Nonstationary trapped lee waves generated by the passage of an isolated-jet

Observational evidence readily demonstrates that trapped lee waves exhibit notable nonstationary behavior. Despite this, the current understanding of the 3D time evolution of these waves is not as advanced as for 2D trapped waves experiencing a steady, horizontally uniform forcing. Changes in wave amplitude, downstream position and extent, and horizontal wavelength are all commonly observed – often occurring on timescales of less than one hour. Further study of lee waves in a more realistic environment is therefore vital to better understand these observations and to understand the drag non-steady trapped waves exert on the large-scale flow.

We present 3D simulations of trapped lee waves forced by the passage of an isolated barotropic jet past a ridge of finite length. Resonant waves are supported by horizontally uniform changes in stability with height. Trapped waves generated within this environment differ significantly in their behavior compared to waves in the more commonly studied 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; 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.

WKB ray tracing shows that spatial gradients in the mean flow, in particular, δu/δx, are the key factor responsible for these behaviors. This conclusion is tested and confirmed through analysis of trapped waves in a time-varying but horizontally-uniform environment, where the waves remain trapped throughout. An example of real-world waves evolving similarly to our modeled waves is discussed using our theory, observations and high-resolution operational model output.