摘要:

Numerical experiments were carried out using a quasigeostrophic baroclinic model to investigate sensitivity of the circulation, eddy variability and jets to the thermal forcing and their meridional structure. Two different flow regimes were revealed: a turbulent regime with strong upscale energy transfer, strong indirect thermal cell and no evident life cycles and a radiatively arrested transfer regime with vanishing non-linear transfer term, indirect thermal circulation cell and clear life cycles. Numerical experiments showed sharp transitions from one to two and from two to three jets when the width of the baroclinically unstable region increased. It was found that the eddy heat fluxes and kinetic energy dropped when a new jet evolved in the system.

ISSN:0309-1929    

DOI:10.1080/03091920008203720

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The normal mode instability of steady Wu-Verkley (1993) wave and modons by Verkley (1984, 1987, 1990) and Neven (1992) is considered. All these flows are solutions to the vorticity equation governing the motion of an ideal incompressible fluid on a rotating sphere. A conservation law for infinitesimal perturbations to each solution is derived and used to obtain a necessary condition for its exponential instability. By these conditions, Fjörtoft's (1953) average spectral number of the amplitude of an unstable mode must be equal to a specific number that depends on the degree of the solution in its inner and outer regions as well as on spectral distribution of the mode energy in these regions. Some properties of the conditions for different types of modons are discussed. The maximum growth (and decay) rate of the modes is estimated, and the orthogonality of the amplitude of each unstable, decaying, or non-stationary mode to the basic solution is shown in the energy inner product.

ISSN:0309-1929    

DOI:10.1080/03091920008203721

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper discusses dynamo action in generalisations of the Ponomarenko dynamo at large magnetic Reynolds number. The original Ponomarenko dynamo consists of a spiralling flow in which the stream surfaces are concentric cylinders of circular cross section, and the flow depends only on distance from the axis in cylindrical polar coordinates.

ISSN:0309-1929    

DOI:10.1080/03091920008203722

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We reconsider thin-disc global asymptotics for kinematic, axisymmetric mean-field dynamos with vacuum boundary conditions. Non-local terms arising from a small but finite radial field component at the disc surface are consistently taken into account for quadrupole modes. As in earlier approaches, the solution splits into a local part describing the field distribution along the vertical direction and a radial part describing the radial (global) variation of the eigenfunction. However, the radial part of the eigenfunction is now governed by an integro-differential equation whose kernel has a weak (logarithmic) singularity. The integral term arises from non-local interactions of magnetic fields at different radii through vacuum outside the disc. The non-local interaction can have a stronger effect on the solution than the local radial diffusion in a thin disc, however the effect of the integral term is still qualitatively similar to magnetic diffusion.

ISSN:0309-1929    

DOI:10.1080/03091920008203723

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We compare the predictions of incompressible, analytic reconnection theory with numerical simulations performed using a time-dependent, doubly periodic code. The properties of the simulated current sheets are shown to be in good quantitative agreement with recent planar analytic reconnection solutions based onX-point merging driven by sheared incompressible flows. We conclude that the analytic treatment is not seriously compromised by assuming an open flow geometry and a one-dimensional current layer. The analytic models, when augmented by nonlinear saturation arguments, are also shown to provide resistive scaling laws that accord well with computed merging rates. By way of contrast, we have found no computational evidence for the osculation of separatrices in current sheets, even for the case of classical head-on reconnection in which all shearing flows are absent. We attribute this to the dynamic nature of the simulated current sheet. Our simulations provide some evidence that dynamic current sheets can break up into magnetic islands, but whether this fragmentation can be attributed to the tearing mode instability remains unclear.

ISSN:0309-1929    

DOI:10.1080/03091920008203724

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor