Dominant Mountain Wave Scales from Complex Terrain: Evidence from DEEPWAVE Flights

The 97 low-stratosphere transects over New Zealand by the NSF/NCAR Gulfstream V aircraft during the DEEPWAVE project in 2014 provide a new data set to study the mountain waves from multi-scale complex terrain. One important puzzle is that different diagnostic quadratic quantities have different wavenumber (or wavelength) weightings: a) w-power, b) momentum flux (MFx) and vertical energy flux (EFz)and c) u-power, p-power and T-power have very different spectra. The dominant wavelength depends on the diagnostic quantity that one chooses. A related puzzle is that the dominant wavelength for MFx decreases in strong flux events. This tendency was called scale “downshifting” by Smith et al. 2016)

To clarify the issue of a “dominant wavelength”, we investigate a family of compact “broad spectrum” terrain shapes. We choose a parabola with an embedded cosine. We identify two mountain wave modes: volume mode and roughness mode. The “volume mode” represents the smooth airflow over the entire Southern Alps massif. The “roughness mode” is associated with flow into and out of interior valleys. The u, T and P-powers are dominated by the long-wave “volume mode”. The w-power is dominated by the “roughness mode”. The MFx and EFz are influenced by both the volume and roughness modes. If the roughness mode increases relative to the volume mode, the dominant MFx wavelength decreases.

With this new framework, we reanalyze the GV aircraft data and WRF wave simulations for New Zealand. The volume mode is nicely seen in the u-power (or T or P-power) but not seen at all by the w-power. Conversely, the roughness mode is seen by the w-power but not by the u-power. The momentum flux has contributions from both modes.