二维多单体雷暴系统中对流单体生成和发展的新机制

在多单体雷暴系统的发展和演变过程中,系统内新对流单体的生成和发展是系统维持最为重要的因素。文中利用中尺度数值模式,研究了环境风场存在低层反向切变(U_z0)情况下,一种新对流单体的生成和发展机制——处于"亚最优切变态"的多单体雷暴系统的维持和发展机制。与以往处于"最优切变态"条件下新对流单体的生成和发展机制不同,由于环境风场存在低层反向切变,此时密度流产生的水平涡度要大于环境风切变产生的水平涡度,密度流向外扩展较快,使两者处于"亚最优切变态",此时多单体雷暴系统中的新对流单体不再是从密度流前沿的阵风锋前上升气流分离出,而是由发展旺盛的主体对流体在其前部中低层激发的垂直上升运动引起。新生成的对流单体因其前部低层湿空气的流入得到快速发展,而其西侧原有的主体对流体因湿空气的流入被切断而开始减弱、消亡并最终被新对流单体所替代。这种新机制的出现与低层环境风反向切变的强度及地表风速有关。在一定的低层反向风切变条件下,这种新机制可与"最优切变态"机制交替作用:当密度流较强时,多单体雷暴系统中新对流单体的生成主要是以"亚最优切变态"机制为主,且新生成的对流单体强度也较弱;当密度流较弱时,多单体雷暴系统中新对流单体的生成主要以"最优切变态"机制为主,此时阵风锋前上升气流得到快速发展,当发展到一定程度时,新对流单体从阵风锋前上升气流中分离出。     

Structures of dynamically unstable shear flows and their implications for shallow internal gravity waves

Summary In this study, the response of a dynamically unstable shear flow with a critical level to periodic forcing is presented. An energy argument is proposed to explain the upshear tilt of updrafts associated with disturbances in two-dimensional stably stratified flows. In a dynamically unstable flow, the energy equation requires an upshear tilt of the perturbation streamfunction and vertical velocity whereU _z is positive. A stability model is constructed using an iteration method. An upshear tilt of the vertical velocity and the streamfunction fields is evident in a dynamically unstable flow, which is required by energy conversion from the basic shear to the growing perturbation wave energy according to the energy argument. The momentum flux profile indicates that the basic flow is decreased (increased) above (below) the critical level. Thus, the shear instability tends to smooth the shear layer. Following the energy argument, a downshear tilt of the updraft is produced in an unstably stratified flow since the perturbation wave energy is negative. The wave energy budget indicates that the disturbance is caused by a thermal instability modified by a shear flow since the potential energy grows faster than the kinetic energy.With 4 Figures     

2010年福建一次早春强降雹超级单体风暴对比分析

利用探空、地面资料以及建阳、龙岩、长乐三部新一代天气雷达资料,对2010年3月5日福建中北部地区5cm强降雹的两个超级单体风暴进行了对比分析。结果表明,干暖盖、强垂直风切变、中高层正涡度区及地面中尺度低压为超级单体的形成提供了良好的环境场。两个超级单体都是由多单体合并后发展起来的,在成熟阶段以右移为主,属长寿命右移风暴:第一个超级单体在发展过程中由于地形作用和新单体的并入经历了3次加强过程,低层出现明显的钩状回波、中高层三体散射特征;第二个超级单体经历了多单体风暴—超级单体风暴—多单体风暴3个阶段,成熟阶段低层呈现出明显的倒"V"形回波特征,中高层有明显向右伸展的云帖。两个超级单体风暴的中气旋都是由中层发展起来,随着中气旋强度不断加强和厚度加大,最强切变中心突降时出现冰雹、大风强对流天气。通过对第一个超级单体中气旋流场分析,发现风暴前、后侧的下沉气流与低层入流形成了明显的辐合旋转作用,下沉的干冷气流进一步推动低层的暖湿入流,形成强烈的上升气流,并在风暴顶形成强辐散,使得风暴长时间维持。第二个超级单体在风暴减弱阶段,风暴右侧出现中气旋分裂,之后减弱、消失。产生强对流天气时,中高层维持高反射率因子,出现三体散射现象、风暴顶强烈辐散以及较大的VIL密度等特征。     

一次长寿命风暴的CINRAD/SA雷达反射率及中气旋产品特征与流场结构分析

利用济南观测站的探空、涡度、地面资料和CINRAD/SA雷达产品,对2004年6月24日发生在山东西部地区的强风暴过程进行了分析。结果表明,风暴低层存在中尺度辐合现象,风暴前侧的入流依靠后部低层冷空气池的向前推进而得到支撑,产生逆环境风切变方向的主上升气流区;中层为双涡旋结构,这种深厚的内部涡旋结构可与环境风相持,形成近似刚体的风暴柱,环境风绕风暴而过,不会吹穿风暴,有利于风暴长时间维持;风暴成熟阶段表现为超级单体结构特征并伴有中气旋;三体散射(TBSS)出现在中高层,TBSS出现10~15 min后地面出现大冰雹,TBSS消失后维持10~20 min的降雹。     

Dynamical effects of environmental vertical wind shear on tropical cyclone motion, structure, and intensity

Summary A series of numerical experiments on an f plane are conducted using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model, version 3 (MM5) to investigate how environmental vertical wind shear affects the motion, structure, and intensity of a tropical cyclone. The results show that a tropical cyclone has a motion component perpendicular to the vertical shear vector, first to the right of the shear and then to the left. An initially axisymmetric, upright tropical cyclone vortex develops a downshear tilt and wavenumber-one asymmetry when embedded in environmental vertical wind shear. In both small-moderate shears, a storm weakens slightly compared to that in a quiescent environment. The circulation centers between 300 hPa and the surface varies from 20 km to over 80 km. The secondary circulation becomes quite asymmetric about the surface cyclone center. As a result, convection on the upshear-right quadrant diminishes, limiting the upward heat transport in the eyewall and thus lowering the warm core and leading to a weakening of the storm. In strong vertical shear (above 12 m s⁻¹), the vertical tilt exceeds 160 km in 48 h of simulation and the secondary circulation on the upshear side is completely destroyed with low-level outflow. The axisymmetric component of eyewall convection weakens remarkably and becomes much less penetrative. As a result, the warm core becomes weak and appears at lower levels and the storm weakens rapidly accordingly. This up-down weakening mechanism discussed in this study is different from those previously discussed. It emphasizes the penetrative role of eyewall convection in transporting heat from the ocean to the mid-upper troposphere, maintaining the warm core structure of the tropical cyclone. The vertical shear is found negative to eyewall penetrative convection.     

Effects of vertical wind shear and storm motion on tropical cyclone rainfall asymmetries over the North Indian Ocean

Tropical cyclone (TC) rainfall asymmetry is often influenced by vertical wind shear and storm motion. This study examined the effects of environmental vertical wind shear (200-850 hPa) and storm motion on TC rainfall asymmetry over the North Indian Ocean (NIO): the Bay of Bengal (BoB) and the Arabian Sea (AS). Four TC groups were used in this study: Cyclonic Storm (CS), Severe Cyclonic Storm (SCS), Very Severe Cyclonic Storm (VSCS) and Extreme Severe Cyclonic Storm (ESCS). The Fourier coefficients for wave number-1 was used to analyze the structure of TC rainfall asymmetry. Results show that the maximum TC rainfall asymmetry was predominantly in the downshear left quadrant in the BoB, while it placed to downshear right quadrant in the AS, likely due to the different primary circulation strength of the TC vortex. For the most intense cyclone (ESCS), the maximum TC rainfall asymmetry was in the upshear left quadrant in the BoB, whereas it was downshear right quadrant in the AS. It is evident for both basins that the magnitude of TC rainfall asymmetry declined (increased) with TC intensity (shear strength). This study also examined the collective effects of vertical wind shear and storm motion on TC rainfall asymmetry. Here, the analysis in case of the strong shear environment (>7 m s^-1) omitted for the AS because the maximum value for this basin was about 7 m s^-1. The result showed that the downshear left quadrant was dominant in the BoB for the maximum TC rainfall asymmetry. In a weak shear environment (<5 m s^-1), on the other hand, downshear right quadrant is evident for the maximum TC rainfall asymmetry in the BoB, while it placed dominantly downshear left quadrant in the AS. In the case of motion-relative wavenumber-1, the maximum TC rainfall asymmetry was dominantly downshear for both basins.     

十堰一次强对流天气雷达回波特征

利用十堰714C天气雷达回波资料,结合其它天气资料,分析了2004年7月6日发生在十堰境内的强对流天气过程。结果表明,这次强对流天气是在有利的大尺度天气形势下产生的,局地环境CAPE指数大,水平风的垂直切变较强。强降雹由多单体强风暴造成,回波强度强,高度较高,顶部有旁瓣假回波,低层存在弱回波区（WER）。雹云移动明显右偏于高空风,属右移风暴,以右后侧和右前侧传播方式发展。强降雹由后一种传播方式造成,初始回波从半空生成,云顶高度较高,强中心位于云体中上层。     

天津雷暴大风天气的环流场及雷达回波特征分析

利用加密自动站、闪电定位仪、FNL和多普勒雷达资料对2018—2020年汛期(4—9月)天津地区出现的47次雷暴大风过程的时空分布、天气形势和雷达回波特征进行统计分析。结果表明：(1)天津地区雷暴大风多出现在北部山区和东部沿海,高发月份为6—8月,多出现在傍晚到前半夜,持续时间多为1～4 h;(2)天津地区雷暴大风的天气形势主要有西北气流型、冷涡型、低槽型和西太平洋副热带高压边缘型,其中冷涡型出现频次最高;(3)造成天津地区雷暴大风的对流风暴类型主要有非线状多单体风暴、线状多单体风暴(不包含飑线)、飑线、弓形回波和普通单体风暴,其中飑线数量最多,飑线和弓形回波是造成雷暴大风极端值的主要风暴类型;(4)当最大反射率因子为61 dBZ、强回波中心下降率为260 m·min^-1上下时发生雷暴大风的可能性最高;(5)根据低层径向速度大值区,可对15.4%的非线状多单体风暴、14.3%的线状多单体风暴和22.2%的飑线雷暴大风提前30 min发布预警。     

一次典型超级单体风暴的多普勒天气雷达观测分析

文中利用位于安徽合肥的S波段多普勒天气雷达资料 ,对 2 0 0 2年 5月 2 7日 1 4～ 2 0时发生在皖北地区的一次典型的超级单体风暴过程进行了详细的分析。此次超级单体南边出现两条明显的出流边界 ,一条位于钩状回波的西南 ,一条位于钩状回波的东南。超级单体左前方的低层反射率因子呈现明显的倒“V”字型结构 ,这也是超级单体风暴的典型特征之一。沿入流方向穿过最强回波位置的反射率因子垂直剖面呈现出典型的有界弱回波区 (穹隆 )、强大的回波悬垂和有界弱回波区左侧的回波墙。最大的回波强度出现在沿着回波墙的一个竖直的狭长区域 ,其值超过 70dBz。相应的中低层径向速度图呈现一个强烈的中气旋 ,旋转速度达到 2 2m/s。风暴顶为强烈辐散 ,正负速度差值达 6 3m/s。相应的垂直累积液态水含量和密度分别超过 70kg/m2 和 5 g/m3 。因此 ,该风暴具有强烈超级单体风暴的典型特征。该超级单体的移动方向在盛行风向的右侧约 30°,属于右移风暴     

A two-moment cloud microphysics parameterization for mixed-phase clouds. Part 2: Maritime vs. continental deep convective storms

Summary A systematic modeling study investigates the effects of cloud condensation nuclei (CCNs) on the evolution of mixed-phase deep convective storms. Following previous studies the environmental conditions like buoyancy and vertical wind shear are varied to simulate different storm types like ordinary single cells, multicells and supercells. In addition, the CCN characteristics are changed from maritime to continental conditions. The results reveal very different effects of continentality on the cloud microphysics and dynamics of the different storms. While a negative feedback on total precipitation and maximum updraft velocity is found for ordinary single cells and supercell storms, a positive feedback exists for multicell cloud systems. The most important link between CCN properties, microphysics and dynamics is the release of latent heat of freezing.     

Structures of dynamically unstable shear flows and their implications for shallow internal gravity waves part II: Nonlinear response

Summary The nonlinear response of a dynamically unstable shear flow with critical level to an initial temperature anomaly is investigated using a nonlinear numerical model. Both nonconstant and constant shear profiles of the basic flow are considered. Effects of the solid lower boundary on the dynamically unstable, nonlinear flow are also studied. It is found that in a dynamically unstable, linear flow with a hyperbolic tangent wind profile, the updraft is tilted upshear. The result in consistent with that of a linear stability model (LC). The upshear tilt can be explained by the Orr mechanism (1907) and the energy argument proposed by LC. In a dynamically unstable, nonlinear flow, the updrafts produced by a sinusoidal initial temperature perturbation are stronger in the lower layer and are more compact and located further apart compared to the corresponding linear flow. In addition, the perturbed wave energy is slightly smaller than the linear case. It is found that the growth rate is smaller during the early stage and much larger during the later stage. For a localized initial temperature perturbation in a dynamically unstable flow, a stronger updraft with two compensated downdrafts are produced. Gravity waves are produced in a dynamically stable flow with both a hyperbolic tangent wind profile and a linear wind profile. For a linear shear flow with Richardson number less than 1/4, the disturbance grows in the early stage and then decays algebraically at later times, similar to that found in other linear theoretical studies. The influence of the solid lower boundary is to suppress the shear instability in a nonlinear flow with a hyperbolic tangent wind profile ofRi<1/4.With 9 Figures     

基于雷达资料四维变分同化和三维云模式对一次超级单体风暴发展维持热动力机制的模拟分析

利用三维云尺度数值模式和雷达资料快速更新循环四维变分同化(4DVar)技术,对京津冀地区一次强降水超级单体风暴发展演变的热动力机制进行了数值模拟和结果分析,并结合雷达、加密探空和自动站资料,揭示了快速变化的近风暴大气环境及风暴自身的热动力三维特征对超级单体形成、发展和演变的影响.雷达回波观测分析表明,这是一次由多单体合...     

Comparison of lightning activity and radar-retrieved microphysical properties in EULINOX storms

A combined analysis of microphysical thunderstorm properties derived by C-band polarimetric Doppler radar measurements and lightning observations from two ground-based systems are presented. Three types of storms, a multicell, a supercell, and a squall line, that were observed during the European Lightning Nitrogen Oxides project (EULINOX) are investigated. Correlations are sought between the mass of rain, graupel, hail, and snow derived form radar observations at different height levels and the electrical activity, represented either by cloud-to-ground or intracloud flashes. These relationships are explained by connecting the radar-derived properties with the non-inductive charging process. For the multicell and the supercell storm, the lightning activity can be linearly correlated to both the hydrometeor total mass and class specific mass in the upper part of the storm. It is shown that the fractions of graupel and hail above the −20 °C-level in these storms positively correlate with the intracloud flash activity in the supercell, and negatively for the cloud-to-ground lightning frequency in the multicell. No such relation can be established for the squall line, indicating that the convective organization plays a crucial role in the lightning development. The analysis of the masses in the different storms shows that lightning activity cannot be parameterized by total mass alone, other parameters have to be identified. The results provide important information for all lightning studies that rely on bulk properties of thunderstorms, e.g., the parameterization of lightning in mesoscale models or the nowcasting of lightning by radar.     

Calculations of hailstone growth in a sloping steady updraft

Abstract

Analytical solutions to the equations of growth and motion of hailstones in updrafts with cloud water contents that vary linearly with height are used to investigate hail growth in a model cloud. The model storm is steady and two‐dimensional and is constructed by compositing sections with linear variations of vertical air velocity and cloud water content so as to approximate the Fleming storm. Hail embryos are introduced at various positions in the updraft and their subsequent history calculated. A strong correlation is found between the embryo starting position and its trajectory and final size. Embryos starting in the central portions of the updraft are carried up rapidly and displaced far from the updraft. Embryos starting near the upshear edge of the updrafr have a simple arcing up and down trajectory and produce small hailstones that can be expected to melt before reaching the ground. Embryos starting near the downshear edge of the updraft give rise to the largest hailstones and have recirculating trajectories. Effects of changing some of the model parameters are investigated.     

新疆阿克苏地区冰雹时空分布及雷达回波特征

利用"新疆气象灾情直报系统"冰雹灾情数据和阿克苏地区人工防雹作业信息,分析了阿克苏地区9县市2010—2019年冰雹天气的时间变化和空间分布特征,选用阿克苏地区9县市2009—2019年10场冰雹天气过程,应用美国Gibson Ridge Software LLC公司GR2Analyst雷达产品处理软件分析二次产品,得...     

Numerical Simulation of Microphysics in Meso-β-Scale Convective Cloud System Associated with a Mesoscale Convective Complex

Numerical simulation of meso-β-scale convective cloud systems associated with a PRE-STORM MCC case has been carried out using a 2-D version of the CSU Regional Atmospheric Modeling System (RAMS) nonhydrostatic model with parameterized microphysics. It is found that the predicted meso-γ-scale convective phenomena are basically unsteady under the situation of strong shear at low-levels, white the meso-β-scale convective system is maintained up to 3 hours or more. The meso-β-scale cloud system exhibits characteristics of a multi-celled convective storm in which the meso-γ-scale convective cells have lifetime of about 30 min. Pressure perturbation depicts a meso-low after a half hour in the low levels. As the cloud system evolves, the meso-low inten-sifies and extends to the upshear side and covers the entire domain in the mid-lower levels with the peak values of 5-8 hPa. Temperature perturbation depicts a warm region in the middle levels through the entire simulation period. The meso-γ-scale warm cores with peak values of 4-8oC are associated with strong convective cells. The cloud top evapo-ration causes a stronger cold layer around the cloud top levels.Simulation of microphysics exhibits that graupel is primarily concentrated in the strong convective cells forming the main source of convective rainfall after one hour of simulation time. Aggregates are mainly located in the stratiform region and decaying convective cells which produce the stratiform rainfall. Riming of the ice crystals is the predominant precipitation formation mechanism in the convection region, whereas aggregation of ice crystals is the predominant one in the stratiform region, which is consistent with observations. Sensitivity experiments of ice-phase microphysical processes show that the microphysical structures of the convective cloud system can be simulated better with the diagnosed aggregation collection efficiencies.     

Numerical simulation of a low-precipitation supercell thunderstorm

Summary Numerical simulations of thunderstorms using initial conditions from an Oklahoma storm event on 26 April 1984 have been carried out using the Klemp-Wilhelmson cloud model. Two thermodynamic and two horizontal wind profiles are mixed to create four initial storm environments. The two initial thermodynamic profiles are designated as moderately and highly unstable. Both hodographs have considerable vertical wind shear, with the extremely unstable profile having substantially stronger storm-relative curvature shear in the lowest few kilometers, as measured by the helicity of the environment. Storms are initialized with two different temperature-perturbation warm bubbles. A variety of storms, qualitatively covering the range observed on this day, are simulated. With a strongly sheared, moderate instability environment, the two different temperature-perturbation simulations differ qualitatively, unlike the other pairs of simulations.Of particular interest is the simulation carried out with the weaker temperature-perturbation. It displays many of the characteristics of observed low-precipitation (LP) supercell storms. Without artificially suppressing rainfall, this storm produces very little precipitation over the first 6000 s of its lifetime. During this time, there is no strong downdraft at the lowest model level and vorticity at that level is much less than the values aloft. We hypothesize that LP storms can be generated by smaller initial impulses than classic supercells in environments typically associated with a supercell storm, as speculated by Bluestein and Parks (1983), since they are more susceptible to turbulent entrainment in highly sheared environments.With 15 Figures     

两类不同风灾个例超级单体特征对比分析

采用分钟级加密自动气象站观测资料,盐城、淮安和岳阳、荆州雷达探测数据,以及欧洲中期天气预报中心（ECMWF）高分辨率的ERA-Interim全球再分析数据,对比分析了2016年6月23日江苏阜宁龙卷灾害和2015年6月1日湖北监利下击暴流大风灾害的环境特征与超级单体的结构特征。结果表明:（1）两次强对流大风灾害发生在相似的低空环流背景下:风灾发生在低空急流出口区左侧的暖区内、850 hPa低涡中心东侧6—7个经距的位置;环境大气的对流有效位能大于2000 J/kg。但是风灾的类型不同,江苏阜宁大风灾害主要由超级单体龙卷造成,监利“东方之星”沉船事故主要是超级单体触发的下击暴流造成。短时强降水中心与风灾中心的相对位置不同:阜宁龙卷移动方向的左侧伴随着最强短时降水;湖北监利沉船事件发生期间,风灾中心与短时强降水中心基本重合。鉴于不同性质的对流大风位置与超级单体母体的中心位置对应关系上存在差异,通过比较地面观测的瞬时大风与瞬时强降水中心的相对位置将有助于区分强对流大风的性质。（2）环境风垂直切变强度对对流风暴结构、发展、维持有重要影响:阜宁龙卷发生时,其上空0—6 km风垂直切变达4×10-3 s-1,超级单体有明显的向前倾斜结构,形成有界弱回波区;而监利强对流沉船位置0—6 km风垂直切变只有2.3×10-3 s-1左右,风暴单体中的上升气流近乎于垂直。阜宁超级单体中气旋,首先出现在0—1.5 km风垂直切变和0—3 km风暴相对螺旋度带状大值区,在向抬升凝结高度更低的环境移动过程中,其底部不断下降,形成龙卷;而在监利沉船区,中低层风切变和风暴相对螺旋度相对要弱得多,对应风暴单体中的中气旋强度、持续性较弱,中气旋底部高度维持在1.6 km左右。（3）环境湿度垂直结构特征不同可能是风暴单体形成不同类型灾害大风的重要环境因子。监利下击暴流造成的风灾发生时,在地面气温迅速下降过程中,气压变化呈现快速跳升又快速下降的“尖锥”形,气压峰值比降水峰值提前4 min出现。它与对流层中高层环境大气中较为深厚的干空气卷入对流风暴中造成水物质强烈蒸发、冷却过程有关。而阜宁风灾过程中,环境大气中层仅存在非常浅薄的干层,加之低层较为深厚的饱和大气环境,对应的地面冷池效应相对较弱。      

两种垂直累积液态含水量产品的应用对比分析

结合雷达体扫工作模式以及风暴单体垂直结构分布，利用CINRAD气象产品软件包提供的基于单体和格点的VIL产品，对滨州SC雷达监测到的几例强对流风暴单体个例进行分析发现，两种VIL产品的变化趋势基本与风暴变化同步，对灾害性天气发生的预警具有较好的指示意义；一般情况下，基于单体的VIL数值大于基于格点的VIL数值，尤其是发展成熟、孤立的强风暴单体表现更加明显，可以作为判断风暴单体强度的定性指标；对于多单体风暴，由于风暴质心识别等因素会造成VIL数值误差增大，而雷达体扫模式的限制可能造成VIL数值失真。     

一次热带气旋减弱远离后出现暴雨的成因分析

利用常规气象资料、加密气象站观测资料、NCEP的1°×1。分析资料以及多普勒雷达资料,对2011年6月12日热带气旋减弱远离后广东发生的一次暴雨过程进行了诊断分析。结果表明：（1）地而辐合、低空切变、中层气旋式环流和高空辐散的共同作用导致了此次的暴雨过程;（2）垂直上升运动在强降水发生前即有明显反映,中低层正涡度、高层负涡度的配置也有利于上升运动的维持和强降水的发生;（3）前期不稳定能量的累积是必要条件,而K、SI指数在维持高度不稳定的条件下,其24h变化量对此次暴雨的发生有一定的提前指示意义;（4）中尺度切变、辐合对强降水的发生时间和落区移动都有较好的指示作用．中低层乖盲风切蛮增大有利于风暴的发展。