The behavior of magneto-acoustic-gravity waves near the cusp resonance in a lossless,compressible, isothermal,stratified, electrically conducting and uniformly magnetized atmosphere. II: The valve effect

Abstract

In the present paper the behavior of (internal) magneto-acoustic-gravity waves near the cusp resonance in a lossless, compressible, isothermal, stratified, electrically conducting atmosphere that is permeated by a uniform, nearly horizontal magnetic field is re-addressed (Kamp, 1989). The previously analyzed linear conversion of long acoustic-gravity waves into short magneto-acoustic waves that carry off the energy from the resonance region along the magnetic field, is re-analyzed with boundary layer techniques that are based on the smallness of the vertical component of the magnetic field. More specifically the existence of the so-called valve effect for the generated magneto-acoustic mode near the critical level is explicitly demonstrated and shown to be governed by two rivalling effects.     

Jet instability and the stabilizing effect of topography on jets in two-layer rotating systems

Abstract

Laboratory experiments concerning azimuthal jets in two-layer rotating systems in the absence and presence of bottom topography aligned along the jets have been conducted. The jets were forced by the selective withdrawal of fluid from the upper layer of a two-fluid system contained in a circular dishpan geometry. The principal parameters measured in the experiments were the jet Rossby number, Ro, and a stratification parameter F = r ₁/(λ₁λ₂)^1/2 where r ₁ is the radius of the circular disc used for the selective withdrawal (i.e., r ₁ is the approximate radius of curvature of the jet) and λ₁,λ₂ are the internal Rossby radii of deformation in the upper and lower fluids, respectively.

The no-topography experiments show that for a sufficiently small F, the particular value depending on Ro, the jet is stable for the duration of the experiment. For sufficiently large F, again as a function of Ro, the jet becomes unstable, exhibiting horizontal wave disturbances from modes three to seven. An Ro against F flow regime diagram is presented.

Experiments are then conducted in the presence of a bottom topography having constant cross-section and extending around a mid-radius of the dishpan. The axis of the topography is in the vicinity of the jet axis forced in the no-topography experiments and the crest of the topography is in the vicinity of the interface between the two fluids (i.e., the front associated with the jet). The experiments show that in all cases investigated the jet tends to be stabilized by the bottom topography. Experiments with the topography in place, but with the interface between the fluids being above the topography crest, are shown to be unstable but more irregular than their no-topography counterparts.

Various quantitative measurements of the jet are presented. It is shown, for example, that the jet Rossby number defined in terms of the fluid withdrawal rate from the tank. Q, can be well correlated with a dimensionless vorticity gradient, V_G , across the upper layer jet. This allows for an assessment of the stability characteristics of a jet based on a knowledge of V_G (which can be estimated given a jet profile) and F.     

Theoretical and experimental study of breaking wave on sloping bottoms

This paper studies the continuous evolution of breaking wave for the surface water waves propagating on a sloping beach. A Lagrangian asymptotic solution is derived. According to the solution coupled with the wave breaking criteria and the equations of water particles motion, the wave deformation and the continuous wave breaking processes for the progressive water waves propagating on a sloping bottom can be derived. A series of experiments are also conducted to compare with the theoretical solution. The results show that the present solution can reasonably describe the plunging or spilling wave breaking phenomenon.     

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。     

强厄尔尼诺事件下2016年1月中国南方超级寒潮的动力学机制:瞬变波对大气长波异常的调制

本文采用Plumb三维波活动通量和局地Eliassen-Palm通量诊断方法,利用欧洲中期天气预报中心的逐日再分析资料ERA-Interim,分析了超强厄尔尼诺背景下2016年1月下旬中国南方超级寒潮的动力学机制：瞬变波对大气长波异常的调制。前期2015年12月的北大西洋海表热通量正异常,有助于后期大西洋阻塞形势的维持。大气长波能量沿大圆路径从大西洋阻高经乌拉尔地区向东亚中低纬度传播并在此辐合,导致了乌拉尔阻高和华北横槽的经向结构,更多强冷空气聚集在异常偏南的纬向槽线附近。寒潮爆发前夕,2支瞬变波列活跃在亚欧大陆。北支瞬变波列调制了北方的大气长波,使横槽转竖;南支瞬变波列协同调控了南方的大气长波,使南支印缅槽减弱;两者共同作用,促使冷平流大举南下,直达华南沿海,南方寒潮发生。     

飞机下投探空在台风探测中的应用

在海洋探测方面,利用飞机进行下投探空拥有巨大的潜力去弥补全球高空探测网的空白。台风形成后,飞机飞入台风中心或绕台风飞行,下投配有全球卫星定位(GPS)装置的"探空仪",探空仪上安装有温度、气压、湿度传感器,通过GPS定位测风,对台风实施立体观测。多年来,飞机下投探空能够充分掌握台风的整体气象要素分布结构,从而提高对台风强度和移动路径等预报的准确性,飞机下投探空在国际上已经从科学试验转变成业务应用,并且形成新型的探测技术——目标观测。本文主要总结了国际上利用飞机搭载下投式探空仪进行台风探测的发展历程,为我国开展下投探空,特别是台风监测提供可借鉴的经验。     

Analysis of wave groups by wave envelope-phase and the Hilbert Huang transform methods

The wave groups are studied by both conventional wave analysis methods and by the non-stationary Hilbert Huang Transform (HHT) method. Full-scale wave records containing abnormal waves are used. Instantaneous quantities, such as envelope, phase and frequency, are adopted to study the wave grouping. A refined definition of wave group is proposed considering that the wave process is simultaneously amplitude and frequency modulated. The validation of the proposed definition is conducted by analysis of numerical simulation data. Group parameters are proposed based on the time-frequency distribution of energy. An attempt is made to find the relationship between the characteristics of abnormal waves and the group characteristics.     

Interaction of cnoidal waves with an array of vertical concentric porous cylinders

An array of large concentric porous cylinder arrays is mounted in shallow water exposed to cnoidal waves. The interactions between waves and cylinders are studied theoretically using an eigenfunction expansion approach. Semi-analytical solutions of hydrodynamic loads and wave run-up on each cylinder are obtained using first approximation to cnoidal waves. The square array configuration of four-legged identical concentric porous cylinder is investigated in present study. Numerical results reveal the variation of dimensionless wave force and wave run-up on individual cylinder with angle of incidence, porosity parameter, spacing between outer and inner cylinders, spacing between concentric porous cylinders and wave parameter. Different mechanism of wave force is found under different range of scattering parameter.