Short-Wave Signatures of Stratospheric Mountain Wave Breaking

Recent stratospheric mountain wave measurements over the Sierra Nevada indicate that downgoing secondary waves may be common or even ubiquitous in large wave events. Because of their short wavelengths, they may dominate the vertical velocity field near the tropopause. and they give a remote indicator of wave breaking farther aloft. Using a 2D numerical model, the authors have simulated the secondary wave generation process with qualitative agreement in the wave location, phase speed, wavelength (i.e., 10-20 km), and amplitude. A key to the analysis was the use of Morlet wavelet cross spectra on both the observational and simulated fields. Several characteristics of the simulated secondary waves were unexpected. First, the secondary waves are generated with good efficiency, approaching 20% of the primary upgoing wave momentum flux. Second. whereas most of the secondary waves are downward, the shorter components reflect upward from the tropopause, giving a kind of lee wave trapping in the lower stratosphere. Long waves are also observed propagating upward and downward away from the wave breaking region. Third, the phase speed of the secondary waves is nearly zero so the Eliassen-Palm relationship between momentum and energy flux is satisfied. While the 2D results are robust to grid size and subgrid parameterization, an extension of the modeling to three dimensions is disappointing. The secondary waves' amplitudes in the 3D runs are much smaller than observed.