Nonstationary Trapped Lee Waves Generated by the Passage of an Isolated Jet

The behavior of nonstationary trapped lee waves in a nonsteady background flow is studied using idealized three-dimensional (3D) numerical simulations. Trapped waves are forced by the passage of an isolated, synopticscale barotropic jet over a mountain ridge of finite length. Trapped waves generated within this environment differ significantly in their behavior compared with waves in the more commonly studied two-dimensional (2D) steady flow. After the peak zonal flow has crossed the terrain, two disparate regions form within the mature wave train: 1) upwind of the jet maximum, trapped waves increase their wavelength and tend to untrap and decay, whereas 2) downwind of the jet maximum, wavelengths shorten and waves remain trapped. Waves start to untrap approximately 100 km downwind of the ridge top, and the region of untrapping expands downwind with time as the jet progresses, while waves downstream of the jet maximum persist. Wentzel–Kramers–Brillouin (WKB) ray tracing shows that spatial gradients in the mean flow are the key factor responsible for these behaviors. An example of real-world waves evolving similarly to the modeled waves is presented. As expected, trapped waves forced by steady 2D and horizontally uniform unsteady 3D flows decay downstream because of leakage of wave energy into the stratosphere. Surprisingly, the downstream decay of lee waves is inhibited by the presence of a stratosphere in the isolated-jet simulations. Also unexpected is that the initial trapped wavelength increases quasi-linearly throughout the event, despite the large-scale forcing at the ridge crest being symmetric in time about the midpoint of the isolated-jet simulation.