Laboratory for internal gravity-wave dynamics: the numerical equivalent to the quasi-biennial oscillation (QBO) analogue

We have extended the classical terrain-following coordinate transformation of Gal-Chen and Somerville to a broad class of time-dependent vertical domains. The proposed extension facilitates modelling of undulating vertical boundaries in various areas of computational fluid dynamics. The theoretical development and the efficient numerical implementation have been documented in the context of the generic Euterian/semi-Lagrangian, non-oscillatory forward in time (NFT), nonhydrostatic model framework. In particular, it allows the simulation of stratified flows with intricate geometric, time-dependent boundary forcings, either at the top or at the bottom of the domain. We have applied our modelling framework in the direct numerical simulation of the celebrated laboratory experiment of Plumb and McEwan creating the numerical equivalent to their quasi-biennial oscillation (QBO) analogue. The QBO represents a conspicuous example of a fundamental dynamical mechanism with challenging detail, which is difficult to deduce from experimental evidence alone. A series of 2D and 3D simulations demonstrate the ability to reproduce the laboratory results. The numerical experiments identify the developing periodically reversing mean flow pattern primarily as a wave-wave mean flow interaction-driven phenomenon. The results not only enhance the confidence in the numerical approach but further elevate the importance of the laboratory setup in its fundamental similarity to the atmosphere, while allowing the study of the principal atmospheric mechanisms and their numerical realizability in a confined 'laboratory' environment. Copyright (c) 2005 John Wiley & Sons, Ltd.