Generation and Deep Propagation of Mountain Waves

Here we review recent progress on the generation and deep propagation of mountain waves including the deposition of wave drag in the stratosphere and higher. Results of the 2014 DEEPWAVE project over New Zealand are included along with work from other investigations. We note that for complex terrain, the mountain wave spectrum is quite broad with wavelengths from 10 to 300km. The shortest of these waves, close to the buoyancy cut-off, have little u-power and carry little momentum flux. Waves on the 20 to 60km range dominate the momentum flux in strong wind and wave drag events. Longer waves in the 60 to 300km range also contribute, especially in the weaker events. The longer waves are more difficult to detect from aircraft due to their smaller w-power but easier to detect from satellites, lidars and balloons due to their larger u-power and T-power.

In mid-latitudes, the wind minimum in the low stratosphere acts like a “valve layer”, where the ambient wind speed controls wave breaking there. If waves survive the valve layer, they can penetrate deep into the upper stratosphere and mesosphere. Wherever they break down, they generate an irregular wake of potential vorticity banners associated with the momentum deposition.

As mountain wave events usually persist for less than 48 hours, transient effects are important. Typically, the shorter waves with their small u-power and fast group velocity will reach and dissipate in the mesosphere within six hours. The longer waves will slowly accumulate in the low stratosphere and may break later. Thus, while the wave energy propagates upward, the wave breaking descends with time.

Finally, we describe the roles of anisotropy and non-linearity in wave generation from complex terrain. We show that the mountain wave momentum flux vector can be well predicted as the product of the low-level wind vector and a positive-definite 2 by 2 drag matrix; computed in Fourier space from the terrain power spectrum.