Mountain Wave Spectra and Fluxes over Complex Terrain

Abstract: Recent airborne gravity wave measurements over New Zealand in the lower stratosphere in the DEEPWAVE project allow improved multi-variable spectral analysis. On most days, the observed spectra are broad and different spectral quantities such as w-power (), momentum flux () and u-power () differ markedly from each other making it impossible to define a dominant wavelength for the wave-field as a whole. To better understand these wave fields, we first mimic New Zealand an idealized complex terrain: a parabola with an embedded cosine. The associated wave field is composed of a "volume mode" and a "roughness mode". The volume mode arises from smooth airflow over the whole massif while the roughness mode arises from flow into and out of interior valleys. The u-power is dominated by the long waves in the volume mode. The w-power is dominated by the short waves in the roughness mode. The momentum flux has contributions from both modes. If the roughness wavelength is too near the buoyancy cut-off value, the u' field vanishes and the disturbance carries no momentum flux. Spectral and wavelet analyses from the DEEPWAVE aircraft transects and WRF numerical simulations are used to test these ideas. Using the u-power, p-power and T-power, we are able to identify the volume mode for New Zealand with a wavelength of about 300km. The roughness mode with wavelengths from 20 to 60km usually dominates the momentum and vertical energy fluxes in spite of the fact that it very near to the buoyancy cutoff. The roughness mode contribution is quite variable and less predictable. These results help to explain observations of the dominant wavelength decreasing in strong flux events, so-called "scale downshifting" (Smith et al. 2016).