Broad spectrum mountain waves

Recent airborne mountain-wave measurements over New Zealand in the lower stratosphere during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) campaign allow for improved spectral analysis of velocities u, υ, and w, pressure p, and temperature T fluctuations. Striking characteristics of these data are the spectral breadth and the different spectral shapes of the different physical quantities. Using idealized complex terrain as a guide, the spectra are divided into the long-wave “volume mode” arising from airflow over the whole massif and the short-wave “roughness mode” arising from flow into and out of valleys. The roughness mode is evident in the aircraft data as an intense band of w power from horizontal wavelength λ = 8–40 km. The shorter part of this band (i.e., λ = 8–15 km) falls near the nonhydrostatic buoyancy cutoff (λ = 2πU/N). It penetrates easily into the lower stratosphere but carries little u power or momentum flux. The longer part of this roughness mode (i.e., λ = 15–40 km) carries most of the wave momentum flux. The volume mode for New Zealand, in the range λ = 200–400 km, is detected using the u-power, p-power, and T-power spectra. Typically, the volume mode carries a third or less of the total wave momentum flux, but it dominates the u power and thus may control the wave breakdown aloft. Spectra from numerical simulations agree with theory and aircraft data. Problems with the monochromatic assumption for wave observation and momentum flux parameterization are discussed.