斜压涡度的变化与台风暴雨的关系研究

台风暴雨作为台风引起的最主要灾害之一,一直被人们关注。台风常被认为是对称结构,但从实际状况来看台风的非对称性非常明显,所以有必要研究斜压性涡度在台风中的表现。在高分辨率数值模拟的基础上,通过引入斜压涡度的概念,分析和总结了斜压涡度在2009年台风“莫拉克”暴雨过程中的表现。通过模拟与分析得到如下的结果:斜压涡度和MPV对比,可以看出在登陆前和登陆后,明显低层斜压涡度有更强的异常信号,围绕台风内核呈现正负正的位相特点;从沿着台风中心时间剖面可以看出,登陆前斜压涡度低层多为负正负的位相,并且随着时间的推移,斜压涡度有从大气的高层向台风的移动中心传递的趋势,即在台风即将到达时原先的正涡度被替换为负涡度,所以对其移动有一定指示意义;在台风“莫拉克”过台湾岛时,其斜压涡度表现为负涡度消失,在山地附近有正涡度生成,完成过岛,台风中心被替换;斜压涡度的异常值主要位于大气的低层时,一般会产生较强的降水。      

Three－dimensional Global Scale Permanent－wave Solutions of the Nonlinear Quasigeostrophic Potential Vorticity Equation and Energy Dispersion

The three-dimensional nonlinear quasi-geostrophic potential vorticity equation is reduced to a linear form in the stream function in spherical coordinates for the permanent wave solutions consisting of zonal wavenumbers from 0 to n and rn vertical components with a given degree n. This equation is solved by treating the coefficient of the Coriolis parameter square in the equation as the eigenvalue both for sinusoidal and hyperbolic variations in vertical direction. It is found that these solutions can represent the observed long term flow patterns at the surface and aloft over the globe closely. In addition, the sinusoidal vertical solutions with large eigenvalue G are trapped in low latitude, and the scales of these trapped modes are longer than 10 deg. lat. even for the top layer of the ocean and hence they are much larger than that given by the equatorial β-plane solutions. Therefore such baroclinic disturb-ances in the ocean can easily interact with those in the atmosphere.Solutions of the shallow water potential vorticity equation are treated in a similar manner but with the effective depth H = RT / g taken as limited within a small range for the atmosphere.The propagation of the flow energy of the wave packet consisting of more than one degree is found to be along the great circle around the globe both for barotropic and for baroclinic flows in the atmosphere.     

关于非纬向基流的特性及其扰动的传播

本文在正压无辐散的条件下,讨论了非纬向基流的特性及其约束条件,指出低纬大气一些周期振荡是非纬向基流在β效应作用下的固有振荡。其次,从β平面上的正压涡度方程出发,在一定条件下导出了非纬向流扰动波速表达式。最后用实例加以验证。      

华南涡旋暴雨的物理对比分析

低涡型暴雨是华南地区一种重要的暴雨类型,它常常发生在对流层低层天气尺度切变线上,虽难从气压场上找到,但它所造成的暴雨却很强。我们发现,常规天气图上似乎很弱的这类涡旋,却有较强的流场涡度和典型的散度分布,构成了强降水过程的必要物理条件。本文把它与热带气旋发展过程中的物理结构、能量变化和转换特征进行了诊断、对比分析,指出它们不仅三维物理量场分布差异较大,其发展过程中能量变化也有很大差异。热带气旋的K,值远大于切变线低涡,但K_φ的变化则相反,无论是其绝对值或它占总动能的比值,都不      

湿位涡和倾斜涡度发展

从完整的原始方程出发，在导出精确形式的湿位涡方程的基础上，证得绝热无摩擦的饱和湿空气具有湿位涡守恒的特性。并由此去研究湿斜压过程中涡旋垂直涡度的发展。结果表明，在湿等熵坐标中，涡旋的发展与对流稳定度的减少，等熵面上的辐合和潜热的释放有关。由于等熵位涡分析的应用受等熵面倾斜的限制，又进而发展了Ｚ坐标及Ｐ坐标中的倾斜涡度发展理论。指出无论是湿对称不稳定或对流不稳定大气，还是湿对称稳定或对流稳定大气，除对流稳定度的影响外，风的垂直切变的增加或水平湿斜压的增加均能因湿等熵面的倾斜而引起垂直涡度的增长。湿等熵面的倾斜越大，这种由干湿斜压性加强所引起的涡旋发展更激烈。在梅雨锋附近及其南侧暖湿区的北端，湿等熵面十分陡立，是涡旋发展及暴雨发生的重要地区。对１９９１年６月１２—１５日江淮流域暴雨过程的湿位涡分析表明，湿位涡分析，尤其是等压面上湿位涡量ＭＰＶ１和ＭＰＶ２的分析不仅在中高纬有效，在低纬度及低对流层也十分有效，是暴雨诊断和预报的有力工具。     

The effects of friction and heating of convective condensation in the baroclinic instability problem

By using the β-plane, two-layer quasi-geostrophic baroclinic model, this paper discusses the baroclinic in-stability problem concerning the effects of friction and heating of convective condensation. By Linear analysis it is shown that the combination of β effect, friction and convective heating brings about the asymmetric phenom-enon of margin curves. The convective heating plays a role in the increased baroclinic instability. As the heating increases (m*→1), the short wave cutoff can increase infinitely. Besides, the numerical integration of the finite-amplitude equations shows that the trajectory on the phase plane oscillates periodically in the case of non-dissipation. When the friction dissipation is considered, the trajectory of phase decays and oscillates to the equilibrium. The stronger convective heating not only makes the unstable wave length shorter and the amplitude of the equilibrium decrease, but also makes multiple equilibrium into single equilibrium.     

CONSERVATION OF MOIST POTENTIAL VORTICITY AND DOWN-SLIDING SLANTWISE VORTICITY DEVELOPMENT*

An accurate form of the moist potential vorticity(MPV) equation was deduced from a complete set of primitive equations.It was shown that motion in a saturated atmosphere without diabatic heating and friction conserves moist potential vorticity.This property was then used to investigate the development of vertical vorticity in moist baroclinic processes.Results show that in the framework of moist isentropic coordinate,vorticity development can result from reduction of convective stability,or convergence,or latent heat release at isentropic surfaces.However,the application of the usual analysis of moist isentropic potential vorticity is limited due to the declination of moist isentropic surfaces.and a theory of development based on z-coordinate and p-coordinate was then proposed.According to this theory,whether the atmosphere is moist-symmetrically stable or unstable,on convective stable or unstable,the reduction of convective stability,the increase of the vertical shear of horizontal wind or moist baroclinity may result in the increase of vertical vorticity,so long as the moist isentropic surface is slantwise.The larger the declination of the moist isentropic surface,the more vigorous the development of vertical vorticity.In a region with a monsoon front to the north and the warm and moist air to the south,or by the north of the front,the moist isentropes are very steep.The is the region most favorable for development of vorticities and formation of torrential rain.For a case of persistent torrential rain occurring in the middle and lower reaches of the Changjiang and Huaihe Rivers in June 11-15,1991,moist potential vorticity analysis,especially the isobaric analysis of its vertical and horizontal components,i.e.MPV1 and MPV2,respectively,is effective for identifying synoptic systems not only in middle and high latitudes,but also in low latitudes and in the lower troposphere.It can serve as a powerful tool for the diagnosis and prediction of torrential rain.     

MOIST POTENTIAL VORTICITY DIAGNOSIS OF A MEIYU FRONTAL HEAVY RAIN PROCESS SIMULATION DURING SUMMER,1991

In this paper,the heavy rain process from June 30 to July 2,1991,has been simulated by MM4.and three-dimensional moist potential vorticity distribution of the simulation results has beencalculated.It is shown that moist potential vorticity is an important physical variable to reveal heavyrain structure and dynamic mechanisms.Negative moist potential vorticity corresponds to the Meiyufront-wind shear line system and the negative center corresponds to the heavy rain center.Negativemoist potential vorticity mainly attributes to the effects of meridional baroclinic term and convectiveunstable term.The former is favourable to the maintenance of zonal precipitation and the latter is themechanism of the heavy rain center propagating along the rain belt.The heavy rain is contributed bythe cooperative effects of conditional convective instability,baroclinic instability and upper air inertialinstability.     

MOIST POTENTIAL VORTICITY DIAGNOSIS OF A MEIYU FRONTAL HEAVY RAIN PROCESS SIMULATION DURING SUMMER, 1991

In this paper,the heavy rain process from June 30 to July 2,1991,has been simulated by MM4.and three-dimensional moist potential vorticity distribution of the simulation results has been calculated.It is shown that moist potential vorticity is an important physical variable to reveal heavy rain structure and dynamic mechanisms.Negative moist potential vorticity corresponds to the Meiyu front-wind shear line system and the negative center corresponds to the heavy rain center.Negative moist potential vorticity mainly attributes to the effects of meridional baroclinic term and convective unstable term.The former is favourable to the maintenance of zonal precipitation and the latter is the mechanism of the heavy rain center propagating along the rain belt.The heavy rain is contributed by the cooperative effects of conditional convective instability,baroclinic instability and upper air inertial instability.     

The effect of aspect ratio on the bifurcation properties of a double parallel-connection Lorenz system

A double parallel-connection (DPC) Lorenz system is developed by performing spectrum truncation of the Galerkin series expansion of the two-dimensional Rayleigh-Benard convection equation. Analyses of the equilibrium states indicate that a convective roll stems from a flow with a given wavenumber first losing its stability for a particular aspect ratioβ after a stable laminar flow gets unstable; whenβ has the valueβ _c able to deprive synchronously two flows with different wavenumbers of stability, occurrences of convective rolls with different wavenumbers depend entirely on the initial conditions, in good agreement with the relevant experimental results. The calculations of the unstablized rolls show that, with a smallerβ (as compared withβ _c), the DPC Lorenz system has the same bifurcation properties as the ordinary Lorenz system; for a moderateβ, the system has very complicated periodic, quasi-periodic and phase-locking motions; for a largerβ, it results in intermittent chaos and causes mean flows with different numbers of vortices to occur alternately with time. All these indicate thatβ has substantial effect on the two Lorenz systems coupled through parallel connection in their interaction and the results.     

The stability of large—scale horizontal air motion in the non—linear basic zephyr flow under the effect of rossby parameter

The stability of large-scale horizontal motion in the atmosphere is discussed in this paper by using qualitative analysis theory of non-linear ordinary differential equations. Both the non-linear distribution of basic Zephyr flow and the variation of geostrophic vorticity along the latitude (f=f0 + βδy ) are all included in this paper's mathematical model so as to analogue the background field of large-scale horizontal air motion more really in the rotating reference frame of the earth. Some significant results are drawn out from this paper and the conclusions of Li(1986)'s and Wan et al.(1990)'s are extended widely.     

双台风“纳沙”和“海棠”登陆后强度变化成因及对比分析

利用NCEP（National Centers for Environmental Prediction）再分析资料对2017年双台风“纳沙”（1709）和“海棠”（1710）先后登陆福建后的强度变化特征进行对比分析。结果表明：低纬水汽输送的强弱变化是造成双台风强度变化的主要原因,而越赤道气流对水汽输送起到关键作用。副热带西风急流与中低空风场强弱变化一致,西风急流增强伴随中低空东风加强是“海棠”陆上强度维持不减的原因之一,反之“纳沙”快速减弱。双台风效应对强度变化也起着重要作用：双台风靠近相吸作用下,“纳沙”残余环流卷入“海棠”环流中,为其输送正涡度因子及斜压能量,也是促使“海棠”强度维持的原因。台风登陆后,其上空三种不同层差的垂直风切变值表现一致减小的变化趋势及低于6 m·s^-1的风切,有利于弱台“海棠”陆上长久维持。海岸锋生及海表通量动热力因子对“海棠”右侧中小尺度对流发展和维持起到重要作用,加剧了“海棠”非对称性结构,促使“海棠”强度维持不减。     

冬季蒙古反气旋和伊朗反气旋的年代际变化特征

根据NCEP/NCAR再分析资料,采用客观判定和追踪方法,研究了1948~2013年欧亚地区冬季温带反气旋的年代际气候变化的活动特征。结果发现,反气旋的高频分布区也是反气旋气候变化最大的区域,其中蒙古高原和伊朗高原的反气旋最活跃。反气旋的频数和强度既有长期趋势也有年代际变化。蒙古高原和伊朗高原的反气旋频数具有明显的年代际变化特征。反气旋频数具有2~6年和16~30年周期,且具有变频特征。EOF分解发现蒙古高原和伊朗高原的反气旋频数分布均在较高纬度和较低纬度地区呈现显著相反的偶子极态分布形式。蒙古高原的反气旋强度的变化基本可以体现欧亚大陆反气旋强度的变化。反气旋分布和强度的年代际变化可以用对流层低层经向温度梯度表示的斜压锋的位置和强度的年代际变化来解释,但斜压锋对欧亚反气旋的影响具有区域性。蒙古高原的反气旋在1960~1975年50°N以北较多,1990~2005年50°N以南较多的偶极子态变化与80°~120°E区域的斜压锋纬度位置自55°N南移到45°N有密切关系,30°~80°E区域的斜压锋纬度位置变化不能单独解释伊朗高原反气旋偶极子态年代际变化。自21世纪00年代中期斜压锋偏强对反气旋强度偏强有重要影响。     

The development and initial tests of an atmospheric model based on a vertical coordinate with a smooth transition from terrain following to isentropic coordinates

An atmospheric model (η model) is developed by modifying the UW θ－σ hybrid model. In the η model, the vertical coordinate transforms smoothly from terrain following to isentropic coordinates. The model is developed to capitalize on the inherent advantage of numerical modeling in isentropic coordinates and to eliminate the interface between the sigma planetary boundary layer and isentropic free atmosphere present in the UW θ－σ model. This formulation provides the potential for the data assimilation and the application of higher order schemes. This paper describes the structure of the η model and presents results from initial numerical experiments. The first experiment tests the capability of the η model for simulating the baroclinic development process. In the 48-hr numerical weather forecast experiment, the η model produces reasonable precipitation and synoptic fields at all levels which are similar to those from the UW θ-σ model. The second and third experiments test the capability of the η model for conserving 1) the joint distribution of isentropic potential vorticity (IPV) and proxy ozone and 2) equivalent potential temperature under frictionless and isentropic conditions. These experiments show that distributions of IPV and proxy ozone in the pure isentropic domain and the distributions of prognostic and diagnostic equivalent potential temperature in the model domain remain highly correlated to day 10.     

Discussion on the complete-form vorticity equation and slantwise vorticity development

The complete form of the vertical vorticity tendency equation (the complete-form vorticity equation) is derived from the Ertel potential vorticity equation to contain thermodynamic factors. In this study, a new complete-form vorticity equation, which has the same form as the original complete-form vorticity equation, is deduced from the absolute vorticity vector equation combined with the continuity equation and the expression of three-dimensional (3D) entropy gradient. By comparing the complete-form vorticity equation with the classical vertical vorticity equation, it is found that regardless of whether or not the isentropic surface is tilting, the two vorticity equations are in essence the same. The “baroclinic term” of the complete-form vorticity equation is exactly equal to the solenoidal term of the classical one, and there is a significant amount of cancellation between the two baroclinic items (the “slantwise term” and the horizontal vorticity change term) in the complete-form vorticity equation. In operational weather analysis, the tilt of the isentropic surface can be diagnosed according to the density of the isotherm on the upper-level isobaric map. For synoptic-scale motion, the vertical vorticity produced by the tilt of the isentropic surface is due to the contribution of atmospheric baroclinicity, which is measured by the solenoid. The 3D solenoid is parallel to the isentropic surface, so the more tilted the isentropic surface, the bigger the projection of the 3D solenoid in the vertical direction. The baroclinic contribution can be interpreted based on the PV thinking theory, but the relationship between the vorticity field and the potential vorticity field is not immediate.     

A STUDY ON THE EXCITATION, ESTABLISHMENT AND TRANSITION OF MULTIPLE EQUILIBRIUM STATES PRODUCED BY NEARLY RESONANT THERMAL FORCING---- PART I: ASYMPTOTIC SOLUTIONS OF MULTIPLE EQUILIBRIUM STATES

A two-layer quasi-geostrophic baroclinic model in a narrow, longitudinally periodic channel on a β-plane is used, which involves near-resonant thermal forcing, frictional dissipation and a uniformly sheared basic current. By means of the multi-scale technique, a system of simplified differential equations or disturbances due to the steady thermal-forcing waves projected on the X-T plane is derived, which contains the nonlinear interactions between forced waves and free waves. The asymptotic solutions of the equilibrium states of these equations are analytically obtained by a singular perturbation method. The results show that these mul-tiple equilibrium state (MES) solutions can exist in a wide range of the parameters used.     

Surface and atmospheric controls on the heating coefficient

Measurements of the surface radiation budgets for three surfaces—grass, soil and a cornfield—are used to evaluate the ‘heating coefficient’β, and its componentsβ _↑(=dL _↑/dR _n ) andβ _↓(=dL _↓/dR _n ). This resolution permits an analysis of the sensitivity of β to surface and atmospheric influences.β _↑ is shown, both theoretically and empirically, to be determined by surface properties. For grass and soil, the parameter functions as an index of surface desiccation.β _↓ values are large (even under clear conditions) and variable, accounting for part of the variance in β and the anomalously small and negative values reported in the literature.β _↓ values for cloudy conditions may be larger or smaller than those for clear skies. It is concluded that, unless a predictive procedure can be developed forβ _↓, the Monteith and Szeicz model is of limited use for the routine estimation of net radiation.     

鄂霍茨克海阻高发展过程的正斜压涡度和涡度拟能的演变特征

用正斜压分解法，对1998年发生在鄂霍茨克海的阻高进行涡度和涡度拟以分析，结果发现：在阻高的不同阶段，正压涡度和正压涡度拟能远大于斜压涡度和斜压涡度拟能且在不同的阶段有着不同的演变过程；在阻塞的酝酿阶段正压和斜压涡度和涡度拟能不稳定增长，且向西南传播；在维持阶段，正压和斜压涡度和涡度拟能的增长达到最大值，阻塞形势建立并维持在阻塞区域内；在消亡阶段，正压涡度和涡度拟能与斜压涡度拟能则呈现出不稳定减少，向东南传播，阻塞崩溃，然后又增长。     

非绝热加热对热带气旋非对称结构影响的数值试验

利用含非绝热加热强迫的正压涡度方程。将非绝热加热作适当的参数化处理。对初始对称 热带气旋作了一系列数值试验,结果表明：不仅β项、平流项在热带气旋非对称结构的形成中有重要作用,而且非绝热加热对热带气旋的非对称结构亦有重要影响,从而验证了非绝热加热是热带气旋非对称结构形成的一种可能机制的结论。     

The influence of the coast on the dynamics of upwelling fronts: Part I. Laboratory experiments

We describe laboratory experiments on the instability and later evolution of a front in a two-layer rotating fluid. In particular, we focus on the influence of a nearby boundary on instability growth and eddy formation. The front is produced through the adjustment of a buoyant fluid initially confined within a bottomless cylinder. Typically a front in quasi-cyclostrophic balance establishes after two rotation periods, after which it becomes unstable. Measurements of the velocity and vorticity fields at the surface are made which provide detailed information on the evolution of the front as the instability grows to finite amplitude. We focus on the time evolution of the vorticity and distinguish between the cyclonic and anticyclonic components. The spatial averages of the cyclonic and anticyclonic vorticity first grow exponentially. This growth saturates when eddies form and are advected across the front. The growth rate depends upon two nondimensional parameters: the width W of the upwelling region in units of the internal radius of deformation and the depth ratio δ between the two layers. Measurements of the growth rates for the average of the cyclonic and anticyclonic vorticity are compared to the values inferred from a simplified model for baroclinic instability. A good agreement is obtained when the front develops far from the boundary (i.e. W1). However, the agreement is only qualitative when the front is near the boundary (i.e. W1). We find that, as W decreases, the growth of cyclonic eddies consisting of dense—“coastal”—water is enhanced compared to that of anticyclonic vorticity consisting of buoyant—“off-shore”—water. This crucial effect of the boundary with respect to the instability of the front has significant impact on exchanges across the front.