The Role of diabatic heating, torques and stabilities in forcing the radial- vertical circulation within cyclones part iii: case study of lee-side cyclones

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On the Forcing of the Radial-vertical Circulation within Cyclones-Part 1: Concepts and Equations

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POTENTIAL VORTICITY STRUCTURE OF SUMMER CYCLONES OVER THE CHANGJIANG-HUAIHE VALLEY*

Based on the theory of Ertel potential vorticity,the isentropic potential vorticity maps and vertical profiles of potential vorticity for two summer cyclones over the Changjiang-Huaihe Valley are analysed.After discussing a possible mechanism for the genesis and development of such systems and their differences from typical extratropical cyclones,a conceptual model for their activities is proposed:A weak disturbance in the midlevel of troposphere originated from around the Qinghai-Xizang Plateau may cause heavy precipitation under favourable conditions and latent heat release in the mid-troposphere leads to downward extension of cyclonic circulation and a wave on the quasi-stationary front.This weak cyclone can develop substantially and become a typical extratropical cyclone only when air from the lower stratosphere flows downslope along isentropic surfaces into the region of interest.     

POTENTIAL VORTICITY STRUCTURE OF SUMMER CYCLONES OVER THE CHANGJIANG-HUAIHE VALLEY

Based on the theory of Ertel potential vorticity,the isentropic potential vorticity maps and vertical pro-files of potential vorticity for two summer cyclones over the Changjiang-Huaihe Valley are analysed.Afterdiscussing a possible mechanism for the genesis and development of such systems and their differences fromtypical extratropical cyclones,a conceptual model for their activities is proposed:A weak disturbance in the mid-level of troposphere originated from around the Qinghai-Xizang Plateau may cause heavy precipitation underfavourable conditions and latent heat release in the mid-troposphere leads to downward extension of cycloniccirculation and a wave on the quasi-stationary front.This weak cyclone can develop substantially and becomea typical extratropical cyclone only when air from the lower stratosphere flows downslope along isentropic sur-faces into the region of interest.     

On the deepening and filling of balanced cyclones by diabatic heating

Summary The results of computations of the radial circulation and associated surface pressure tendencies, needed to retain gradient wind balance in a model of an axisymmetric cyclone, due to the action of diabatic heating and boundary layer pumping, are presented. These computations show that diabatic heating will not induce further deepening (i. e. intensification) of the cyclone when this cyclone is weak and has a cold core. On the other hand, a relatively intense warmcore balanced cyclone will deepen appreciably, depending on the degree of baroclinicity and on where exactly the heat sources are located.These results underline the fact that Conditional Instability of the Second Kind (CISK) must be interpreted as a finite-amplitude instability. CISK cannot explain the genesis of a cyclone, such as a polar low or hurricane.With 18 Figures     

夏季江淮气旋的Ertel位涡诊断分析

本文应用Ertel位势涡度的理论,分析了两例夏季江准气旋活动的等熵面位涡图和位涡垂直廓线,讨论了这类系统发生发展的一种可能机制以及它们与典型温带气旋的区别,从而得出了夏季江淮气旋活动的一个概念模式:从高原一带东移的对流层中层弱的扰动在有利条件下引起江淮地区较强降水,中层潜热释放导致气旋性环流向下延伸,最终可在地面静止锋上形成波动。当副热带锋区北侧平流层下部空气沿等熵面南下时,气旋波可能发展成为典型的温带气旋。     

台风Matsa(2005)和Wipha(2007)变性过程对比分析

利用日本气象厅热带气旋年鉴资料和JRA-25再分析资料,对0509号台风Matsa和0712号台风Wipha变性过程进行了对比分析。结果表明,台风Matsa和Wipha均是在我国登陆后转向东北方向运动过程中发生变性,但Matsa嵌入中纬度高空锋区,变性为温带气旋后有再加强过程,而Wipha仅外围环流与锋区接触,变性为温带气旋后无再加强过程。通过等熵面位涡分析进一步表明,Matsa变性加强表现为高层正位涡与低层暖平流的耦合,以及高层正位涡下传至中低层;Wipha的变性过程中,高层正位涡并未与低层暖平流耦合,高层正位涡无明显的下传。     

Analysis of conveyor belts in winter Mediterranean cyclones

The relevance of the midlatitude conveyor belt model to Mediterranean cyclones (MCs) is examined using data from two winters. Eight MCs, which exhibit typical midlatitude cyclone structure, were scrutinized and their conveyor belts were examined. The analysis was based on satellite imagery, isentropic wind maps, vertical cross-sections of potential and equivalent potential temperatures, and air back-trajectories. The conveyor belts found in the studied MCs were similar to the common features of midlatitude cyclones, except for three aspects. First, the warm conveyor belt was not associated with massive organized cloudiness in five of the eight cyclones since it consisted of dry air originated from the Saharan desert. Second, the anticyclonic branch of the cold conveyor belt was not found in half of the MCs. Third, the dry air intrusion originated north of the cyclone and extended southward around it, unlike its common midlatitudinal northwest–southeast orientation. This is consistent with the relatively small baroclinic vertical-westward tilt of the cyclones analyzed.     

Effect of tropospheric temperature change on the zonal mean circulation and SH winter extratropical cyclones

This study aims to understand the mechanisms which cause an overall reduction of SH extratropical cyclone activity with a slight increase in the high latitudes in a warmer climate simulated in general circulation models (GCMs) with increasing CO₂. For this purpose, we conducted idealized model experiments by forcing warm temperature anomalies to the areas where climate change models exhibit local maximum warming—the tropics in the upper troposphere and the polar regions in the lower troposphere—simultaneously and separately. The Melbourne University atmospheric GCM (R21) coupled with prescribed SST was utilized for the experiments. Our results demonstrate that the reduction of SH extratropical cyclone frequency and depth in the midlatitudes but the slight increase in the high latitudes suggested in climate change models result essentially from the tropical upper tropospheric warming. With this tropical warming, the enhanced static stability which decreases baroclinicity in the low and midlatitudes turns out to be a major contributor to the decrease of cyclone activity equatorward of 45°S whereas the increased meridional temperature gradient in the high latitudes seems an important mechanism for the increase of cyclone activity over 50°–60°S.     

夏季进入北极温带气旋的活动特征及其影响因素北大核心

进入北极地区的气旋在移动过程中往往伴有大风、强降水等特征,对北极气候变化有深刻的影响。基于NCEP2再分析资料,识别并跟踪了北半球夏季(6—8月)从中纬度进入北极的温带气旋,考察了其年际变化特征和影响因素。结果表明:1979—2019年夏季进入北极的温带气旋共867个,其中消失在北极边缘区域和中心区域的数量分别为688个和161个,且后者平均强度更大、平均持续时间更长。分区域研究发现,夏季从陆地进入极区的气旋个数较多,而从海洋进入极区的气旋强度更大,活动更为剧烈。对进入北极的气旋年际时间序列进行分析发现,夏季进入北极的气旋个数和强度均存在年际变率,其中气旋个数的年际变率尤为显著。气旋个数年际变率主要周期为5 a,强度的主要周期约为2.7 a。进一步分析发现,引导气流是影响气旋向北移动的重要因素。此外,夏季北大西洋气旋强度与同期北大西洋涛动(North Atlantic Oscillation,NAO)指数存在较好相关。研究还表明,进入极区气旋活动的年际变化受大气斜压不稳定性的影响,在北太平洋地区区域平均的Eady增长率与气旋个数和强度的相关性均最强,相关系数分别为0.4和0.5。     

2019年春季海洋天气评述

2019年春季（3—5月）的大气环流特征为：北半球极涡呈偶极型分布,两个低值中心分别位于东、西半球,中高纬度呈4 波型。3月,亚洲中东部中高纬度的经向型环流利于冷空气南下。4月和5月,冷空气势力减弱,温带气旋活动增加。我国近海出现了17次8级以上大风过程,其中冷空气大风过程6次,冷空气和温带气旋共同影响的大风过程有3次,入海温带气旋大风过程8次。春季共有13次海雾过程,3月3次,4月6次,5月4次。近海浪高在2 m以上的海浪过程有14次,台湾海峡周边海域的浪高较大。西北太平洋和南海没有台风生成,全球其他各大洋共有热带气旋 12个。我国近海的海面温度整体呈上升趋势,且北方海域的上升幅度大于南方海域。     

The relation between circulation intensity in the temperate latitude cyclone and air temperature and precipitation anomalies

The relationship between air temperature, precipitation, and circulation intensity in extratropical cyclones at subsequent stages of their evolution is studied. The cyclones observed in the Atlantic and Pacific sectors in winters from 1991 to 2000 are considered. The NCEP/NCAR reanalysis data, GPCP database, and cyclone trajectory archive of the Shirshov Institute of Oceanology, Russian Academy of Sciences, are used. It is demonstrated that there are significant connections between circulation intensity, air temperature, and precipitation in cyclones. The intensity of circulation along the cyclone contour reflects both the precipitation fall in the cyclone center and a synoptic structure of frontal precipitation. It was concluded that speed circulation along the cyclone contour can be recommended as a characteristic of cyclone intensity together with the currently used pressure in the cyclone center.     

4-6月江淮流域不同尺度温带气旋发展率及移动特征

基于改进的温带气旋客观识别方法,统计分析1986—2015年4—6月不同空间尺度江淮流域温带气旋的发展率及移动特征。结果表明:1)整体气旋发展率呈先上升、后下降趋势,较大空间尺度气旋发展率呈显著上升趋势,中间尺度气旋发展率无明显变化趋势。2)较大尺度气旋源地集中在湖北、湖南交界处,中间尺度气旋源地位于安徽等江淮流域东部,各尺度气旋源地位于整层大气非绝热加热项大值区域内。3)气旋初生阶段,江淮流域存在风速切变线,淮北地区风速小于淮南地区,江淮流域上空对流层低层位势高度为负距平,中高层则为正距平,属浅薄低压系统。4)不论何种尺度气旋,均存在东移和东北移动路径,但较大尺度气旋东北移动路径的比例较大。     

A climatology of Southern Hemisphere extratropical cyclones

A climatology of extratropical cyclones determined by an objective automatic scheme applied to 15 years (1975–89) of once-daily Australian Bureau of Meteorology hemispheric analyses is presented. Contour maps of the positions of formation (cyclogenesis), dissipation (cyclolysis) together with other cyclone statistics are presented. The distribution of cyclones through the hemisphere was found to be dominated by a permanent high latitude core coincident with the circumpolar trough. During the winter and intermediate seasons, two mid latitude branches are evident in the cyclone density originating in the Tasman Sea and South American sectors, both spiraling poleward and merging with the circumpolar core in the Southern Oceans. Systems were observed to move in an castsouth-east direction, away from their location of formation, exhibiting peak speeds of migration in the mid latitudes. Little seasonality was evident in the densitiy distribution of cyclones through the Southern Oceans, but a considerable amount was found in their central pressure. Correspondence to: I Simmonds     

登陆台风等熵面位涡演变的数值模拟研究

选取1997年第11号台风温妮为研究个例,通过中尺度模式MM5模拟再现了该台风登陆后经历初期减弱、变性及变性后再次发展的演变过程.引入Ertel等熵面位涡收支方程,深入分析了登陆台风结构演变的过程中绝热与非绝热作用对对流层低层位涡局地变化的影响.研究表明:台风温妮深入内陆的过程中,对流层低层台风中心西北侧位涡增长,且大值中心不再与台风中心重合;由于摩擦和非绝热加热的存在,对流层低层位涡不守恒,其局地变化主要决定于位涡的水平平流(守恒项)、位涡的垂直平流、加热的垂直微分(非守恒项)的分布;台风温妮变性前后,对流层低层位涡的守恒性逐渐减弱,非守恒项尤其是加热的垂直微分对位涡的局地增长的正贡献不断增强直至占有主导地位.     

2010—2019年春季影响大连的温带气旋特征及爆发性气旋成因分析

利用常规和加密气象观测、NCEP再分析、云图等资料,对2010—2019年春季影响大连的温带气旋特征及爆发性气旋造成的极端天气的物理机制进行分析。结果表明：春季进入到渤海、黄海北部的气旋平均每月2.4个;气旋一般先进入黄海,进入黄海和经渤海进入黄海的温带气旋总计有84.5%进入黄海北部,且春季进入黄渤海的气旋73%会给大连地区带来大风或降水天气,影响大连东部沿海的几率远高于其他地区;产生较强灾害性天气的爆发性气旋多发生在春季,路径基本都是由西南向东北方向移动。爆发性气旋主要是因为温带气旋经过黄渤海后短时间快速降压,到大连陆地发生爆发性发展,这种温带气旋的发展一般从低层开始,具有较强的锋区和斜压性,爆发阶段位于正涡度平流最大的高空急流出口区,对应低空位于低空急流左前方辐合区。较强的冷、暖温度平流是造成极端降水和大风天气的主要因素,暴雨的形成主要是温带气旋带来的暖湿气流持续输送,并伴有较强上升运动促使的水汽垂直输送,整层水汽充沛;当低空急流发展和冷、暖空气交绥时,出现了在高湿、高温的湿斜压锋区上的强降水;而北路强冷空气与黄、渤海上爆发性发展的温带气旋形成极强气压梯度,是出现极端大风的主要原因。     

热带气旋“Malakas”与中纬度系统的相互作用及其结构变化

本文利用中尺度数值模式WRF和LAGRANTO轨迹模式对2010年变性台风"Malakas"进行数值模拟和轨迹分析,分析了Malakas在变性过程中与中纬度系统的相互作用,以及在相互作用过程中Malakas的结构变化特征。结果表明:Malakas变性过程经历了三个阶段:(1)高层扰动加强期,高层的正位涡产生的气旋性环流使低层Malakas中心北部的斜压带西侧产生负的温度平流,表现为冷空气的入侵;(2)Malakas和中纬度系统相互作用时期,台风北上导致斜压带出现,深对流的爆发使低层暖湿气流沿着斜压带上升,快速上升气流中的潜热释放导致低PV空气向对流层上部净输送,在其北部高层重新构建出一个脊;(3)Malakas变性成温带气旋,残存的台风内核与斜压带逐渐合并,负的位涡平流带着非绝热外出流驱动了下游最初脊的构建,加速并且固定了中纬度急流,并整体放大了上层Rossby波模式。     

Buoyancy in tropical cyclones and other rapidly rotating atmospheric vortices

Motivated primarily by its application to understanding tropical-cyclone intensification and maintenance, we re-examine the concept of buoyancy in rapidly rotating vortices, distinguishing between the buoyancy of the symmetric balanced vortex or system buoyancy, and the local buoyancy associated with cloud dynamics. The conventional definition of buoyancy is contrasted with a generalized form applicable to a vortex, which has a radial as well as a vertical component. If, for the special case of axisymmetric motions, the balanced density and pressure distribution of a rapidly rotating vortex are used as the reference state, the buoyancy field then characterizes the unbalanced density perturbations, i.e. the local buoyancy. We show how to determine such a reference state without approximation.The generation of the toroidal circulation of a vortex, which is necessary for vortex amplification, is characterized in the vorticity equation by the baroclinicity vector. This vector depends, inter-alia, on the horizontal (or radial) gradient of buoyancy evaluated along isobaric surfaces. We show that for a tropical-cyclone-scale vortex, the buoyancy so calculated is significantly different from that calculated at constant height or on surfaces of constant σ (σ = (p − p^*)/(p_s − p^*), where p is the actual pressure, p^* some reference pressure and p_s is the surface pressure). Since many tropical-cyclone models are formulated using σ-coordinates, we examine the calculation of buoyancy on σ-surfaces and derive an expression for the baroclinicity vector in σ-coordinates. The baroclinic forcing term in the azimuthal vorticity equation for an axisymmetric vortex is shown to be approximately equal to the azimuthal component of the curl of the generalized buoyancy. A scale analysis indicates that the vertical gradient of the radial component of generalized buoyancy makes a comparatively small contribution to the generation of toroidal vorticity in a tropical cyclone, but may be important in tornadoes and possibly also in dust devils.We derive also a form of the Sawyer–Eliassen equation from which the toroidal (or secondary) circulation of a balanced vortex may be determined. The equation is shown to be the time derivative of the toroidal vorticity equation in which the time rate-of-change of the material derivative of potential toroidal vorticity is set to zero. In analogy with the general case, the diabatic forcing term in the Sawyer–Eliassen equation is shown to be approximately equal to the time rate-of-change of the azimuthal component of the curl of generalized buoyancy.Finally, we discuss the generation of buoyancy in tropical cyclones and contrast the definitions of buoyancy that have been used in recent studies of tropical cyclones. We emphasize the non-uniqueness of the buoyancy force, which depends on the choice of a reference density and pressure, and note that different, but equivalent interpretations of the flow dynamics may be expected to arise if different reference quantities are chosen.