强垂直风切变环境下对流单体对飓风强度的影响

利用高分辨率模式输出资料,诊断分析强垂直风切变环境下飓风Bonnie(1998)中风暴相对螺旋度的分布特征,再现了Molinari等(2008)利用下投式探空仪获得的该飓风内部风暴相对螺旋度的离散观测结果。通过对比不同垂直风切变环境下,不同区域风暴的相对螺旋度、对流有效位能及风速的水平分布,揭示出与高值风暴相对螺旋度相联系的强对流单体的分布与环境垂直风切变的密切联系。基于风暴相对螺旋度和对流有效位能的配置分析,研究强环境垂直风切变时段,眼壁附近的深厚涡旋对流以及螺旋雨带中的小型对流单体的三维结构和演变特征。分析表明,环境垂直风切变较强时,在眼壁附近的顺切变区存在典型的深厚涡旋对流系统,这类深厚涡旋系统能够激发二级垂直环流,有利于旋转上升运动的维持,并在近眼心区域引发补偿性的干暖下沉气流,有助于飓风暖心的维持和加强;同时,螺旋雨带中也存在以涡度为特征的小型对流单体,这些对流单体随着平流不断移入飓风中心,使得飓风中心垂直涡度增加,最终导致飓风强度的增强。      

1992年Andrew飓风眼壁区倾斜上升运动发展的可能机制——非线性对流对称不稳定

在1992年Andrew飓风数值试验输出高分辨资料的中尺度扰动结构分析基础上,对眼壁区倾斜上升运动进行了动力分析,指出存在一种非线性对流-对称不稳定的发展机理.梯度风不平衡是涡旋大气中提供对称不稳定的物理基础,并用模式资料诊断研究了倾斜上升气流存在的这种发展机理的可能.     

9012号台风中尺度螺旋雨带与暴雨的观测分析

研究９０１２号台风中尺度螺旋雨带结构特征及其与闽东北沿海地区特大暴雨的关系。中尺度螺旋雨带起源于不对称的台风眼壁，以脉冲形式径向向外传播，它的前方是中尺度低压，中低压的优动形式和惯性重力小相当一致。螺旋雨带受海陆和地形作用有加强趋势，在某些特殊地形作用下，降水增幅可有１－３倍。     

1996年第8号台风中尺度结构的数值研究

用非静力平衡的中尺度模式ＭＭ５模拟研究了从１９９６年７月３１日０８时至８月１日２０时３６小时内９６０８号台风的移动路径、中尺度热力动力学结构及云和降水物理结构及其变化。这段时间覆盖了９６０８号台风在台湾和福建省福清市登陆全过程。结果表明：该模式比较成功地模拟预报了这次台风登陆过程,模拟了登陆前后台风中心附近的一些重要中尺度结构和发展演变特征。模拟预报的台风路径与实况接近,预报的８月１日０８时（积分２４小时）台风环流中心位于２小时后台风登陆点福建省福清市附近的海面上。台风中心附近低层呈气旋式辐合、高层呈气旋式辐散。台风眼壁具有由低层辐合形成的强上升气流,台风眼中心为下沉气流。台风中心周围动力结构表现出明显的不对称特征。台风眼壁的雨水分布呈现由中尺度雨团构成的环形雨带结构。台风在台湾滞留期间,台风中心由于低层云的发展使得卫星云图上原先清晰可见的台风眼变得模糊不清。高分辨的模式预报表明,在对流层中层具有强上升气流和云雨水集中的眼壁比台风眼中心更为暖湿。     

Chanchu台风(0601)精细结构中的熵流特征分析

在耗散结构理论的基础上,根据热力学第二定律推导出了熵平衡方程。利用高分辨率模式输出资料通过对比Chanchu台风(0601)螺旋雨带上游、中游和下游及眼壁附近不同区域对流单体和熵流分布情况,揭示出负熵流值与台风的强对流单体有密切联系。基于负熵流与台风精细结构的配置分析,研究中尺度范围内熵流随Chanchu台风发生、发展、消亡各阶段的演变特征。分析表明,对流单体在从雨带上游至下游的演变过程中,熵流分布特征也会发生相应的变化,强对流单体与负熵流大值区相对应;当对流单体减弱,负熵流也随之减弱;当单体最后合并并汇入眼墙时,负熵流彼此合并旋入眼墙,有助于眼墙中深厚对流的维持和发展;此外,负熵流对于Chanchu台风在各发展阶段的强度变化也有一定的指示意义,揭示了负熵流对大气系统的组织化作用。      

台风莫拉菲(2009)登陆前后电荷结构演变的模拟研究

利用中尺度起电放电模式以及卫星和闪电定位等观测资料,对比分析了台风莫拉菲（2009）在登陆前后以及衰亡阶段的电荷结构及形成。结果表明：莫拉菲在登陆前存在近海加强过程,加强中逐渐形成清晰的台风眼并伴随眼壁区闪电活动的多发。眼壁区对流在近海加强阶段呈现正的三极性电荷结构,主负电荷区位于-25℃——10℃层,其上下各有一个正电荷区。而在台风达到最大强度后呈现负的偶极性电荷结构,仅存在云中部的负电荷区和下部的正电荷区。眼壁区对流的电荷结构同台风强度变化密切相关而不受登陆直接影响。在台风发展的不同阶段,外螺旋雨带对流主要表现为正的三极性或正的偶极性电荷结构,之前的研究一般认为外雨带对流只能呈现正的偶极性电结构。外雨带三极性电结构的形成可以类似于眼壁区三极性结构的形成,也存在其他形成机制,即在霰粒子与冰晶组成的正偶极性电荷结构下存在一个由雹粒子组成的正电荷区,从而形成正的三极性电荷结构。台风衰亡阶段对流主要表现负的偶极性电荷结构,对流活动较弱,类似于陆地雷暴消散阶段的特性。不同类型的电荷结构所对应对流的相对强度也在文内进行了讨论。     

强台风Chanchu（0601）的数值研究：转向前后内核结构和强度变化

使用FY卫星TBB资料和新一代非静力中尺度模式WRF分析南海强台风Chanchu(0601)"急翘"转向前后内核结构和强度变化过程。结果表明:转向后内核结构非对称特征明显。WRF数值模式较好地模拟出Chanchu强度和异常路径变化过程,再现了内核结构演变:转向前,垂直切变较弱,有利于快速加强,内核结构较为对称;转向后,垂直切变明显增大,强回波位于垂直切变下风方向的左侧,显示为内核非对称结构。使用傅立叶变换方法分解模拟结果中的雷达回波,发现眼壁和内螺旋雨带的2波非对称沿方位角移速与涡旋罗斯贝波(VRWs)的理论波速一致,Chanchu快速加强过程中断和强度维持的可能原因为:眼壁传播的VRWs受到外螺旋雨带的扰动以及涡旋倾斜加剧引起眼壁非对称性加强导致"急翘"时眼壁破裂,此后眼区和眼壁区水平混合过程加强,850 hPa眼区相当位温明显增加,抑制高层相对暖干空气和低层相对冷湿空气相互交换,使得随眼壁内侧下沉气流向下输送的暖干空气减少,低层增温作用减弱,快速加强过程中断;VRWs径向内传导致高值涡度由眼壁内侧向眼心传播,引起最大风速半径(RMW)内侧切向风速增大,RMW随时间向眼心延伸,眼壁进一步收缩,一定程度上抵消了垂直切变加大的负面影响,Chanchu维持强度。     

热带风暴中波动特征的研究进展和问题

在分析热带风暴眼壁和螺旋雨带中尺度波动特征最新研究的基础上，指出这些研究所忽略的问题，其中包括重力惯性波和涡旋Rossby波波解存在的前提条件和约束、理论分析与观测研究存在的差异等。提出一种基于准平衡动力条件下，热带风暴内中尺度扰动涡散运动共存时，区别于标准模混合的不可分的混合涡旋Rossby-重力惯性波，并讨论了位涡守恒条件下这一类不可分混合波的可能成波机制。利用高分辨率的模式大气资料，采用非对称波分量的分解方法分析了Bonnie飓风中的中尺度波动特征，结果表明，热带风暴中1波型扰动既具有涡旋波性质，但也存在散度扰动的变化，而2波型扰动则体现了明显的不可分混合波的特性。     

9015号台风登陆后其周围的若干中尺度特征

利用加密观测资料详细分析了登陆并穿过浙江省的９０１５号台风所造成的风雨分布及其变化与台风周围的一些中尺度特征以及地形作用的关系。分析表明：台风登陆后气压眼与风眼存在偏高，这种偏离状况是与风雨强度相对应的。此外，在台风登陆后不久，由于入流增强，中心眼区范围内降雨会突然增强；台风造成的雨区成带状分布，带宽约４０─６０ｋｍ，间隔为３０─１００ｋｍ，雨带内主要雨团相对于台风中心的移动轨迹大都呈顺时针旋转；较强的风雨区往往出现在台风周围的中尺度低压和中尺度涡旋及辐合线附近，前者易在倒槽内生成，后者则与地形影响有关。     

大尺度凝结加热与中尺度位温扰动对冷锋锋生的作用

文中导得简化的包括大尺度凝结潜热的半地转锋生模式，利用该模式讨论了大尺度凝结加热和中尺度位温扰动对冷锋锋生的影响，给出了锋生过程中各物理量的演变图。计算结果表明，大尺度凝结加热对冷锋锋生具有加强作用，增大锋生率，增大上升运动速度，缩小上升运动区的范围，使之更具有中尺度系统特征，使非地转越锋环流增强并发生倾斜，凝结加热和中尺度位温扰动的结合是锋前暖区多重雨带形成的可能机制之一。     

The Dynamical Characteristics and Wave Structure of Typhoon Rananim （2004）

Typhoon Rananim (2004) was one of the severest typhoons landfalling the Chinese mainland from 1996 to 2004. It brought serious damage and induced prodigious economical loss. Using a new generation of mesoscale model, named the Weather Research and Forecasting (WRF) modeling system, with 1.667 km grid horizontal spacing on the finest nested mesh, Rananim was successfully simulated in terms of track, intensity, eye, eyewall, and spiral rainbands. We compared the structures of Rananim to those of hurricanes in previous studies and observations to assess the validity of simulation. The three-dimensional (3D) dynamic and thermal structures of eye and eyewall were studied based on the simulated results. The focus was investigation of the characteristics of the vortex Rossby waves in the inner-core region. We found that the Rossby vortex waves propagate azimuthally upwind against the azimuthal mean tangential flow around the eyewall, and their period was longer than that of an air parcel moving within the azimuthal mean tangential flow. They also propagated outward against the boundary layer inflow of the azimuthal mean vortex. Futhermore, we studied the connection between the spiral potential vorticity (PV) bands and spiral rainbands, and found that the vortex Rossby waves played an important role in the formation process of spiral rainbands.     

SATELLITE-BASED ANALYSIS ON THE CONCENTRIC EYEWALL REPLACEMENT CYCLES OF SUPER TYPHOON MUIFA (1109)

Multisatellite data is used to analyze the characteristics of three eyewall replacement cycles (ERCs) during the lifetime of Typhoon Muifa (1109). Spiral rainbands evolutions, concentric eyewall (CE) structure modes, CE durations, and intensity changes are discussed in detail. In addition, an ERC evolution model of Typhoon Muifa is given. There are four main findings. (1) The outer spiral rainband joins end to end to form the outer eyewall after it disconnects from the original (inner) eyewall. The inner eyewall weakens as the outer eyewall becomes axisymmetric and is intensified. The contraction of the outer eyewall causes the inner eyewall to dissipate rapidly. Finally, the ERC ends with an annular eyewall or spiral rainbands. (2) Although the CE duration times of Typhoon Muifa’s three ERCs covered a large range, the CE structures were all maintained for approximately 5 h from the formation of the axisymmetric outer eyewall to the end of the cycle. (3) There is no obvious precipitation reflectivity in the eye or moat region for the subsidence flow. The convection within the two eyewalls is organized as a radially outward slope with increasing height. (4) Typhoon intensity estimation results based on ADT may not explain the intensity variations associated with ERC correctly, while the typhoon’s warm core data retrieved from AMSU-A works well.     

Finescale Spiral Rainbands Modeled in a High-Resolution Simulation of Typhoon Rananim （2004）

Finescale spiral rainbands associated with Typhoon Rananim(2004)with the band length ranging from 10 to nearly 100 km and band width varying from 5 to 15 km are simulated using the Fifth-Generation NCAR/Penn State Mesoscale Model(MM5).The finescale rainbands have two types:one intersecting the eyewall and causing damaging wind streaks,and the other distributed azimuthally along the inner edge of the eyewall with a relatively short lifetime.The formation of the high-velocity wind streaks results from the interaction of the azimuthal flow with the banded vertical vorticity structure triggered by tilting of the horizontal vorticity.The vertical advection of azimuthal momentum also leads to acceleration of tangential flow at a relatively high altitude.The evolution and structures of the bands are also examined in this study. Further investigation suggests that the boundary inflection points are related tightly to the development of the finescale rainbands,consistent with previous findings using simple symmetric models.In particular,the presence of the level of inflow reversal in the boundary layer is a crucial factor controlling the formation of these bands.The near-surface wavy peaks of vertical vorticity always follow the inflection points in radial flow.The mesoscale vortices and associated convective updrafts in the eyewall are considered to strengthen the activity of finescale bands,and the updrafts can trigger the formation of the bands as they reside in the environment with inflow reversal in the boundary layer.     

台风“珍珠”(2006)螺旋雨带发展机制的数值模拟研究

基于2 km分辨率的ARW-WRF数值模拟资料,讨论了台风"珍珠"(2006)螺旋雨带中对流单体及内雨带的发展机制。结果表明:模式很好地再现了台风的路径和强度。作为雨带中仅仅存在于眼壁外侧的内雨带,其传播机制与重力波、涡旋Rossby波及混合波没有联系,其可能发展机制仅与低层出流、水平风场和变形场有关。低层出流使得内雨带径向向外运动,而低层的水平风场和变形场使其形成螺旋结构。同时,就螺旋雨带中精细对流单体的发展而言,涡度收支方程定量分析表明,其主要通过两种方式获得垂直涡度:水平涡度倾斜为垂直涡度;上升运动拉伸垂直涡度。随着平流输送,对流单体在眼壁附近合并和汇聚。     

Lightning Activity and Precipitation Characteristics of Typhoon Molave （2009） Around Its Landfall

Cloud-to-ground （CG） lightning data,storm intensity and track data,and the data from a Doppler radar and the Tropical Rainfall Measuring Mission （TRMM） satellite,are used to analyze the temporal and spatial characteristics of lightning activity in Typhoon Molave （0906） during different periods of its landfall （pre-landfall,landfall,and post-landfall）.Parameters retrieved from the radar and the satellite are used to compare precipitation structures of the inner and outer rainbands of the typhoon,and to investigate possible causes of the different lightning characteristics.The results indicate that lightning activity was stronger in the outer rainbands than in the eyewall and inner rainbands.Lightning mainly occurred to the left （rather than ＂right＂ as in previous studies of US cases） of the moving typhoon,indicating a significant spatial asymmetry.The maximum lightning frequency in the tropical cyclone （TC） eyewall region was ahead of that in the whole TC region,and the outbreaks of eyewall lightning might indicate deepening of the cyclone.Stronger lightning in the outer rainbands is found to be associated with stronger updraft,higher concentrations of rain droplets and large ice particles at elevated mixed-phase levels,and the higher and broader convective clouds in the outer rainbands.Due to the contribution of large cloud nuclei,lightning intensity in the outer rainbands has a strong positive correlation with radar reflectivity.The ratio of positive CG lightning in the outer rainbands reached its maximum 1 h prior to occurrence of the maximum typhoon intensity at 2000 Beijing Time （BT） 18 July 2009.During the pre-landfall period （0300 BT 18 July-0050 BT 19 July）,the typhoon gradually weakened,but strong lightning still appeared.After the typhoon made landfall at 0050 BT 19 July,CG lightning density rapidly decreased,but the ratio of positive lightning increased.Notably,after the landfall of the outer rainbands at 2325 BT 18 July （approximately 1.5 h prior to the landfall of the TC）,significantly higher ice particle density derived from the TRMM data was observed in the outer rainbands,which,together with strengthened convection resulted from the local surface roughness effect,might have caused the enhanced lightning in the outer rainbands around the landfall of Molave.     

特大眼台风Winnie（1997）的高分辨率数 值模拟

台风Winnie 1997的眼直径为370 km,是有观测以来发现的最大台风眼之一。应用Penn State/NCAR高分辨率中尺度模式MM5,成功地模拟了Winnie的路径、强度、台风眼及其双眼壁结构。由此根据模式输出结果分析了台风眼及内外眼壁附近的流场和热力场特征。发现Winnie台风的眼壁及其周围风场都显示了明显的非对称性结构。Winnie的外眼壁对应一个极大风速环,也是暖湿环和正涡度环。内眼壁对应一个次极大风速环、暖湿环。上升运动控制整个内眼壁和海平面2 km以上的外眼壁区域,下沉运动则控制眼区和内外眼壁之间。径向入流集中在外眼壁和内外眼壁之间的边界层,流出则位于外眼壁的对流层中上层。     

Sensitivity of the simulation of tropical cyclone size to microphysics schemes

The sensitivity of the simulation of tropical cyclone(TC) size to microphysics schemes is studied using the Advanced Hurricane Weather Research and Forecasting Model(WRF). Six TCs during the 2013 western North Pacific typhoon season and three mainstream microphysics schemes–Ferrier(FER), WRF Single-Moment 5-class(WSM5) and WRF Single-Moment6-class(WSM6)–are investigated. The results consistently show that the simulated TC track is not sensitive to the choice of microphysics scheme in the early simulation, especially in the open ocean. However, the sensitivity is much greater for TC intensity and inner-core size. The TC intensity and size simulated using the WSM5 and WSM6 schemes are respectively higher and larger than those using the FER scheme in general, which likely results from more diabatic heating being generated outside the eyewall in rainbands. More diabatic heating in rainbands gives higher inflow in the lower troposphere and higher outflow in the upper troposphere, with higher upward motion outside the eyewall. The lower-tropospheric inflow would transport absolute angular momentum inward to spin up tangential wind predominantly near the eyewall, leading to the increment in TC intensity and size(the inner-core size, especially). In addition, the inclusion of graupel microphysics processes(as in WSM6) may not have a significant impact on the simulation of TC track, intensity and size.     

Characteristics and possible formation mechanisms of severe storms in the outer rainbands of Typhoon Mujiga (1522)

To better understand how severe storms form and evolve in the outer rainbands of typhoons, in this study, we investigate the evolutionary characteristics and possible formation mechanisms for severe storms in the rainbands of Typhoon Mujigae, which occurred during 2–5 October 2015, based on the NCEP–NCAR reanalysis data, conventional observations, and Doppler radar data. For the rainbands far from the inner core (eye and eyewall) of Mujigae (distance of approximately 70–800 km), wind speed first increased with the radius expanding from the inner core, and then decreased as the radius continued to expand. The Rankine Vortex Model was used to explore such variations in wind speed. The areas of strong stormy rainbands were mainly located in the northeast quadrant of Mujigae, and overlapped with the areas of high winds within approximately 300–550 km away from the inner core, where the strong winds were conducive to the development of strong storms. A severe convective cell in the rainbands developed into waterspout at approximately 500 km to the northeast of the inner core, when Mujigae was strengthening before it made landfall. Two severe convective cells in the rainbands developed into two tornadoes at approximately 350 km to the northeast of the inner core after Mujigae made landfall. The radar echo bands enhanced to 60 dBZ when mesocyclones occurred in the rainbands and induced tornadoes. The radar echoes gradually weakened after the mesocyclones weakened. The tops of parent clouds of the mesocyclones elevated at first, and then suddenly dropped about 20 min before the tornadoes appeared. Thereby, the cloud top variation has the potential to be used as an early warning of tornado occurrence.