| The breakdown of vertically propagating planetary waves in the stratosphere is investigated using an ultra-high horizontal resolution Contour Dynamics with Surgery model. In the model, planetary waves are forced at the tropopause and propagate upwards through the stratosphere and into an absorbing sponge (the first of its kind for such a model). In the context of wave breaking, two aspects of the system are questioned, namely, (1) what is the sensitivity to upper-boundary conditions? and (2) given perfect upper-boundary conditions what controls wave breaking?; (1) In a Boussinesq environment. wave breaking is compared using: (a) a rigid upperboundary condition (as in previous work) and (b) an absorbing sponge (preventing spurious reflections). In (a) both local (to the forcing) and remote breaking is evidenced for weak forcing while only local breaking is observed for sufficiently strong forcing. In (b) remote breaking is absent and local breaking, which occurs for sufficiently strong forcing, has quite a different character to that seen in it). Compressibility effects are also investigated.; (2) A quasi-linear theory is developed which predicts wave breaking if the zonal mean flow decelerates by more than one-half of its initial value (via positive group-velocity/zonal-mean-flow feedbacks). This so-called “one-half” rule for planetary wave breaking is confirmed through fully-nonlinear simulations. Numerical simulations detail the precise sequence of events leading up to and after wave breaking. |
