Momentum transport and low-level drag generated by trapped gravity lee waves

Inaccurate representation of the low-level drag processes within trapped gravity lee wave trains is known to result in overestimates of the vertical flux of horizontal momentum aloft. Current parameterizations of mountain wave drag tend to be focused on momentum transport by vertically propagating waves in a steady flow, which leads to these poor results. To address these issues, 3D simulations of trapped waves in a time-evolving barotropic flow are performed to illustrate drag mechanisms within the waves and the resultant momentum profile. Waves generated in this study are compared to earlier 2D and steady work on trapped lee waves, and to observations.

Trapped waves are forced by a 2-layer stability profile. All dynamical aspects of the wave train are analyzed to highlight areas of drag generation and momentum transport. The relative importance of trapped waves in generating these processes is considered in relation to wave breaking and flow blocking. Drag generated by these waves and their 3D momentum transports are considered in relation to their impacts on the large-scale flow – both at mountain top, and aloft.

A background flow that changes over a 1.5 day timescale forces a trapped lee wave train that depends on more than just the mountain top speed that generates the waves. Results are not as may be expected from a simple evolution of the steady case. Trapped waves are seen to propagate relative to the flow dependent upon the group velocity they were forced with. Early in the simulation, the trapped waves develop and grow whilst moving downstream. As the flow decelerates, waves gradually decay and may either propagate upstream or continue to be carried further downstream by the background flow – depending upon their relative position and group velocity.