"98.7"突发性特大暴雨中尺度切变线低涡发展的涡源诊断

1998年7月20～23日(简称“98．7”)发生在武汉周边地区的特大暴雨过程与沿低涡切变线相继生成和强烈发展的MαCS及MβCS直接关联。利用MM5模拟提供的高空间分辨(20km)输出资料，对这次突发性特大暴雨中尺度切变线低涡发展的动力学机制进行了诊断。涡度分析指出，高、低空正涡度中心在武汉周边地区上空的叠加和耦合是该低涡切变线持续发展的主要物理机制之一。总涡源的诊断揭示，在突发性暴雨强烈发生发展期，武汉周边地区上空从低层到高层有一近乎垂直的涡源高值区生成和维持，其垂直结构的发展演变与涡度场垂直结构的发展演变相一致。这一结果表明，大气总涡源对该中尺度低涡切变线的生成和发展起着决定性作用，也是该暴雨中尺度系统持续发展的重要动力学机制。对贡献于总涡源的诸分量计算表明，在650hPa以下，散度项对大气总涡源的正贡献最大，但在此层以上至200hPa之间，垂直涡度平流项的贡献要比散度项大，同时水平平流项也为正贡献；在整个对流层，扭转项对总涡源为负贡献，散度项只在450～250hPa之间为负贡献。在近地层，垂直涡度平流项和水平平流项基本对总涡源不作任何贡献。时间平均涡源和纯扰动涡源对低涡切变线的生成很重要；在强烈发展期，相互作用涡源作用最大，纯扰动涡源贡献次之；随着非线性相互作用涡源贡献的减小，低涡切变线东移减弱。在该期间时间平均涡源和纯扰动涡源仍为正贡献。     

位涡理论及其应用

位涡是近代天气动力学的重要概念之一。本文主要对位涡理论的某些要点,包括位涡的概念、位涡的守恒性、位涡的分析、位涡思想、位涡反演、湿位涡及位涡理论的发展和应用等作一简要介绍。     

活跃涡与准静止涡能量场特征差异

对盛夏７,８两个月西南低涡个例θｓｅ能量场综合分析后发现：活跃类西南低涡（活跃涡）与准静止类西南低涡（准静止涡）能量场特征具有明显差别。     

陇南涡与汉水上中游秋季（9～10月）降水的关系

分析陇南低涡发生，发展的特征，揭示其对汉水上中游秋季降水的影响，并提出预报判据。     

夏季青藏高原低涡研究进展述评

高原低涡是造成我国暴雨的重要天气系统之一。本文在简要介绍第一次青藏高原科学试验及以前研究工作的基础上,重点介绍了第二次青藏高原科学试验及近年来的一些主要研究成果,包括高原低涡的活动特征、高原低涡发展东移的机理、高原低涡发展东移的大尺度条件及高原低涡的结构特征等。并指出了目前高原低涡研究的局限性和待改进的方向。     

青岛区域性低涡暴雨的气候特征及预报

对青岛1961～2002年的区域性暴雨个例(3个以上基本测站出现≥50mm降水)进行了分析,指出西南涡、西北涡以及切变涡是造成青岛区域性暴雨最主要的影响系统,其路径不同,所产生暴雨的区域及范围不同,急流的位置及动向和切变线位置是判断暴雨落区的重要因素。     

一次西南涡暴雨的位涡分析

利用NCEP提供的FNL再分析资料对2013年5月25—26日发生在我国中东部西南涡暴雨过程的初期阶段进行了位涡分析,结果表明:高原槽附近异常高位涡,促进了低层西南涡前期的发展,这对西南涡前期的预报有很好指导意义;西南涡发展前期,强降雨中心湿位涡正压子项自低层至高层呈现出"负—正"的分布特征,湿位涡斜压子项在中低层有强负值中心,反映出低层对流不稳定和垂直风切变对于对流性降水的促进作用;西南涡发展阶段,湿位涡正压子项正值中心呈现出倾斜漏斗状,斜压子项在降雨中心低层出现了强负值中心;强降水中心与低层扰动湿位涡负值有较好的对应关系,对于强降雨落区的预报提供了一种参考。     

基于西南涡加密探空资料同化的一次奇异路径耦合低涡大暴雨数值模拟研究

利用WRF模式及WRFDA同化系统,引入业务探空资料和西南涡加密探空资料,对一次四川盆地奇异路径低涡耦合大暴雨过程进行了数值试验,对比检验不同同化试验对本次过程降水和低涡移动路径的模拟能力,分析了加密探空资料同化对西南涡结构及其降水演变的影响。结果表明:在同化业务探空资料的基础上,引入西南涡加密探空资料能改善模式对本次降水和低涡移动路径的模拟,而仅同化业务探空资料对模拟结果的改善作用有限;引入西南涡加密探空资料,一方面能在初始风场上产生气旋式扰动,增加初始高原涡和西南涡的强度,另一方面通过调整初始四川盆地上空大气温、湿度结构,使模式在积分初期就能产生出实况量级的降水;西南涡加密探空资料的同化试验揭示了仅靠高层的高位涡不足以激发和维持700 hPa的西南涡,需要通过低层水平辐合引起正涡度增加并向上输送来增强700 hPa的气旋式环流,进而促进西南涡的移动和发展,而模拟初期降水的潜热释放也起重要作用,加深了对西南涡及其降水成因的认识。      

一次引发川东暴雨的西南涡特征分析

利用NCEP再分析格点资料、常规观测资料、自动站降水资料、0.1°×0.1°的FY-2E云顶亮温资料,对2013年6月29日至7月1日发生在四川东部的大暴雨过程进行分析,结合涡度收支方程重点分析了引发这次大暴雨的西南涡结构。结果表明：在西南低涡发生发展过程中,对低涡发展起直接作用的是水平辐合辐散项和水平平流项,低涡形成前水平辐合辐散项起主要贡献,低涡形成后水平平流项贡献增大,并在对流层中低层以正贡献为主,扭转项贡献最小,而垂直输送项在低涡形成前期以正贡献为主,低涡减弱阶段以负贡献为主;在西南低涡形成前期,对流层高层有位涡大值区向下传输至中层,中高层正位涡叠加在低层负位涡之上,有利于低层低涡的发展及不稳定能量的存储与释放,是低涡维持发展的重要因素。     

冬季乌拉尔山月平均阻塞形势的统计分析和数值试验

根据冬季（１２，１，２月）乌拉尔山地区月平均阻塞形势集中出现的范围和强度定义了一个阻塞指数。相关分析表明：乌拉尔山月平均阻塞与中国月平均气温有较大的负相关关系；它和全球海表温度之间存在三个明显的高相关海区；北太平洋热带东部海区，北大西洋热带和中纬度海区以及阿留申群岛西南方的海区。应用两层原始方程谱模式，在以上３个海区上空加入理想的非绝热孤立热源强迫分别做数值试验。结果表明：３个海区上空的非绝热孤立     

Local kinetic energy budget of high-frequency and intermediate-frequency eddies: winter climatology and interannual variability

The local budget of eddy kinetic energy (EKE) for both high-frequency (HF, 2–6 days) and intermediate-frequency (IF, 7–29 days) eddies are evaluated for Northern Hemisphere boreal winter using the 31-year (1979/80–2010/11) NCEP-DOE reanalysis. A new form of EKE equation is used to isolate the kinetic energy generation/destruction due to interactions among eddies of different timescales. The main source of HF EKE is baroclinic conversion that is concentrated in the mid-lower troposphere. Barotropic conversion mainly damps HF EKE and shows positive contributions to IF EKE on the northern flank of the winter-mean tropospheric jet. Interaction between HF and IF eddies acts as a sink for HF EKE and a main source for IF EKE, especially over the eastern ocean basins, confirming the substantial role of synoptic-scale transients in the development of IF phenomena such as atmospheric blocking. Large interannual variability is found for various EKE budget terms. The HF EKE response to El Niño is characterized by a dipole (tri-pole) anomaly over the North Pacific (North Atlantic). Baroclinic conversion is the main driver of the observed changes in HF EKE while barotropic conversion, interaction between HF and IF eddies, and energy flux convergence all play non-negligible roles in determining the final meridional structure of the HF EKE anomalies. Associated with El Niño, IF EKE generally decreases over the North Pacific and increases over the North Atlantic, which mainly result from changes in baroclinic conversion and EKE conversion due to eddy–eddy interactions. The latter is dominated by interaction between IF and LF (low-frequency, 30–90 days) eddies over the North Pacific, and by interactions between HF and IF eddies, and between IF and LF eddies over the North Atlantic.     

A Perspective on the Evolution of Atmospheric Blocking Theories: From Eddy-Mean flow Interaction to Nonlinear Multiscale Interaction

In this paper, we first review the research advancements in blocking dynamics and highlight the merits and drawbacks of the previous theories of atmospheric blocking. Then, the dynamical mechanisms of atmospheric blocking are presented based on a nonlinear multi-scale interaction(NMI) model. Previous studies suggested that the eddy deformation(e.g., eddy straining, wave breaking, and eddy merging) might lead to the formation and maintenance of atmospheric blocking.However, the results were specu...     

The Role of Warm North Atlantic SST in the Formation of Positive Height Anomalies over the Ural Mountains during January 2008

The most severe snowstorm and freezing-rain event in the past 50 years hit central and southern China in January 2008. One of the main reasons for the anomalous climate event was the occurrence of atmospheric circulation anomalies over middle and high latitudes, particularly the persistent blocking that occurred over the Ural Mountains. Along with atmospheric anomalies, a strong La Nia event in the Pacific and warm sea surface temperature anomalies (SSTAs) in the North Atlantic were the most significant in the lower boundary. Since a brief analysis suggests that La Nia exerts no significant impact on the Urals, the key point of focus in this study is on the role of the warmer SSTAs in the North Atlantic. Based on an observational composite, North Atlantic SSTAs pattern when the height anomaly over the Urals is strongly positive is found similar to that in January 2008, but no significant SSTAs occurred elsewhere, such as the Pacific. Using an atmospheric general circulation model, ECHAM5, the impact of North Atlantic SSTAs on the extratropical atmosphere circulation in the event was investigated. The results show that the warm SSTAs strengthened the blocking high over the Urals, through anomalous transient eddies. The consistency between the study model and the observational composite indicates that the warm SSTAs in the North Atlantic were indeed an important factor in the formation of the snowstorm disaster of January 2008.     

黑潮延伸体海洋中尺度涡旋:四套数据的比较

在不同的中尺度涡数据中,涡旋的识别,跟踪方法以及物理参数的定义存在差异,因此需要对涡旋结果进行相互比较和验证.本文比较了Chelton,GEM-M,Faghmous和Dong四套中尺度涡数据在黑潮延伸体区域的基本特征.结果表明,各数据的中尺度涡数量,特征和轨迹均存在差异.经过归一化处理后,涡特征的地理分布和时间变化高度相似,特别是Chelton,Fahgmous和Dong三套数据.     

Interpreting Reynolds stress from perspective of eddy geometry

Geometric features in oceanic mesoscale eddies such as tilt and anisotropy can influence the properties of the Reynolds stress that provides feedback between the eddies and the background flow. By regarding an eddy as a wave, previous studies have parameterized the Reynolds stress based on the equivalence in the tilt angle between the phase of the eddy stream functions and the variance ellipse for the Reynolds stress (RS-ellipse). However, the wave assumption cannot predict the anisotropy of the RS-ellipse, and also largely simplifies the eddy geometry, which would naturally be an ellipsoid rather than a wave. The present study explores the shape relation between elliptical eddies and the RS-ellipse, by mathematically reformulating the Reynolds stress based on the eddy shape. The new formula reveals that the shape relation is regulated by the horizontal extent of the occurrence probability distribution (PDF) of the eddy, and that the shape of the eddy and RS-ellipse are identical at the place of maximum PDF when the horizontal scale of the PDF is sufficiently larger than the size of the eddy. A similar tendency is found in eddies detected by satellite altimetry in the Kuroshio Extension jet region. A detailed analysis of the PDF in this region shows that the tilts of the eddies are likely to be consistent with the destabilization effect on the jet, suggesting a strong relation between the eddy geometry and the jet's stability in this region. These findings may open a path toward a new method to parameterize the Reynolds stress with the background state, exploiting the shape equivalence between the eddies and the RS-ellipse.     

瞬变天气涡旋对北大西洋涛动的增强效应

使用NCEP/NCAR再分析资料计算了冬季北大西洋瞬变涡旋活动强度与北大西洋涛动(NAO)逐日指数的时间序列,结果发现:当涡旋活动强度出现峰值后会伴随NAO模态增强现象;而随着NAO的增强,涡旋能量同落.为了判断是否涡流相互作用将天气尺度的能量转换为低频尺度的能量,使用瞬变涡度通量来研究涡度与能量的传输.通过分析瞬变涡...     

Characteristics and vertical structure of oceanic mesoscale eddies in the Bay of Bengal

The signatures of mesoscale eddies induced surface and subsurface changes have not been comprehensively quantified for the Bay of Bengal (BoB) region. This study quantifies the statistical properties and three-dimensional (3D) eddy structures in the BoB. To accomplish this, the satellite altimetry data combined with automated eddy detection and tracking algorithm is used. Horizontal distribution of surface characteristics of eddies is analyzed by using 24 years (1993–2016) of AVHRR infrared satellite sea surface temperature (SST) and 7 years (2010–2016) of sea surface salinity (SSS) of SMOS satellite data. Surface eddy centric composite analysis reveals the existence of warm (cold) and diverse SSS anomalies for anticyclonic (cyclonic) eddies. During winter, it is important to note that the eddy induced SST and SSS anomalies show the dipole patterns show opposite phases for the cyclonic and anticyclonic eddies. Observed diploe structures are consistent with the eddy rotation and background large-scale meridional gradient of temperature and salinity fields. The 3D structure of eddies is investigated by using the ARMOR3D and Argo float profiles. The horizontal distribution of temperature and salinity anomalies from ARMOR3D signify the monopole structure of eddies in the subsurface layers. Further, the analysis of composite averages of 241 (200) Argo temperature profiles indicates the core of anticyclonic (cyclonic) eddies centered at about ∼140 m (∼100 m). However, salinity profiles depict the existence of core at ∼65 m (∼50 m). This study have practical relevance to a variety of stakeholders and finds profound importance in the validation of eddy-resolving ocean models for the BoB region.     

Oceanic mesoscale eddy in the Kuroshio extension: Comparison of four datasets: 黑潮延伸体海洋中尺度涡旋: 四套数据的比较

Some relatively mature mesoscale eddy products have been released for scientific purposes in recent decades. However, the metrics used to identify eddies, the tracking methods, and the definition of the physical parameters are all different across the different datasets, so intercomparisons and validation of these datasets are badly needed. Here, the authors intercompare the basic features of ocean mesoscale eddies in the Kuroshio extension region from four eddy datasets—namely, Chelton, GEM-M, Faghmous, and Dong. In the case study, eddy numbers and locations as well as the eddy tracks identified by the four datasets are compared for a specific date. The authors find that all the datasets have different eddy numbers, but more than 50% of identified eddies coincide. GEM-M, with the so-called “segmentation” algorithm, can identify considerably more eddies than others, while Chelton identifies fewer eddies due to tracking errors, which also lead to a long lifespan. From the analysis of the probability distribution function of eddy features, GEM-M eddies tend to have a larger amplitude and radius and Chelton tends to have long-life eddies. It is further found that the geographic distributions and temporal variation of normalized eddy features are highly similar among the four datasets—particularly among Chelton, Fahgmous, and Dong. In addition, the mean trajectories of the four datasets are generally overlapped initially, and then spread after 245 days. The findings help toward better understanding the uncertainties of eddy features in the Kuroshio extension region.摘要在不同的中尺度涡数据中, 涡旋的识别,跟踪方法以及物理参数的定义存在差异, 因此需要对涡旋结果进行相互比较和验证.本文比较了Chelton,GEM-M,Faghmous和Dong四套中尺度涡数据在黑潮延伸体区域的基本特征.结果表明, 各数据的中尺度涡数量,特征和轨迹均存在差异.经过归一化处理后, 涡特征的地理分布和时间变化高度相似, 特别是Chelton,Fahgmous和Dong三套数据.     

非均匀风场与急流强迫的水体涡旋动力特征模拟

通过数值模拟有限区域水气界面由强迫作用驱动形成的水体涡旋及环流动力结构特征,分析非均匀风场、水体急流、两者叠加以及环境边界和地转偏向力等因子的综合影响,探讨此类水体涡旋结构和动力特征。风应力驱动的水体涡旋尺度大,相对深厚,正涡旋具有下凹表面,负涡旋具有上凸表面。水体急流驱动的涡旋形成在急流两侧,对应急流所在深度及厚度尺度相对较小,也较浅,但流速与强度均大于风场驱动的涡旋环流。地形阻挡起着引导涡旋环流走向的作用;同时在北半球地转偏向力对急流侧向负涡旋形成和强度增强更为有利。此外正涡旋对应的辐合辐散势函数强于负涡旋,有利于正涡旋区垂直上升运动强于负涡旋中垂直下沉运动。非均匀风场及水体急流两种强迫叠加作用下,涡旋数量增加、尺度减小,底层的流场形态及强度与表层差异增大。形成的水体涡旋结构呈现多种形态：深厚的整层一致;浅薄的仅维持在上层,或上下层环流相反等。风应力驱动的涡旋以正压性为主,水体急流驱动的涡旋因急流的垂直强切变而具有强的斜压性,在正斜压动能的转换中,正压性涡旋区有斜压动能向正压动能转换,斜压性涡旋区有正压动能向斜压动能转换,均有利于这两个区域正负涡旋的维持。