A study of partitioning Q vector on background conditions of a torrential rainfall over Shanghai, China on 25 August 2008

A rainfall that occurred during 0200–1400 Beijing Standard Time(BST)25 August 2008 shows the rapid development of a convective system,a short life span,and a record rate of 117.5 mm h-1for Xujiahui station since 1872.To study this torrential rainfall process,the partitioning method of Q vector is developed,in which a moist Q vector is first separated into a dry ageostrophic Q vector(DQ)and a diabatic-heating component.The dry ageostrophic Q vector is further partitioned along isothermal lines in the natural...     

METEOROLOGICAL CHARACTERISTICS DURING THE SECOND MEIYU EPISODE IN 1998

This article examined the meteorological features of the second Meiyu/Baiu episode(hereafterMeiyu Ⅱ)in 1998 and the results are summarized as follows:(1)The Meiyu Ⅱ period in 1998 hasbeen the longest and the most delayed ending one since 1885,which caused the great flood aroundthe Yangtze River Basin.The circulation situation in Meiyu Ⅱ is so typical that the averagegeopotential height and wind fields at 500 hPa and 850 hPa are very similar to monthly mean inJune.The abnormal circulation in Meiyu Ⅱ is associated with an adjusting of the Okhotsk high andshort period south-north oscillation of the subtropical high.(2)The heavy rainfall around theYangtze River during Meiyu Ⅱ period is closely related to a persistent shear line,a clear eastwardmoving vortex structure and a very strong jet flow at lower levels.It is very clear that the strongdivergence at higher levels and convergence at lower levels occurred in situ during Meiyu Ⅱ period.(3)It is very clear that the convective activity propagates eastward quickly with a period about 7days during Meiyu Ⅱ period.     

Typical Structure, Variety, and Multi-Scale Characteristics of Meiyu Front

Meiyu front plays an important role in summer rainfall in central China. Based on the GMS-5 satellite images, NCEP reanalyses （2.5°×2.5°） and final analyses （1°×1°） data, and meteorological conventional sounding observations, the horizontal and vertical structures of the Meiyu front were summarized using multiple diagnostic variables, including winds, temperature, jet stream, front, pseduo-equivalent potential temperature, divergence, vertical motion, static instability, etc. In this paper, four cases were selected and analyzed, two of which are in 26-28 June and 23 July 2002 during the Experiment on Heavy Rain in the Meiyu period in the lower reaches of the Yangtze River, and the others are in May and July 1998. The two cases in July 1998 and July 2002 are the secondary Meiyu front cases. The results show that the structures and characteristics of the Meiyu front are different for various cases, or at various places and time, or at various stages of one case, and the frontal characteristics can be converted from the polar front to the equatorial front. Because of the interaction of the different scale circulations in the high and low latitudes, the horizontal structure of the Meiyu front has various forms.
The results in this paper also show that the typical Meiyu front consists of a narrow band with a high gradient of potential equivalent temperature below 500 hPa, south of which is warm and moist air mass, and north of which is the transformed air mass from the midlatitude ocean or polar continent. Below the mid troposphere, south of the front blows southwesterlies, while north blows easterlies. The ascending motion and precipitation usually occur ahead of the Meiyu front. In the upper troposphere, the subtropical front is above the Meiyu front, but two fronts are separated. In addition, the upper westerly jet stream and the easterlies to the south of the Meiyu front result in the upper divergent flow field.
The multi-scale characteristics of the horizontal structure of the Meiyu front can     

A Case Study on a Quasi-Stationary Meiyu Front Bringing About Continuous Rainstorms with Piecewise Potential Vorticity Inversion

A 4-day persistent rainstorm resulting in serious flooding disasters occurred in the north of Fujian Province under the influences of a quasi-stationary Meiyu front during 5-8 June 2006. With 1°× 1° latitude and longitude NCEP reanalysis data and the ground surface rainfall, using the potential vorticity （PV） analysis and PV inversion method, the evolution of main synoptic systems, and the corresponding PV and PV perturbation （or PV anomalies） and their relationship with heavy rainfall along the Meiyu front are analyzed in order to investigate the physical mechanism of the formation, development, and maintenance of the Meiyu front. Furthermore, the PV perturbations related to different physics are separated to investigate their different roles in the formation and development of the Meiyu front. The results show： the formation and persistence of the Meiyu front in a quasi-WE orientation are mainly due to the maintenance of the high-pressure systems in its south/north sides （the West Pacific subtropical high/ the high pressure band extending from the Korean Peninsula to east of North China）. The Meiyu front is closely associated with the PV in the lower troposphere. The location of the positive PV perturbation on the Meiyu front matches well with the main heavy rainfall area along the Meiyu front. The PV inversion reveals that the balanced winds satisfying the nonlinear balanced assumption represent to a large extent the real atmospheric flow and its evolution basically reflects the variation of stream flow associated with the Meiyu front. The unbalanced flow forms the convergence band of the Meiyu front and it mainly comes from the high-pressure system in the north side of the Meiyu front. The positive PV perturbation related to latent heat release in the middle-lower troposphere is one of the main factors influencing the formation and development of the Meiyu front. The positive vorticity band from the total balanced winds is in accordance with the Meiyu front band and the magnitude of the posit     

METEOROLOGICAL CHARACTERISTICS DURING THE SECOND MEIYU EPISODE IN 1998*

This article examined the meteorological features of the second Meiyu/Baiu episode(hereafter Meiyu Ⅱ) in 1998 and the results are summarized as follows:(1) The Meiyu Ⅱ period in 1998 has been the longest and the most delayed ending one since 1885,which caused the great flood around the Yangtze River Basin.The circulation situation in Meiyu Ⅱ is so typical that the average geopotential height and wind fields at 500 hPa and 850 hPa are very similar to monthly mean in June.The abnormal circulation in Meiyu Ⅱ is associated with an adjusting of the Okhotsk high and short period south-north oscillation of the subtropical high.(2) The heavy rainfall around the Yangtze River during Meiyu Ⅱ period is closely related to a persistent shear line,a clear eastward moving vortex structure and a very strong jet flow at lower levels.It is very clear that the strong divergence at higher levels and convergence at lower levels occurred in situ during Meiyu Ⅱ period.(3) It is very clear that the convective activity propagates eastward quickly with a period about 7 days during Meiyu Ⅱ period.     

INTERACTION BETWEEN TROPICAL CYCLONE AND MEIYU FRONT

Generally speaking,the convection activities are inactive over western Pacific warm pool andtropical cyclone(TC)activity seldom occurs over the offshore of East Asia during the period ofMeiyu rainfall.However,if a TC is active in this area,the Meiyu rainfall will often weaken or endup.Based on a statistical study with the data from 1980 to 1995,it is found that about 91% of 23TC activities affected the intensity of Meiyu rainfall,and 50% of the end-up of Meiyu events wererelated to the active TCs and the change of subtropical high.The present paper simulates the effectof TC on Meiyu circulation by using MM4 model,and the results agree with the observations.From the point of view of vapor and energy transport,the landing of TC cuts not only thetransport of the water vapor to Changjiang-Huaihe River basin from the Bay of Bengal but also theconversion of the mean flow energy to the Meiyu circulation because of the TC forcing to the zonalcirculation.These two effects make the convection and perturbation existing in Meiyu region lackthe supply of the vapor and energy for their maintenance and lead to the end of Meiyu rainfall.     

A modified moist ageostrophic <Emphasis Type="Italic">Q</Emphasis> vector

The quasi-geostrophic Q vector is an important rainfall associated with large-scale weather systems diagnostic tool for studying development of surface and is calculated using data at single vertical level. When ageostrophic Q vector was introduced, it required data at two vertical levels. In this study, moist ageostrophic Q vector is modified so that it can be calculated using data at a single vertical level. The comparison study between the original and modified moist ageostrophic Q vectors is conducted using the data from 5 to 6 July 1991 during the torrential rainfall event associated with the Changjiang-Huaihe mei-yu front in China. The results reveal that divergences of original and modified moist ageostrophic Q vectors have similar horizontal distributions and their centers are almost located in the precipitation centers. This indicates that modified moist ageostrophic Q vector can be used to diagnose convective development with reasonable accuracy.     

Mesoscale Analysis of a Heavy Rainfall Event Along the Huaihe River Valley During 8-9 July 2007

During 8-9 July 2007,several successively developed rainstorms along the Meiyu front produced heavy rainfall in the Huaihe River Valley,which led to the most catastrophic flooding in this region since 1954.Through mesoscale analysis of both conventional and intensive observations from upper air and surface stations,automatic weather stations,Doppler radars,and the FY-2C satellite,the current study examines the developing style and environmental conditions of the mesoscale convective systems(MCSs)that led to the development of the rainstorms.Our analysis showed that this event went through three phases.The first phase of the heavy rainfall(Phase Ⅰ)was caused by a meso-α-scale wind shear in the lower troposphere during 0200-1700 BT(Beijing Time)8 July.Phase Ⅱ was characterized by a reduction in rain rate and the formation of a low-level vortex between 1700 BT 8 and 0200 BT 9 July.In Phase Ⅲ,the well-organized mature meso-α-scale low-level vortex brought about intensified rains during 0200-0800 BT 9 July.Satellite and raclar observations showed a backward development of MCSs(new convective cells were generated at the back of the system)in PhaseⅡ,a forward development in Phase Ⅲ,and a spiral organization of the convective lines in Phase Ⅱ.The heavy rainstorm systems were initiated continuously along a surface mesoscale dew-point front with a horizontal scale of～300 km(as part of the Meiyu front)in the upper reaches of the Huaihe River Valley near Fuyang City,Anhui Province and then gradually decayed in the middle and lower reaches.It is hypothesized that lifting by strong low-level convergence is sufficient to trigger convection in the high CAPE(convective available potential energy)environment.     

Diagnostic analyses and application of the moist ageostrophic vector<Emphasis Type="Italic">Q</Emphasis>

Considering the main thermal forcing factor, which is critical for the development of synoptic systems,the concept of the moist ageostrophic vector Q is introduced. A formula of the moist ageostrophic Q and the ageostrophic diabatic equation, in which the divergence of the moist ageostrophic Q is taken as a single forcing term, is derived. Meanwhile, the moist ageostrophic Q is applied to diagnose a torrential rain process in North China. The results suggest that the moist ageostrophic Q can clearly reveal the system development during the torrential rain process; the corresponding relationship between the divergence of the moist ageostrophic Q and the rainfall area is better than that of the vertical velocity (ω) and the divergence of the dry Q; the 6-h rainfall region can be correctly drawn according to the negative area of the divergence of the moist ageostrophic Q, and its precipitation is positively correlated to the magnitude of the divergence of the moist ageostrophic Q. The research provides valuable information for improving short-term weather forecast.     

A Simulation Study of the Mesoscale Convective Systems Associated with a Meiyu Frontal Heavy Rain Event

In this study, evolution of the mesoscale convective systems （MCSs） within a Meiyu front during a particularly heavy rainfall event on 22 June 1999 in East China was simulated by using a nonhydrostatic numerical model ARPS （Advanced Regional Prediction System）. Investigations were conducted with emphasis on the impact of the interaction among multi-scale weather systems （MWSs） on the development of MCSs in the Meiyu frontal environment. For this case, the development of MCSs experienced three different stages. （1） The convections associated with MCSs were firstly triggered by the eastward-moving Southwest Vortex （SWV） from the Sichuan Basin, accompanying the intensification of the upper-level jet （ULJ） and the low-level jet （LLJ） that were approaching the Meiyu front. （2） Next, a low-level shear line （LSL） formed, which strengthened and organized the MCSs after the SWV decayed. Meanwhile, the ULJ and LLJ enhanced and produced favorable conditions for the MCSs development. （3） Finally, as the MCSs got intensified, a mesoscale convective vortex （MCV）, a mesoscale LLJ and a mesoscale ULJ were established. Then a coupled-development of MWSs was achieved through the vertical frontal circulations, which further enhanced the MCV and resulted in the heavy rainfall. This is a new physical mechanism for the formation of Meiyu heavy rainfall related to the SWV during the warm season in East China. In the three stages of the heavy rainfall, the vertical frontal circulations exhibited distinguished structures and played a dynamic role, and they enhanced the interaction among the MWSs. A further examination on the formation and evolution of the MCV showed that the MCV was mainly caused by the latent heat release of the MCSs, and the positive feedback between the MCSs and MCV was a key characteristic of the scale interaction in this case.     

“海棠”台风（2005）结构对其降水影响的Q矢量分解研究

摘要： 基于WRF模式对2005年台风“海棠”登陆降水过程的成功模拟, 本文初步尝试利用修改后的非地转干Q矢量(QN矢量)PG分解, 定量揭示台风结构对台风降水和台风雨强差异形成的影响。结果表明： （1）在台风登陆过程的不同阶段, 对台风降水起主要贡献的台风结构因子是不同的。在台风登陆过程前12 h期间, 对降水贡献最为显著的为QNshdv, 其次是QNalst和QNcrst, 而QNcurv的贡献最小; 在后12 h期间, 对降水贡献最为显著的为QNcrst, 其次是QNcurv, QNshdv的贡献列第三, 而QNalst的贡献最小。（2）各台风结构因子QNalst、 QNcurv、 QNshdv及QNcrst对台风降水发生的贡献都存在明显的时、 空变化。（3）在台风登陆降水过程中, 对每个时刻暴雨雨强形成有贡献的台风结构因子是不同的。相对来讲, QNcurv对暴雨、 大暴雨及特大暴雨之间雨强差异形成的贡献最为显著, QNalst与QNcrst的贡献情况较为接近, 而QNshdv的贡献则相对最小。通过QN矢量PG分解, 可以定量揭示出登陆台风结构对台风降水的影响, 这也是总的QN矢量（即QN矢量）难以揭示的潜在物理机制。     

Q矢量旋度：一个与暴雨相关的新诊断量

As a powerful tool to diagnose vertical motion, frontogenesis, and secondary circulation, the Q vector and its divergence are widely used. However, little attention has been given to the curl of Q vector. In this paper, a new set of analyses combining the divergence of the Q vector (DQ) with the vertical component of the curl of the Q vector (VQ) is applied to a Northeastern cold vortex rainfall case. From the derivation, it was found that the expressions of the Q vectors and their divergences in saturated moist flow (DQm) differ from those of dry and unsaturated moist atmosphere (DQ), while the VQs of various background flows are exactly the same, which largely simplified the analyses. This case study showed that, compared with the DQ, not only can the DQm diagnose precipitation more effectively, but the VQ may also be indicative of precipitation (especially for heavy rainfall and strong convection) because of its direct, close relationship with ageostrophic motion. Thus, the VQ may be computed and analyzed with ease, and may serve as a useful tool for analyses of precipitation and strong convective systems. Citation: Yang, S., and D. H. Wang, 2008: The curl of Q vector: a new diagnostic parameter associated with heavy rainfall, Atmos. Oceanic Sci. Lett., 1, 36-39     

定量分析几种Q矢量

结合1991年7月5日20:00～6日20:00一次典型的江淮梅雨锋暴雨过程，从台站实际业务工作需要考虑，细致、具体地比较分析了850 hPa、700 hPa及500 hPa 3个层次的准地转 Q 矢量散度场、半地转 Q 矢量散度场、非地转 Q 矢量散度场及湿 Q 矢量散度场与相应时刻地面降水场对应关系的差异，同时还针对每一种 Q 矢量，将其在850 hPa、700 hPa及500 hPa 3个层次的散度场对同时刻降水场的反映能力进行了比较。在定量比较的基础上，得到了对4种 Q 矢量诊断特性的具体认识:（1）在整个梅雨锋暴雨过程中，3个层次的半地转 Q 矢量散度场及准地转 Q 矢量散度场对雨区的反映能力较小，而非地转 Q 矢量的散度场和湿 Q 矢量的散度场对雨区的反映能力明显较前二者大，尤其是湿 Q 矢量散度场在每个层次的诊断能力基本都大于相应层次的其它 Q 矢量的散度场。（2）对于每一种 Q 矢量而言，基本都是在700 hPa的散度场与雨区的对应关系好于各自在850 hPa和500 hPa的散度场，尤其是700 hPa湿 Q 矢量散度辐合场对同时刻梅雨锋暴雨的强度及落区都有很好的指示作用。最后，基于理论的角度对各 Q 矢量的诊断特性进行了较为深入地探讨和比较分析，明确地指出了4种 Q 矢量存在理论前提上的差异。     

昆明准静止锋的准地转Q矢量分析

应用Ｑ矢量理论,对１９９５年２月一次与云南寒潮天气过程有关的昆明准静止锋进行了诊断分析。结果表明：在８５０ｈＰａ上,云南东部地区存在明显的的非地转风辐合场,从而引起昆明准静止锋锋生;当７００ｈＰａ上Ｑ矢量锋生函数值增大,说明儿后冷空气加强,昆明准静止锋容易南下;低层Ｑ矢量辐合带与昆明准静止锋雨区有较好的对应关系。     

湿Q矢量释用技术及其在定量降水预报中的应用

新发展了一种湿Q矢量释用技术:利用松弛法迭代求解以非地转干Q矢量散度为强迫项的方程得到垂直运动场ω ₁, 然后由ω ₁计算湿Q矢量散度场, 接着再利用松弛法迭代求解以湿Q矢量散度场为强迫项的ω方程得到垂直运动ω ₂, 最后由ω ₂结合水汽条件进行降水量计算, 得到湿Q矢量释用降水场。结合一次典型的江淮梅雨锋暴雨过程研究表明, 湿Q矢量释用降水场对同期观测降水场水平分布特征、极端降水强度都具有一定的反映能力, 反映出湿Q矢量释用技术具备实际应用的可行性和一定合理性。将此释用技术应用于华东区域数值预报模式 (基于MM5 V3.6而建立, 以下简称MM5) 产品, 得到湿Q矢量释用定量降水预报 (QPF) 场, 其独立于模式本身输出的QPF场, 但与模式QPF场具有相同的时空分辨率。针对2004年6—8月汛期华东地区一次梅雨锋降水过程和一次登陆台风降水过程, 结合实况雨量资料, 比较分析了湿Q矢量释用QPF场和MM5模式QPF场对实际观测降水场的反映能力, 结果表明, 前者对有无降水、10.0 mm/24 h以上明显降水的反映能力明显优于后者。进一步进行预报统计检验表明, 湿Q矢量释用预报有无降水、小雨及10.0 mm/24 h以上降水的TS评分、正确率都明显高于MM5模式, 而漏报率、空报率则是前者明显低于后者。这也充分反映出湿Q矢量释用技术应用于QPF研究的有效性。最后, 探讨了数值预报产品释用技术对数值预报模式性能的依赖性, 并指出未来对湿Q矢量释用技术进一步改进的方向及其广泛应用前景。     

吉林省一次强对流天气的中尺度分析

作者对１９９３年８月１２日发生在吉林省境内的一次龙卷风天气进行了分析。结果表明：在出现龙卷风之前和形成初期，地面风场，高空环流形势，Ｑ↑→矢量散度，对流稳定度和能量场诸方面都有表现，可提供预报信息。     

台风“韦帕”(0713)引发华东暴雨过程的诊断比较

采用湿Q矢量、螺旋度、湿位涡对给华东造成严重灾害的超强台风"韦帕"(0713)强降水过程进行诊断比较分析,结果表明:登陆前,上述物理量均能提前指示强降水落区,湿位涡有更多提前指示时间,螺旋度次之,湿Q矢量最少;登陆后,三者均能表征台风主体云系降水,湿位涡、螺旋度与强降水有较好对应关系,而湿Q矢量指示强降水位置偏北,但湿位涡会出现一定程度空报现象;台风深入内陆后,螺旋度预报指示明显不如湿位涡、湿Q矢量好,螺旋度指示强降水位置偏东,而湿Q矢量指示强降水范围略偏小;对台风后部强降水,湿Q矢量和螺旋度均未能预报出降水落区,而湿位涡仍有较好的预报效果。从区域平均看,螺旋度与湿Q矢量的预报指示时效小于12 h,而湿位涡超过12 h。     

1998年7月河套气旋强烈发展时的暴雨过程分析

采用位涡理论对1998年7月4—7日的一次河套气旋强烈发展中的暴雨过程进行分析。结果表明:此次夏季河套气旋的强烈发展是在高层正位涡平流和低层暖平流的共同作用下产生的。高空双急流结构产生的强烈辐散加强了低层辐合，有利于气旋的加强。强降水出现在河套气旋强烈发展过程中，是由高层冷空气与季风涌带来的西南暖湿气流辐合而引起的大尺度降水过程。在这次气旋强烈发展过程中，对流层低层到中上层均出现强的上升气流，使得南方深厚的暖湿空气不断随西南风流入暴雨区上空。暴雨发生时，华北地区处于地面Ω型的θ_se高能舌之中，其上空500 hPa存在一个由大尺度动力强迫形成的东北—西南向的非地转湿 Q 矢量辐合带，对流云带与 Q 矢量辐合中心有非常好的对应关系。