The effect of ageostrophy on the stability of thin oceanic vortices

This paper examines the stability of vortices in a two-layer ocean on the f-plane. The mean depth of the upper layer is assumed to be much smaller than the depth of the lower layer. Using the primitive equations, we derive an asymptotic criterion for baroclinic instability of compensated (i.e. confined to the upper layer) vortices. Surprisingly, it coincides exactly with a similar criterion derived from the quasigeostrophic equations [Benilov, E.S., 2003. Instability of quasigeostrophic vortices in a two-layer ocean with thin upper layer. J. Fluid Mech. 475, 303–331]. Thus, to leading order in , ageostrophy does not affect the stability properties of thin compensated vortices. As a result, whether a vortex is stable or not, depends on its shape, not amplitude (although the growth rate of an unstable vortex does depend on its amplitude).     

The influence of the coast on the dynamics of upwelling fronts: Part II. Numerical simulations

We investigated the dynamics of upwelling fronts near a coast. This work was first motivated by laboratory experiments [Bouruet-Aubertot, Linden, Dyn. Atmos. Oceans, 2002] in which the front is produced by the adjustment of a buoyant fluid initially confined within a bottomless cylinder. It was shown that cyclonic eddies consisting of coastal waters are enhanced when the front is unstable near the coast (the outer vertical boundary). The purpose of this paper is to provide further insights into this process. We reproduced the experimental configuration using a three-dimensional model of the primitive equations. We first show that for coastal fronts more potential energy, in terms of the maximum available potential energy, is released than for open-ocean fronts. Therefore, waves of larger amplitude are generated during the adjustment and the mean flow that establishes has a higher kinetic energy in the former case. Then as baroclinic instability starts and wave crests reach the boundary, cyclonic eddies are enhanced as in the laboratory experiments and in a similar way. However, in contrast to the laboratory experiments, offshore advection of cyclonic eddies can occur in two stages, depending on the spatial organization of the baroclinic wave. When the baroclinic wave consists of the sum of different modes and is thus highly asymmetric, the offshore advection of cyclonic eddies occurs just after their enhancement at the boundary, as in the laboratory experiments. By contrast, when a single-mode baroclinic wave develops, neighboring cyclonic eddies first merge before being advected offshore. Very different behavior is observed for open-ocean fronts. First a mixed baroclinic–barotropic instability grows. Then the eddies transfer their energy to the mean flow and the barotropic and baroclinic instabilities start again. An excellent agreement is obtained with the main result obtained in the laboratory experiments: the ratio between growth rates of surface cyclonic and anticyclonic vorticity increases as the instability develops nearer to the coast.     

山西省大雾的中尺度数值模拟

利用中尺度模式MM5（The Fifth—Generation NCAR／Penn State Mesoscale Model）对山西省2009年发生的3场典型雾个例进行了数值模拟,探讨了物理过程参数化方案对雾数值模拟的影响,确定了基于模式模拟数据的雾判别指标,为该地区大雾数值预报系统的研制提供了理论基础。结果表明,综合考虑边界层方案和辐射方案对地表温度、高空温度、2m温度及相对湿度、10m风速、雾的空间分布、雾的生消过程、雾的发展高度等要素数值模拟的影响,边界层方案选用high-resolution planetary boundary layers cheme（HIR）方案、辐射方案选用Cloud方案时,雾数值模拟的结果与实况更为一致。综合分析多个典型雾个例的模拟结果,山西省境内雾的预报指标为：20m液态水含量为0．13～0．6g·kg^（-1）,20～1500m高度大气层存在逆温层,10m风速小于4m·s^（-1）。     

斜压不稳定理论的发展历程分析

斜压不稳定理论是中高纬度地区天气尺度扰动生成和发展的机制,是继长波理论之后大气动力学上的又一重大进展,对现代天气预报具有重要的指导意义。按照斜压不稳定理论发展的时间脉络,阐述了赵九章、查尼和伊迪在斜压不稳定方面开展的研究工作,分析了3位科学家各自的研究特点及历史贡献。赵九章1946年发表在《Journal of Meteorology》上的论文,最早提出了"斜压不稳定"这一概念,给出了不稳定的临界波长,并阐述了在不稳定扰动情况下能量的转换,以及不稳定波对大气环流带来的可能影响,尽管得出的不稳定临界波长与观测差别较大,但其对波-流相互作用的讨论在当时是超前的。查尼于1947年采用滤波和尺度分析等方法,将大气扰动方程简化为一个可以求解的系统,推导出大气稳定状态的判据,建立了斜压不稳定理论,其结果与实况比较接近;并据此把准地转模式成功应用于数值天气预报实践中,促使数值天气预报获得首次成功。1949年,伊迪在查尼研究工作的基础上通过合理的简化方法,得到了更为简洁的模型。最后,通过对比他们的研究思路,重点分析了赵九章未能使得斜压不稳定理论提前一年建立的原因:由于其研究思路始终局限在大气水平运动上,忽略了斜压系统发展中散度项的贡献,因而未能抓住天气系统发生、发展的本质,致使其最终与斜压不稳定理论的成功建立失之交臂,其论文本身存在的一些亮点也因此被后人忽视。      

台风榴莲(2001)在季风槽中生成的机制探讨

利用NCEP 1°×1°分析资料、TMI海温资料、卫星云图资料对季风槽中南海台风榴莲(2001)生成机制进行了分析,揭示了大尺度环境流场、温暖洋面、中尺度对流活动对热带气旋(TC)生成的控制作用.结果表明,水平风速垂直切变的演变在一定程度上指示着TC在暖湿洋面上生成的时间,水平风速垂直切变由强向弱转变,在TC发生前18小时迅速减小到10 m/s,随后在10 m/s以下维持少变,垂直切变的变化主要反映了对流层高层环流形势的演变;在对流层中低层,季风槽的形成和加强对TC的生成有重要作用,由于热带温暖洋面作用,季风槽首先表现出有利于单体对流和带状对流发生发展的条件性对流不稳定特征,随着季风槽的加强,季风槽进一步表现出有利于中尺度扰动发生发展的正压不稳定特征;季风槽槽线南侧的低空急流的经向分布很宽广,由105°E越赤道气流和中南半岛偏西气流(其源头是索马里越赤道低空急流)汇合而成,急流的加强活动具有经向差异,由于边界层高θ_e空气辐合抬升产生两条经向距离约300 km的显著带状对流云系,槽线南侧风速分布的经向差异导致两条带状云系发生追赶,并逐步在季风槽底部槽线附近合并加强为MCC,进而导致中尺度涡旋(MCV)的产生并最终发展成为TC.分析结果还表明,为深对供应丰富对流有效位能的主要是来自台风发生区域本地南海暖洋面的地面热通量,南海暖洋面对TC生成有重要贡献.台风榴莲的生成是一个多尺度相互作用过程,主要包括涡旋对流热塔、与带状对流云系伴随的涡度带的升尺度,涡度带合并成长为MCV,以及大尺度条件对TC在季风槽中生成的时间及地点的控制作用等.     

热带气旋生成过程的中尺度涡旋活动数值模拟

热带气旋生成过程中包含不同尺度环流及其相互作用。为此,本文将热带气旋生成数值模拟的起点提前到模拟中尺度涡旋(MCV)的生成,从而利用高分辨率数值试验结果,对热带气旋过程中的不同尺度涡旋活动进行分析。模式首先模拟了季风涡旋的东南侧增强的西南气流中出现低形变旋转性扰动,随着扰动的旋转性增强,中层出现水平尺度为200 km左右的MCV。在扰动区内的不同高度上还发现10~20 km尺度不等的中γ气旋性涡旋扰动,其中部分涡旋扰动具有热塔的特征,中γ气旋性涡旋扰动在MCV的旋转环境内不断组织化,低层气旋性涡旋扰动的分布比中层更加集中。模拟表明这些较小尺度的气旋性中尺度涡旋扰动对热带气旋的生成有重要作用。     

登陆台风内中尺度强对流系统演变机制的湿位涡分析

2005年05号台风"海棠"登陆福建后,在外围云系里有个明显发展的中尺度对流云团经过温州东部及北部地区,引起了强降水,从而造成比热带风暴环流本身更具破坏力的强烈天气,因此研究台风内中尺度对流系统(M(2S)的发展机制能够为预报台风灾害提供依据.文中使用中尺度静力模式WRF对台风"海棠"登陆过程进行了模拟,模式很好地模拟了台风登陆过程的路径、强度变化趋势和降水分布,尤其是模拟出了台风环流内的一次中尺度对流系统的发展过程,并利用模拟结果对台风环流内的这次中尺度对流系统进行了与之相关联的湿位涡分析,从而揭示了台风环流内中尺度对流系统发展演变的湿位涡特征.结果表明,在对流形成阶段,MPV1即对流不稳定为MCS的形成提供背景不稳定条件,由MPV2即湿等熵面的倾斜和水平风的垂直切变而引起的涡旋发展作为强迫机制:MCS形成的区域及东南区域中低层是强对流不稳定层,蕴含丰富的不稳定能量,倾斜上升运动把对流不稳定区具有强不稳定能量的暖湿卒气向西北中层的中性层结区输送.由于θep的减小,气旋性涡度增强,有利于形成对流,另一方面,由于湿等熵面倾斜和低空急流加强而引起的涡旋发展作为一种强迫机制激发对流不稳定能晕得到释放,从而形成对流;在对流系统的发展阶段,由于低层的对流不稳定性进一步减弱,θep一步减小,气旋性涡度进一步增强,有利于MCS的增强,中层等θe线的倾斜度比绝对动量M等值线的倾斜度大,对应有条件对称不稳定区域,满足条件对称不稳定(CSI)条件,在湿等熵面倾斜和台风低空急流作用下引起的涡旋发展强迫对称不稳定能量释放,从而使得对流得以维持和加强.通过以上的分析给出了台风环流内中尺度对流系统发生发展的概念模型.     

A simulation of heat flow in the lower troposphere by a laboratory model

A laboratory model of the troposphere has been constructed in which the heat flow above the ground in the lowest kilometer may be simulated. The scaling criteria, derived from the governing equations with a number of approximations, are 104 for length, 200 for velocity and 0.2 for temperature. Water is the simulating fluid, which moves through a closed-circuit water-tunnel. A set of electrically heated aluminum plates simulate the heating of the ground by insolation. Temperature and velocity fine structure are measured. The fluid isotherms are inferred from shadowgraph photography. All observations are confined to conditions of natural and free convection.A comparison between the temperature and velocity profiles within the prototype and the model, for identical boundary conditions, indicates that the model is a good representation of the prototype to heights of 1000 meters.It is found that the vertical transfer of warm air is along cylindrical columns, which take on different forms in natural and in free convection. In the former, the columns carry heat aloft into caps which rise at some 1.2 meter per second, or which may merge with neighbouring columns to continue upwards to heights of several hundred meters. If an elevated temperature-inversion layer is present, these convective columns perturb it, initiating horizontally-travelling waves. In free convection, the columns are inclined downwind, with their upper extremities extending almost horizontally; the cap is absent and the wave-like tail may separate and move across the fluid independently of its parent.     

一次人工增雨作业效果的中尺度数值模拟

本文根据AgI播撒影响机制,把AgI催化过程耦合到初始条件、边界条件更完善且有业务化能力的中尺度WRF模式中,并利用该模式对溧阳天目湖积云增雨过程进行中尺度催化数值模拟分析。实况资料及数值模拟分析结果表明,该模式成功地模拟了溧阳天目湖地区的对流云系,作业后催化云的强回波面积扩大,平均回波强度增加,地面降水量增加,且微物理过程分析表明,作业后催化云中高空冰晶、雪、霰冰相大粒子含量增加,相应地低空雨水含量增加,导致地面降水量增加。     

中尺度强对流云系相互作用与热带气旋形成的数值模拟

本文是热带气旋形成的多尺度组合理论的续篇它以新的方法——数值模拟的结果支持了这一理论。特别是它进一步证实了“热带大气涡旋增幅效应”的存在,也进一步定量地解释了热带气旋前期低压环流的形成。     

A modeling study on the effects of MJO and equatorial Rossby waves on tropical cyclone genesis over the western North Pacific in June 2004

This study investigates the influence of the Madden Julian oscillations (MJO) and equatorial Rossby (ER) waves on tropical cyclone (TC) formation in western Pacific during June 2004 through one control and three wave experiments for each of the five TCs. The control experiment reasonably simulates the formation of five TCs. In the corresponding wave experiments, the MJO, ER waves, and both the MJO and ER waves are removed, from the initial fields and lateral boundary conditions, respectively. The differences of simulated TC intensity between the control and corresponding wave experiments provide a quantitative assessment of the relative contribution of each wave to TC formation.In the wave experiments with the MJO removed, three of the five TCs are weakened, and the remaining two (TC A and B) grow stronger due to an altered background flow that steered the TCs into more favorable oceanic regions. For the wave experiments with ER waves removed, three of the five simulated TCs become weaker (TC A, C, and E). TC D develops into a tropical storm because of a dominant influence from active synoptic-scale disturbance. The results indicate that both the MJO and ER waves have an important modulating effect on TC formation. In addition to the influence from the MJO, ER and synoptic-scale waves, local processes may dominate in TC formation; for the example of TC B, none of the waves positively influence the formation in significant ways. The present modeling approach provides a quantitative assessment of the relative contribution of tropical wave disturbances to TC formation.     

2007年3月3—5日辽宁省暴雪和大风天气的中尺度分析

使用中尺度数值模式WRFV2.2.1对辽宁省2007年3月3-5日的暴雪和大风天气过程进行了数值模拟,结合10min一次的地面自动观测站资料和数字化多普勒天气雷达探测资料,研究了中尺度重力波的结构及其环境场特征,探讨了波动的激发机制.对流层上层中尺度重力波生成在350-250 hPa(约9-11 km),周期为2-3h,水平波长30-40 km,波动沿水平方向传播约9h.地面气压扰动振幅约为2 hPa,周期为2-3 h,波动由西南向东北方向传播,方向与地面风向相反.沿波的传播方向,地面观测的逐时降水量呈波动特征,周期约为2h.对流层上层中尺度重力波减弱后,雷达降水回波强度出现显著的波动特征.对流层上层中尺度重力波生成在朝向脊区传播的高空急流出口区下方,300 hPa环境场具有显著的切变不稳定特征.波动生成在理查逊数小于0.25的地区,在中尺度重力波生成的高度上,暖平流强,风速低,风切变大.中尺度重力波生成地区出现显著的不平衡气流,拉格朗日罗斯贝数大于0.7,水平散度倾向出现明显的大值,其中-▽w·(e)v/(e)v的量级明显大于其他各项,表明对流层上层重力波的生成及发展与环境场的显著风切变有关.     

2013年6月26—29日江西暴雨过程的中尺度数值模拟分析

利用NCEP再分析资料和WRF模式,对2013年6月26—29日江西大范围暴雨过程进行了数值模拟分析。结果表明,西太平洋副热带高压脊线稳定维持在21°N附近,副高北侧强盛西南气流将水汽向江南北部地区输送是暴雨产生和稳定维持的主要原因。超低空偏南急流的建立、发展和维持是这次连续暴雨过程产生的一个重要因素,同时低空低涡南侧出现一串近似东西向排列的30～60km更小尺度的强对流系统,它们与大暴雨区相吻合;整层水汽通量密集区的南北界位置和暴雨区南北界位置基本吻合,整层水汽的大值中心的范围和大暴雨中心的范围具有明显的正相关关系;水汽通量散度最大辐合中心为暴雨的产生输送了大量的水汽,水汽辐合中心与暴雨的落区有很好的一致性;强降水落区与假相当位温最大值区相对应。     

东亚地区南北两个风暴源地中风暴的结构和动力学差异

基于欧洲中心中期天气预报再分析资料(ERA-40),使用涡旋追踪和合成技术、多尺度子空间变换以及局地多尺度能量分析方法研究了东亚地区南北两个风暴源地中风暴的差异。结果发现,南、北两个源地风暴在结构上和内部动力过程上均存在着显著不同。南支源地(40°N以南)风暴底层比高层强,与线性斜压模式中的最不稳定模态结构相似;而北支源地(40°N以北)风暴则正好相反,与下游发展理论所描述的斜压波结构相似。并且发现,南支源地风暴的非地转风场比北支源地风暴的强。能量学诊断结果显示,南支源地风暴的能量源除了斜压不稳定外,有很大一部分来自正压不稳定,而北支源地风暴中则是存在弱的动能逆尺度传输。此外,南支源地风暴的浮力转换和非绝热做功均比北支源地风暴的强,其主要原因是南支源地风暴的垂直运动更强,风暴中的水汽更加充足。      

Controlling the open ocean forcing of baroclinic coastal jets with a 4D-var method

An optimal control approach was carried out to investigate the influence of perturbations of baroclinic coastal jets which are located upstream of the observation site. Two academic numerical experiments were considered: the intrusion of the buoyancy anomaly within an established density current along a straight coast (experiments A), and the interaction of a coastal current with a shelf topography (experiments B). We explored the sensitivity of the forcing retrieval to the spatial distribution of observations (experiments A) and to the temporal sampling of the forcing (experiments B), using a 4D-var formalism. The sensitivity of numerical simulations is discussed in the framework of analytical solutions. The results provide insight into how observational network and data assimilation method would impact the realistic modelling of coastal jets and the shelf slope exchanges.     

基于CWRF的中国近海热带气旋集合模拟及其对物理参数化方案的敏感性研究

为评估CWRF模式的降尺度能力和其热带气旋模拟对物理参数化方案的敏感性,本文利用ERI再分析资料驱动CWRF在30km网格上对1982-2016年中国近海热带气旋开展了一次集合模拟.结果表明:CWRF与ERI均能模拟出热带气旋的季节变化和年际变化形势且均存在低估,但相较ERI,CWRF的降尺度技术和集合模拟可以再现更多...     

北京\

使用常规观测资料、卫星云图、雷达回波资料、自动气象站降水量以及0.25°×0.25°的NCEP/NCAR再分析资料,对2017年8月1日发生在黑龙江南部的暖区暴雨过程的中尺度特征及成因进行了分析。结果表明：暴雨发生在副高加强西伸北抬及有台风活动的背景下,副高外围的水汽输送为暴雨提供了充沛的水汽条件;低层西南风的增大导致暖锋锋生,暖锋的辐合抬升作用加强,造成较大范围的暴雨天气;锋生区附近存在CSI,锋生作用及CSI的释放,加强了沿着锋面倾斜向上的斜升气流及锋面次级环流,CSI导致的斜升气流的发展进一步触发对流不稳定,导致大范围的垂直上升运动,降水显著加强;暖锋云带内部探空分析显示大气处于不稳定状态,有利于以短时强降水为主的对流发展。暴雨是由云团的后向传播造成的,强降水以暖云降水为主,降水效率高,雨强大,暖锋稳定少动,由暖锋锋生所致的对流单体在同一区域重复新生,并沿暖锋自西向东传播,形成列车效应,暴雨中心一直有最大反射率因子超过45 dBz且降水效率高的强回波活动,持续时间超过4 h,导致强降水持续时间长,降水累积量大。

     

Growing interfacially coupled oscillations of the ocean–atmosphere

A stability analysis of the coupled ocean–atmosphere is presented which shows that the potential energy (PE) of the upper layer of the ocean is available to generate coupled growing planetary waves. An independent analysis suggests that the growth of these waves would be maintained in the presence of oceanic friction. The growing waves are a consequence of relaxing the rigid lid approximation on the ocean, thus allowing an upward transfer of energy across the sea surface. Using a two and a half layer model consisting of an atmospheric planetary boundary layer, coupled with a two layer ocean comprising an active upper layer and a lower layer in which the velocity perturbation is vanishingly small, it is shown that coupled unstable waves are generated, which extract PE from the main thermocline. The instability analysis is an extension of earlier work [Tellus 44A (1992) 67], which considered the coupled instability of an atmospheric planetary boundary layer coupled with an oceanic mixed layer, in which unstable waves were generated which extract PE from the seasonal thermocline. The unstable wave is an atmospheric divergent barotropic Rossby wave, which is steered by the zonal wind velocity, and has a wavelength of about 6000 km, and propagates eastward at the speed of the deep ocean current. It is argued that this instability, which has a multidecadal growth time constant, may be generated in the Southern Ocean, and that its properties are similar to observations of the Antarctic Circumpolar Wave (ACW).     

青藏高原上中尺度对流系统(MCS)的数值模拟

A mesoscale convective system (MCS) developing over the Qinghai-Xizang Plateau on 26 July 1995 issimulated using the fifth version of the Penn State-NCAR nonhydrostatic mesoscale model (MM5). Theresults obtained are inspiring and are as follows. (1) The model simulates well the largescale conditionsin which the MCS concerned is embedded, which are the well-known anticyclonic Qinghai-Xizang PlateauHigh in the upper layers and the strong thermal forcing in the lower layers. In particular, the modelcaptures the meso-α scale cyclonic vortex associated with the MCS, which can be analyzed in the 500 hPaobservational winds; and to some degree, the model reproduces even its meso-β scale substructure similarto satellite images, reflected in the model-simulated 400 hPa rainwater. On the other hand, there aresome distinct deficiencies in the simulation; for example, the simulated MCS occurs with a lag of 3 hoursand a westward deviation of 3-5° longitude. (2) The structure and evolution of the meso-α scale vortexassociated with the MCS are undescribable for upper-air sounding data. The vortex is confined to thelower troposphere under 450 hPa over the plateau and shrinks its extent with height, with a diameter of4° longitude at 500 hPa. It is within the updraft area, but with an upper-level anticyclone and downdraftover it. The vortex originates over the plateau, and does not form until the mature stage of the MCS. Itlasts for 3-6 hours. In its processes of both formation and decay, the change in geopotential height fieldis prior to that in the wind field. It follows that the vortex is closely associated with the thermal effectsover the plateau. (3) A series of sensitivity experiments are conducted to investigate the impact of varioussurface thermal forcings and other physical processes on the MCS over the plateau. The results indicatethat under the background conditions of the upper-level Qinghai-Xizang High, the MCS involved is mainlydominated by the low-level thermal forcing. The simulation described here is a good indication that itmay be possible to reproduce the MCS over the plateau under certain large-scale conditions and with theincorporation of proper thermal physics in the lower layers.