making landfall along the southeast coast of China, leading to an increase in the landfall TC when the EAWJ was located north of its average latitude. Meanwhile, anomalous easterly wind shear and positive anomaly in low-level relative vorticity along TCs landfall-track favored TC development. In addition, anomalous water vapor transport from westerly wind in the South China Sea resulted in more condensational heating and an enhanced monsoon trough, leading to the maintenance of TC intensity for a longer time. All of these environmental factors increase the TC potential hazard in Southeast China. Furthermore, the EAWJ may affect tropical circulation by exciting meridional propagation of transient eddies. During a low EAWJI phase in July-August, anomalous transient eddy vorticity flux at 200 hPa propagates southward over the exit region of the EAWJ, resulting in eddy vorticity flux convergence and the weakening in the zonal westerly flow to the south of the EAWJ exit region, producing a favorable upper-level circulation for a TC making landfall. 相似文献
6.
使用中国气象局热带气旋资料中心的热带气旋最佳路径数据集和NCEP/NCAR再分析资料提供的月平均数据,对北上影响山东的热带气旋（tropical cyclone,TC）及其造成的极端降水进行统计分析,并揭示了有利于 TC北移影响山东的大气环流特征。结果表明：影响山东的 TC主要出现 于 6—9 月,其中盛夏时节（7、8 月）TC对山东影响最大;TC影响山东时,强度主要为台风及以下等 级,或已发生变性;TC会引发山东极端降水事件,TC极端降水多出现在夏秋季（7—9 月）,其中8月的占比最大,9月次之,TC降水在极端降水事件中的占比约为 10%,但年际变化大,有些年份占比达60%以上,特别是1990 年以来 TC对极端降水的贡献显著增强;影响山东的 TC主要生成于西 北太平洋,多为转向型路径;当500 hPa位势高度异常场呈太平洋一日本遥相关型的正位相时,TC更易北上影响山东,此时西北太平洋副热带高压位置偏北,其外围气流会引导TC北上转向,对华东地区造成影响;850 hPa上,南海至西北太平洋存在异常气旋式环流,对流活跃,夏季风环流和季风槽加强,有利于TC的生成和发展,同时,华东、华南上空有异常上升运动,涡度增大,垂直风切变减小,水汽充沛,TC登陆后强度能得到较好的维持。 相似文献
7.
Based on high-resolution reanalysis data of the European Centre for Medium-Range Weather Forecasts, several samples of tropical cyclones (TCs), including tropical storms, severe tropical storms, and typhoons, in the South China Sea (SCS), were selected for composite analysis. The structures of these three types of vortices and their differences with ‘bogus’ vortices were investigated. Results showed that TCs in the SCS have characteristics that are distinctly different from vortices formed by the bogussing scheme used at Guangzhou Institute of Tropical and Marine Meteorology, such as no anticyclone in higher layers, strong convergence concentrated at the bottom of the troposphere, and strong divergence happening in higher layers instead of at 400 hPa. These differences provide clues for constructing a more realistic structure for TCs in the SCS. It was also found that the three types of vortices have some structural features in common. The area with high wind speed is fan-shaped in the north around the TC center, the maximum vorticity appears at 925 hPa, the strongest convergence appears at 1000 hPa, and strong divergence is located from 150 to 100 hPa. On the contrary, significant differences between them were revealed. The warm cores in tropical storms, severe tropical storms, and typhoons are located at 600–400 hPa, 400−300 hPa, and 400−250 hPa, respectively. Among the three types of TCs, the bogus vortex of tropical storms has the largest errors in structure and suffers the largest errors in track forecasts. However, typhoons have the largest errors in the forecast of intensity. This may be related to the great impacts of ocean on TC intensity. 相似文献
8.
A CLIMATOLOGY OF EXTRATROPICAL TRANSITION OF TROPICAL CYCLONES IN THE WESTERN NORTH PACIFIC 总被引：1，自引：1，他引：0
Based on best-track data and JRA-25 reanalysis, a climatology of western North Pacific extratropical
transition (ET) of tropical cyclone (TC) is presented in this paper. It was found that 35% (318 out of 912) of all
TCs underwent ET during 1979–2008. The warm-season (June through September) ETs account for 64% of all ET events
with the most occurrence in September. The area 120°E–150°E and 20°N–40°N is the most favorable region for ET
onsets in western North Pacific. The TCs experience ET at latitudes 30°N–40°N and have the greatest intensity in
contrast to other latitude bands. The distribution of ET onset locations shows obviously meridional migration in
different seasons. A cyclone phase space (CPS) method was used to analyze the TC evolution during ET. Except for
some cases of abnormal ET at relatively high latitudes, typical phase evolution paths—along which TC firstly
showed thermal asymmetry and an upper-level cold core and then lost its low-level warm core—can be used to
describe the main features of ET processes in western North Pacific. Some seasonal variations of ET evolution paths
in CPS were also found at low latitudes south of 15°N, which suggests different ET onset mechanisms there. Further
composite analysis concluded that warm-season ETs have generally two types of evolutions, but only one type in cold
season (October through next May). The first type of warm-season ETs has less baroclinicity due to long distance
between the TC and upper-level mid-latitude system. However, significant interactions between a mid-latitude upper
-level trough and TC, of either approaching or being absorbed into the trough, and TC’s relations with downstream
and upstream upper-level jets, are the fingerprints for both a second type of warm-season ETs and almost all the
cold-season ETs. For each type of ETs, detailed structural characteristics as well as precipitation distribution
are illustrated by latitude. 相似文献
9.
THE INTENSITY VARIATION OF THE SUMMER INTERTROPICAL CONVERGENCE ZONE IN WESTERN NORTH PACIFIC AND ITS IMPACT ON TROPICAL CYCLONES 总被引：3，自引：1，他引：2
Based on the satellite data from the National Oceanic and Atmospheric Administration and the NCEP/NCAR reanalysis data, the variation of the intensity of convection over the Intertropical Convergence Zone(ITCZ) in summer and its impacts on tropical cyclones are studied. In this paper, an intensity index of the ITCZ is proposed according to Outgoing Longwave Radiation(OLR) in the region of(5°–20°N, 120°–150°E) in the western North Pacific(WNP). Then strong and weak ITCZ years are classified and different variables during the strong/weak ITCZ years are analyzed. The composite results show that the ITCZ anomaly is connected to the general atmospheric circulation and SST distribution. In the strong ITCZ years, the subtropical anticyclone weakens and shifts northward. Besides, there is salient cyclonic anomaly at the low level and anticyclonic anomaly at the high level. SST patterns in the preceding winter resemble to those of La Nina. It could persist into the succeeding summer. However, it is opposite in the weak ITCZ years. The impact of the ITCZ anomaly on the tropical cyclone(TC) formation and track is also discussed. There are more TCs over the WNP(5°–20°N, 120°–150°E) in the strong ITCZ years and there is a significant increase in the northward recurving TCs. In the weak ITCZ years, fewer TCs occur and the frequency of the northwestward track is higher. 相似文献
10.
利用中国气象局热带气旋（TC）资料、NCEP/NCAR 再分析资料和美国 NOAA 向外长波辐射（OLR）等资料,分析了2010年西北太平洋（WNP）及南海（SCS）热带气旋活动异常的可能成因,讨论了同期大气环流配置和海温外强迫对TC生成和登陆的动力和热力条件的影响。结果表明,2010年生成TC频数明显偏少,生成源地显著偏西,而登陆TC频数与常年持平。导致7～10月TC频数明显偏少的大尺度环境场特征为：副热带高压较常年异常偏强、西伸脊点偏西,季风槽位置异常偏西,弱垂直风切变带位置也较常年偏西且范围偏小,南亚高压异常偏强,贝加尔湖附近对流层低高层均为反气旋距平环流,这些关键环流因子的特征和配置都不利于 TC 在WNP的东部生成。影响TC活动的外强迫场特征为：2010年热带太平洋经历了El Ni?o事件于春末夏初消亡、La Ni?a事件于7月形成的转换;7～10月,WNP海表温度维持正距平,140°E以东为负距平且对流活动受到抑制;暖池次表层海温异常偏暖,对应上空850 hPa为东风距平,有利于季风槽偏西和TC在WNP的西北侧海域生成。WNP海表温度和暖池次表层海温的特征是2010年TC生成频数偏少、生成源地异常偏西的重要外强迫信号。有利于7～10月热带气旋西行和登陆的500 hPa风场特征为：北太平洋为反气旋环流距平,其南侧为东风异常,该东风异常南缘可到25°N,并向西扩展至中国大陆地区;南海和西北太平洋地区15°N以南的低纬也为东风异常;在这样的风场分布型下,TC容易受偏东气流引导西行并登陆我国沿海地区。这是2010年生成TC偏少但登陆TC并不少的重要环流条件。 相似文献
11.
利用NCEP 1°×1°的6 h再分析资料和常规气象观测资料,对2012年7月21日发生在北京地区的一次大暴雨天气过程进行非地转湿Q矢量(Q*)和湿位涡等物理量诊断分析,研究暴雨期间Q*散度、锋生函数和湿位涡的时空分布特征,以及它们与强降水之间的关系。结果表明,Q*在850 hPa高度层上对暴雨表现出良好的诊断特性,冷、暖气流的汇聚加强了锋生作用,强锋生中心出现几小时后即出现暴雨。暴雨区位于Q*辐合区内,Q*散度对6 h后暴雨的落区有很好的指示意义。暴雨落区基本位于MPV1正、负值交界处的等值线密集带上以及MPV2负值区内。暴雨区上空,从近地面到对流层低层的对流性不稳定与条件性对称不稳定同时存在,两者共同作用,这很可能是此次暴雨的中尺度对流系统发生发展的重要条件之一。 相似文献
12.
CONSERVATION OF MOIST POTENTIAL VORTICITY AND DOWN-SLIDING SLANTWISE VORTICITY DEVELOPMENT* 
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下载免费PDF全文 An accurate form of the moist potential vorticity(MPV) equation was deduced from a complete set of primitive equations.It was shown that motion in a saturated atmosphere without diabatic heating and friction conserves moist potential vorticity.This property was then used to investigate the development of vertical vorticity in moist baroclinic processes.Results show that in the framework of moist isentropic coordinate,vorticity development can result from reduction of convective stability,or convergence,or latent heat release at isentropic surfaces.However,the application of the usual analysis of moist isentropic potential vorticity is limited due to the declination of moist isentropic surfaces.and a theory of development based on z-coordinate and p-coordinate was then proposed.According to this theory,whether the atmosphere is moist-symmetrically stable or unstable,on convective stable or unstable,the reduction of convective stability,the increase of the vertical shear of horizontal wind or moist baroclinity may result in the increase of vertical vorticity,so long as the moist isentropic surface is slantwise.The larger the declination of the moist isentropic surface,the more vigorous the development of vertical vorticity.In a region with a monsoon front to the north and the warm and moist air to the south,or by the north of the front,the moist isentropes are very steep.The is the region most favorable for development of vorticities and formation of torrential rain.For a case of persistent torrential rain occurring in the middle and lower reaches of the Changjiang and Huaihe Rivers in June 11-15,1991,moist potential vorticity analysis,especially the isobaric analysis of its vertical and horizontal components,i.e.MPV1 and MPV2,respectively,is effective for identifying synoptic systems not only in middle and high latitudes,but also in low latitudes and in the lower troposphere.It can serve as a powerful tool for the diagnosis and prediction of torrential rain. 相似文献
13.
利用高分辨率观测资料和ERA5再分析资料, 分析造成2021年11月7—8日东北极端暴雪的温带气旋结构特征及爆发性发展机制, 结果表明:温带气旋发生在高空冷涡背景下, 地面气旋在黄海形成后出现爆发性快速增强并沿东北地区东部北上。地面降雪区主要分布在气旋西侧, 且降雪强度与气旋的发生发展密切相关;地面气旋在爆发性发展后由叶状云系演变为逗点涡旋云系, 并表现出明显的锋面断裂和暖锋包卷;其垂直结构也先后出现高空锋区断裂、干暖核形成和中性锢囚锋区加强;西伯利亚高压脊、华北高空槽和东北高压脊3个异常中心构成Rossby波列, 随着高度异常中心不断东移及波能量向下游地区频散, 华北高空槽区的波作用通量明显增大导致华北冷涡快速增强, 涡度因子的急剧增大有利于地面气旋爆发性发展;随着平流层位涡高值区沿等熵面不断向南发展和向下传播, 导致中层冷涡快速发展并向下伸展, 诱发地面气旋爆发性增强。 相似文献
14.
15.
北半球阻塞高压的维持Ⅰ:准地转和Ertel位涡分析 总被引：2，自引：3，他引：2
从北半球不同地区的４个阻塞高压个例，研究了阻塞高压维持机制及其地域性差异。对阻塞高压区域３００ｈＰａ时间平均准地转让涡低值区的维持机制的分析表明，该位涡低值区的维持机制存在显著的地域性差异。其中，在２个太平洋阻高个例中，时间平均流的位涡平流使低位涡向阻高西北部输送，从而有利于位涡低值区经向度的维持；时变扰动的位涡输送则有抵消平均流作用的倾向，从而对位涡低值区的维持起耗散作用。在大西洋和东亚阻高个例中，平均流的位涡平流使低位涡区向东移动，而时变扰动的位涡输送则有抵消平均流位涡平流的作用，从而有利于位涡低值区在原地稳定维持。以上主要是相对涡度输送的差异所致。等熵面Ｅｒｔｅｌ位涡分析表明，阻高区域３３０Ｋ时间平均等熵位涡低值区的维持机制与３００ｈＰａ时间平均准地转位涡低值区的维持机制十分相似，从而表明以上等压面准地转位涡分析可以近似用来代表等熵Ｅｒｔｅｌ位涡分析。 相似文献
16.
Three typhoons, Goni, Morakot and Etau which were generated in Western Pacific in
2009, are successfully simulated by the WRF model. The horizontal and vertical vorticity and
their interaction are analyzed and diagnosed by using the simulation results. It is shown that
their resultant vectors had a fixed pattern in the evolution process of the three typhoons:
The horizontal vorticity converged to the tropical cyclone (TC) center below 900 hPa level,
flowed out from it at around 900 to 800 hPa, and flowed in between 800 hPa and 700 hPa. If
multiple maximum wind speed centers showed up, the horizontal vorticity converged to the
center of the typhoon below the maximum wind speed center and diverged from the TC center
above the maximum wind speed center. At low levels, the three typhoons interacted with each
other through vertical circulation generated by the vortex tube. This circulation was mainly
generated by the eastward or westward horizontal vorticity vectors. Clouds and precipitation
were generated on the ascending branch of the vertical circulation. The vortex tubes often
flowed toward the southwest of the right TC from the northeast of the left TC. According to
the full vorticity equation, the horizontal vorticity converted into the vertical vorticity
near the maximum wind speed center below 850 hPa level, and the period of most intense
conversion was consistent with the intensification period of TC, while the vorticity advection
was against the intensification. The vertical vorticity converted into the horizontal
vorticity from 800 hPa to 600 hPa, and the wind speed decreased above the maximum wind speed
region at low levels. 相似文献
17.
Four observed blocking anticyclones in different regions of the Northern Hemisphere are in-vestigated.Analyses show that there exist distinct differences in the maintenance of the time-meanquasi-geostrophic potential vorticity(PV)low in 300 hPa within blocking areas.In two Pacificblocking cases,the PV advection by time-mean flow tends to flow the PV low to northwestern partof the blocking highs,and thus is beneficial to the maintenance of the blockings'strength.Thetransfer by transient eddies acts to balance the effect of the time-mean flow.In the Atlantic andAlaska blocking cases,however,the advection of mean flow tends to flow the PV low eastward.The PV transfer by transient eddies acts to flow potential vorticity low to the western part of theblocking ridges and also to balance the time-mean flow's effect.Thus,in the latter two cases,it isthe transfer by the transient eddies that acts to maintain the blockings. 相似文献
18.
In this study, we first show that tropical cyclone (TC) Usagi evolved from a mid-level vortex over the South China Sea (SCS) in August 2001. The initial disturbance of TC Usagi had a maximum potential vorticity (PV) near 500 hPa, and an anticyclonic circulation with a cold core near the surface. The cyclonic circulation and its warm core of the mid-level vortex developed gradually downward toward the surface when environmental easterly and dry air intruded from the upper troposphere; finally, the mid-level vortex evolved into TC Usagi under favorable environment conditions such as weak vertical wind shear, deep moist layer, etc. To investigate the dynamic and thermodynamic processes during TC Usagi genesis, the technique of piecewise PV inversion is employed. The results show that the actions of upper-layer PV and potential temperature anomalies were not important in TC Usagi genesis. Surface-layer thermal anomalies mainly produced negative disturbances of temperature at the vortex center below 800 hPa, which was unfavorable to the genesis of a cyclonic circulation near the surface. Middle-to-lower-layer latent heat played a key role in TC Usagi genesis and downward development of dynamic and thermodynamic processes. The actions of dry air intrusion from the upper troposphere, environmental westerly changing into easterly in the middle and lower troposphere, and baroclinic structure of the vortex were also important. The cyclonic circulation of the mid-level vortex could develop downward quickly from the middle troposphere toward the surface. However, whether the warm core of the vortex developed near the surface depended on the combined actions of surface-layer thermal anomaly and middle-to-lower-layer latent heat. Finally, we present a conceptual model of TC Usagi genesis induced by a mid-level vortex over the SCS. 相似文献
19.
Four observed blocking anticyclones in different regions of the Northern Hemisphere are investigated.Analyses show that there exist distinct differences in the maintenance of the time-mean quasi-geostrophic potential vorticity(PV) low in 300 hPa within blocking areas.In two Pacific blocking cases,the PV advection by time-mean flow tends to flow the PV low to northwestern part of the blocking highs,and thus is beneficial to the maintenance of the blockings'strength.The transfer by transient eddies acts to balance the effect of the time-mean flow.In the Atlantic and Alaska blocking cases,however,the advection of mean flow tends to flow the PV low eastward.The PV transfer by transient eddies acts to flow potential vorticity low to the western part of the blocking ridges and also to balance the time-mean flow's effect.Thus,in the latter two cases,it is the transfer by the transient eddies that acts to maintain the blockings. 相似文献
20.
A Statistical Analysis on the E ect of Vertical Wind Shear on Tropical Cyclone Development 下载免费PDF全文
下载免费PDF全文 Using tropical cyclone (TC) best track and intensity of the western North Pacific data from the Joint TyphoonWarning Center (JTWC) of the United States and the NCEP/NCAR reanalysis data for the period of 1992-2002, the effects of vertical wind shear on TC intensity are examined. The samples were limited to the westward or northwestward moving TCs between 5°N and 20°N in order to minimize thermodynamic effects. It is found that the effect of vertical wind shear between 200 and 500 hPa on TC intensity change is larger than that of the shear between 500 and 850 hPa, while similar to that of the shear between 200 and 850 hPa. Vertical wind shear may have a threshold value, which tends to decrease as TC intensifies. As the intensifying rate of TC weakens, the average shear increases. The large shear has the obvious trend of inhibiting TC development. The average shear of TC which can develop into typhoon (tropical depression or tropical storm) is below 7 m s-1 (above 8 m s-1). 相似文献