A STUDY ON MESOSCALE VORTICITY PROPAGATION IN A TYPHOON-LIKE VORTEX*

By using an f-plane barotropic quasi geostrophic model in the rectangular coordinates with a grid spacing of 5 km,ten experiments whose integration time is 36 hours are performed in order to study the interaction between a typhoon vortex and a mesoscale vortex whose initial center position is located at 2 r_m northwest to the typhoon center,where r_m is the radius of maximum wind of the typhoon vortex. Results show that the interaction can create a pair of smaller scale vortices or lumps,which extend from the outside region of the typhoon to near its center,resulting in the inward propagation of mesoscale vorticity.In this process,the vorticity concentration of the mesoscale vortex may appear.The coexistence of the propagation and the concentration makes the increase of vorticity in the inside region i.e.a more intensive typhoon.Meanwhile,the intensity of the lump with positive vorticity oscillates with time,with the oscillation period being several hours,the distance from the typhoon center to the lump center also has a similar oscillation period,which reduces the oscillation of typhoon intensity.In the case of stronger circular basic flow,the interaction can make the intensification of typhoon more obviously. In addition,in some parametric conditions,the interaction may break down the continuous vorticity zone,exhibiting a cluster of smaller vorticity lumps.     

An experimental study of global instabilities due to the tidal (elliptical) distortion of a rotating elastic cylinder

Abstract

Broad band secondary instability of elliptical vortex motion has been proposed as a principal source of shear-flow turbulence. Here experiments on such instability in an elliptical flow with no shear boundary layer are described. This is made possible by the mechanical distortion in the laboratory frame of a rotating fluid-filled elastic cylinder. One percent ellipticity of a 10 cm diameter cylinder rotating once each second can give rise to an exponentially-growing mode stationary in the laboratory frame. In first order this mode is a sub-harmonic parametric Faraday instability. The finite-amplitude equations represent angular momentum transfer on an inertial time scale due to Reynolds stresses. The growth of this mode is not limited by boundary friction but by detuning and centrifugal stabilization. On average, a generalized Richardson number achieves a marginal value through much of the evolved flow. However, the characteristic flow is intermittent with the cycle: rapid growth, stabilizing momentum transfer from the mean flow, interior re-spin up, and then again. Data is presented in which, at large Reynolds numbers, seven percent ellipticity causes a fifty percent reduction in the kinetic energy of the rotating fluid. In the geophysical setting, this tidal instability in the earth's interior could be inhibited by sub-adiabatic temperature gradients. A near adiabatic region greater than 10 km in height would permit the growth of tidally destabilized modes and the release of energy to three-dimensional disturbances. Such disturbances might play a central role in the geodynamo and add significantly to overall tidal dissipation.     

热带气旋边界层精细结构研究综述

热带气旋边界层中包含许多小尺度环流系统,其精细结构与台风灾害紧密联系。随着观测资料的丰富、理论研究的深入、计算能力的提升及国家防台减灾的迫切需要,近20 a热带气旋边界层精细结构的研究日益受到国内外学者的重视,并取得了显著进展,目前该方向成为当前热带气旋研究的热点领域之一。本文对近20 a热带气旋边界层精细结构的研究从观测和模拟两方面进行了回顾,主要集中在近年来发现的热带气旋边界层滚涡和龙卷尺度涡旋,并对热带气旋边界层精细结构的研究进行展望。     

Convective dynamos with intermediate and strong fields

Abstract

The weak-field Benard-type dynamo treated by Soward is considered here at higher levels of the induced magnetic field. Two sources of instability are found to occur in the intermediate field regime M ～ T ^1/12, where M and T are the Hartmann and Taylor numbers. On the time scale of magnetic diffusion, solutions may blow up in finite time owing to destabilization of the convection by the magnetic field. On a faster time scale a dynamic instability related to MAC-wave instability can also occur. It is therefore concluded that the asymptotic structure of this dynamo is unstable to virtual increases in the magnetic field energy.

In an attempt to model stabilization of the dynamo in a strong-field regime we consider two approximations. In the first, a truncated expansion in three-dimensional plane waves is studied numerically. A second approach utilizes an ad hoc set of ordinary differential equations which contains many of the features of convection dynamos at all field energies. Both of these models exhibit temporal intermittency of the dynamo effect.     

The evolution of sub-mesoscale,coherent vortices on the β-plane

Abstract

An investigation is made of the evolution of small-scale, axisymmetric vortices in a stratified fluid with spatially variable Coriolis parameter. The criteria for smadness are a horizontal scale less than or equal to the first internal radius of deformation and a vertical scale less than or equal to that of the ambient stratification. These circumstances match those of Sub-mesoscale, Coherent Vortices frequently observed in the oceans. The dynamical model is the balance equations, which include various effects of finite Rossby number. The principal topics are the regime of nearly uniform propagation, the development of an equilibrium ratio of vertical and horizontal scales (i.e., Burger number selection), and the occurrence of various types of instability for vortices with extremes in amplitude or shape.     

Modons and monopoles on a γ-plane

Abstract

The gamma plane approximation introduced in this study corresponds to a nonlinear horizontal shallow flow in a plane where, in addition to the familiar linear variation of f (i.e., β), there is a quadratic variation with latitude. Such a plane may have some application to the mesoscale oceanic flow in the immediate vicinity of the North Pole because at the pole the linear gradient (β) vanishes so that the quadratic variation (γ) is the dominant gradient. It is also applicable to the flow near the center of a rotating (laboratory) tank.

Exact analytical solutions analogous to the stationary barotropic mid-latitude modons (Stern, 1975) are constructed. First, it is shown that, for a modon situated slightly off the pole (i.e., both β and γ are present) the condition of stationarity (in a resting ocean) takes the form β ∫∫ Ψ dxdy — 2γ ∫∫ yΨ dxdy = 0, where Ψ is the streamfunction and x and y are Cartesian coordinates pointing eastward and northward, respectively. Secondly, it is shown that due to the presence of γ, the cyclonic cell situated to the north increases in size and engulfs the southern anticyclone which decreases in size. Namely, as the pole is approached the engulfing cyclone grows whereas the anticyclone shrinks. Ultimately, when the center of the modon (whose diameter is R) reaches a critical distance from the pole (0.1227 R) the anticyclone diminishes to merely a point. Modons that are closer than this critical distance to the pole cannot contain an anticyclone. Far away from the pole our solution reduces to the familiar mid-latitude β-plane modon as should be the case.

In contrast to these dramatic effects of γ on modons, the migration of monopoles (i.e., isolated cyclones or anticyclones) is almost unaffected by γ even though γ is of the same order of (or larger than) β. This results from the fact that the γ-induced perturbations are symmetrical (with respect to north and south) whereas those due to β are asymmetrical. It is shown that, as in other eddies, self-propulsion is primarily caused by asymmetrical perturbations so that disturbances due to γ have almost no influence on the migration.     

Vortex lattice oscillations in rotating neutron stars with quark “CFL” cores

Collective elastic oscillations of a lattice of nonabelian quark semisuperfluid vortex filaments in the superfluid core of a rotating neutron star are examined. It is shown that in the incompressible fluid approximation, transverse long wavelength oscillations (Tkachenko oscillations) owing to shear deformation of the vortex lattice propagate in a plane perpendicular to the axis of rotation. The periods of these oscillations are consistent with rotational variations on the order of 100-1000 days observed in the pulsars PSR B0531+21 and PSR B1828-11. Translated from Astrofizika, Vol. 52, No. 1, pp. 165–169 (February 2009).     

不同形状初始涡旋和热力强迫能量频散特征的数值研究

用谱方法求解球面无辐散正压涡度方程，讨论了不同形状初始涡旋和热力强迫能量频散特性的差异。结果表明：１．在不同结构的环境风场下，差异是没的。环境风场为零或西风时，不不形状的初始涡旋或热力强迫能量频散方向，１速度，强度的差异均较大，但在半球模式中经向频散均很小，能量以向东传播为主。有东风风场存在时，只表现为频散强度的差异，且向极传播的波形波长较长，向南传播的波列波长较短。２．以上线性结果中用波射线跟踪     

台风环流区域内中尺度涡量传播特征的研究

用一个高分辨率f平面直角坐标系的正压准地转模式,实施了10组积分时间为36 h的试验,研究了初始位于台风外区的一个中尺度涡旋与台风涡旋的相互作用。结果表明:这种相互作用可以激发一个从外区伸展到内区的较小尺度的涡旋对,以此方式将涡量内传至台风中心附近。同时,中尺度涡旋呈现涡量集中化的特征。涡量内传与涡量集中化共存,使内区涡量增多,导致台风增强。此外,在一定条件下,这种相互作用还可以使涡量带破碎和断裂,形成一系列空间尺度更小的涡块。     

Identification of Von Karman Vortices in the Surface Winds of Heard Island

Heard Island (73°30' E, 53°05' S) is an isolated island inthe sub-Antarctic located in a region of strong westerly winds. The dominanttopographic feature is a large glaciated volcano, which peaks at 2,745 m inaltitude. Only limited meteorological information exists for the island, withincomplete records from a station at Atlas Cove covering the period 1948 to1954, and from a station at The Spit from 1992 onwards. We present the resultsof wind observations that were conducted on Dovers Moraine at the eastern endof Heard Island over the 2000/2001 summer, with the aim of characterising localwinds at this location on the island. Wind was measured using a Woelfle typemechanical wind recorder. Wind speed was comparable, and on occasions stronger,at this location compared to simultaneous records at the western end of the island.Winds were predominantly from the south-south-west, or north-north-west to north.A number of periods of relatively low wind speed coincided with repeated winddirection patterns, which we associate with the surface signature of von Karmanvortices. Results from this study suggest there are significant topographically-generated differences in wind characteristics between the eastern and western ends of the island.