摘要:

For the idealised geometry in Figure 2, the results of Buchwald and Kachoyan (1987) for a barotropic ocean are extended to include the diffraction of shelf waves by a strait. Assuming a known flux through the strait together with an independent incident shelf wave, the amplitudes of the scattered shelf wave modes are calculated by the adoption of a modal expansion in the strait. The boundary condition at the strait mouth is then satisfied by the use of a Galerkin technique. The results show that higher mode amplitudes are underestimated by Buchwald and Kachoyan (1987). These results are extended to the case of a strait of finite length joining two semi-infinite oceans. An incident shelf wave in one ocean gives rise to a pressure gradient along the strait, and strait oscillations ensue. For a range of geometries and frequencies, calculations show that up to half the incident energy is diffracted through the strait, with some transfer into higher modes. The generation of oscillations in the strait by a periodic wind-stress along the strait is also considered. For parameters relevant to the Bass Strait and the ACE observations the results indicate that incident shelf waves on the south coast are the more likely source of the waves observed on the east coast of Australia.

ISSN:0309-1929    

DOI:10.1080/03091929308203605

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The formation of a thermocline in a water column, where shear-free turbulence is generated from both the surface and the bottom, and a stabilizing buoyancy flux is imposed on the surface, was studied using a numerical model with the aim of understanding the formation of a tidal front in the coastal area. The time evolutions of the distributions of density and turbulent kinetic energy calculated from the model shows that the emergence of a thermocline depends on the conditions determined by the buoyancy flux at the surfaceQ, the eddy diffusivities maintained at the bottom and at the surfaceKbandKsand the height of the water columnH.The criterion for the formation of a thermocline was predicted asRdelta4∼ constant for large δ (δ > 0.5), but the dependence on δ decreases as δ tends to 0, whereR=H4Q/K3b, δ = 1 −D0/His the depth of a thermocline in the absence of bottom mixing. The depth of a thermocline was found to decrease as the bottom mixing increases for a given value ofD0. The results were interpreted in comparison with previous studies.

ISSN:0309-1929    

DOI:10.1080/03091929308203606

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Recently some doubt has arisen about the accuracy of a finite difference approach used in the linear stability analysis of velocity profiles representative of tropical cyclones. There are conflicting results in the literature concerning the stability of the azimuthal wavenumber-one mode, based on the f-plane non-divergent barotropic vorticity equation in cylindrical coordinates. In this paper it is shown via finite difference codes that this mode is stable, for five such velocity profiles. A better representation of the outer boundary condition also yields more accurate growth rates. A tighter semicircle bound than that used in some shooting methods is derived, incorporating wavenumber dependence. A global matrix (“pseudo-spectral”) method, the discrete ordinate method, also appears to work remarkably well—especially for lower unstable wavenumbers at coarse domain resolutions.

ISSN:0309-1929    

DOI:10.1080/03091929308203607

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The developed turbulence in the Ekman layer affects atmospheric flows in two ways: it essentially modifies the beta-effect and leads to the quadratic (in velocity) surface drag. Equations governing the global atmospheric dynamics are obtained for the turbulent planetary boundary layer.

ISSN:0309-1929    

DOI:10.1080/03091929308203608

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An explanation is put forward for the instability observed within a precessing, rotating spheroidal container. The constant vorticity solution for the flow suggested by Poincaré is found to be inertially unstable through the parametric coupling of two inertial waves by the underlying constant strain field. Such resonant couplings are due either to the elliptical or shearing strains present which elliptically distort the circular streamlines and shear their centres respectively. For the precessing Earth's outer core, the shearing of the streamlines and the ensuing shearing instability are the dominant features. The instability of some exact, linear solutions for finite precessional rates is established and used to corroborate the asymptotic analysis. A complementary unbounded analysis of a precessing, rotating fluid is also presented and used to deduce a likely upperbound on the growth rate of a small disturbance. Connection is made with past experimental studies.

ISSN:0309-1929    

DOI:10.1080/03091929308203609

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The problem of three-dimensional convection in spherical shells is considered in the case of spherically symmetric physical conditions for a Boussinesq fluid. While the question of the preferred patterns of convection has been treated in earlier papers, general rules for the flow direction have not yet been found. The present analysis addresses this problem and presents numerical solutions for convection in a fluid of infinite Prandtl number with octohedral or with dodecahedral symmetry as a function of the Rayleigh number.

ISSN:0309-1929    

DOI:10.1080/03091929308203610

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We examine topographical coupling between the Earth's fluid outer core and solid overlying mantle as a possible explanation of decadal period variations in the length-of-day. The topographical torque can be calculated straightforwardly, provided that the flow at the core surface and the topography of the core-mantle boundary are known. Such calculations generally give torques which are larger than required to explain the length-of-day variations; we show however that these calculations are extremely sensitive to the flow, and so cannot be taken as providing any observational evidence either for or against topographical coupling. However, observations of the magnetic field do provide useful constraints on the problem of topographical coupling, in particular constraining the timescale and the nature of the response of the core to topographical coupling.

ISSN:0309-1929    

DOI:10.1080/03091929308203611

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this paper we consider two limiting cases of the vertical propagation of hydromagnetic waves in a viscous and thermally conducting isothermal atmosphere permeated by a uniform horizontal magnetic field. It will be assumed that the viscosity, thermal conductivity, and magnetic field strength are small, but that the effect of thermal conduction dominates in the first case, while in the second case it is the effects of the viscosity and the magnetic field which dominate. Since the equilibrium density decreases exponentially with height, all the diffusivities increase exponentially. In the first case it is convenient to consider the atmosphere divided into three distinct regions. In the lowest region the thermal diffusivity, kinematic viscosity and Alfvén speed are small whereas in the uppermost region all these quantities are large. In the middle region the thermal diffusivity is large while the Alfvén speed and the kinematic viscosity are still small. Therefore, in the lowest region the motion is adiabatic and in the middle region it is isothermal. The exponential increase of the thermal diffusivity with height transforms the oscillatory process from an adiabatic one in the lowest region, to an isothermal one in the middle region and creates a semitransparent reflecting layer between the two regions. The upper reflecting layer, which connects the two upper regions, is of a different type because it acts as an absorbing and reflecting layer. The existence of two reflecting layers will influence the reflection process in the two lower regions and the final conclusions depend on the relative effects of the kinematic viscosity with respect to that of the Alfvén speed and on the relative locations of the two reflecting layers. In the second case the viscosity and the magnetic field combine to create a reflecting and absorbing layer. Thus there are two regions connected by a reflecting and absorbing layer. Below this layer all the diffusivities are small and the motion is adiabatic, while above it the motion will decay to a constant value before it is influenced by the effects of the thermal conduction. Reflection coefficients are derived for both cases and it is shown that resonances will occur.

ISSN:0309-1929    

DOI:10.1080/03091929308203612

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The stability properties of magnetic flux tubes in stellar convection zones including overshoot regions is of considerable interest in connection with the problems of magnetic flux storage and hydromagnetic dynamo action in the Sun and other cool stars. We have developed a general formalism based on the approximation of thin flux tubes which provides a basis for a linear stability analysis of arbitrary flux tube equilibria. As a first application, the stability of axisymmetric, toroidal flux tubes (flux rings) located in the equatorial plane of a star under the influence of differential rotation and stratification has been considered. Arbitrary angular velocity differences between the interior of the flux ring and its environment are permitted. It is found that the linear evolution of radial and azimuthal perturbations (i.e., within the equatorial plane) is decoupled from that of latitudinal perturbations (perpendicular to the plane). The latitudinal instability (‘poleward slip’) is found to be suppressed if the matter within the flux tube rotates faster than its environment by a sufficient amount. For perturbations within the equatorial plane, both stratification (sub-order superadiabatic) of the external gas and rotation are crucial. Angular momentum conservation tends to suppress axisymmetric modes. This effect is enhanced by a faster rotation of the gas within the flux tube. Non-axisymmetric modes are more unstable since the constraint of angular momentum conservation is broken. For these modes, a slower internal rotation rate has a stabilizing effect. Within a certain range of magnetic field strengths, a second region of stability exists within the region of unstable configurations, which can extend into the superadiabatically stratified (convectively unstable) region. The character of the different modes is discussed in conjunction with the topology of the stability diagram.

ISSN:0309-1929    

DOI:10.1080/03091929308203613

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A general method for solving the two-dimensional time-dependent ideal MHD equations is developed when the Alfven-Mach number is small and the Mach number large in comparison with unity. The method is applied to the problem of the time-dependent nonlinear collapse of an X-type neutral point. A current sheet is formed which grows in a self-similar manner.

ISSN:0309-1929    

DOI:10.1080/03091929308203614

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor