Inferring Stratospheric Mountain Wave Breaking through Observations at the Tropopause

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 further aloft. Using a 2-D numerical model, we have simulated the secondary wave generation process with qualitatively agreement in the wave location, phase speed, wavelength (i.e. 10 to 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 upgoing wave momentum flux. Second, while 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. Third, the phase speed of the secondary waves is nearly zero so that they satisfy the Eliassen-Palm relationship between momentum and energy flux. While the 2-D results are robust to grid size and sub-grid parametrization, an extension of the modeling to three dimensions is disappointing. The secondary waves amplitudes in the 3-D runs are much smaller than observed.