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The quasi-geostrophic ellipsoidal vortex model

Published online by Cambridge University Press:  21 April 2004

DAVID G. DRITSCHEL
Affiliation:
School of Mathematics and Statistics, University of St Andrews, St Andrews, UK
JEAN N. REINAUD
Affiliation:
School of Mathematics and Statistics, University of St Andrews, St Andrews, UK
WILLIAM J. McKIVER
Affiliation:
School of Mathematics and Statistics, University of St Andrews, St Andrews, UK

Abstract

We present a simple approximate model for studying general aspects of vortex interactions in a rotating stably-stratified fluid. The model idealizes vortices by ellipsoidal volumes of uniform potential vorticity, a materially conserved quantity in an inviscid, adiabatic fluid. Each vortex thus possesses 9 degrees of freedom, 3 for the centroid and 6 for the shape and orientation. Here, we develop equations for the time evolution of these quantities for a general system of interacting vortices. An isolated ellipsoidal vortex is well known to remain ellipsoidal in a fluid with constant background rotation and uniform stratification, as considered here. However, the interaction between any two ellipsoids in general induces weak non-ellipsoidal perturbations. We develop a unique projection method, which follows directly from the Hamiltonian structure of the system, that effectively retains just the part of the interaction which preserves ellipsoidal shapes. This method does not use a moment expansion, e.g. local expansions of the flow in a Taylor series. It is in fact more general, and consequently more accurate. Comparisons of the new model with the full equations of motion prove remarkably close.

Type
Papers
Copyright
© 2004 Cambridge University Press

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