Inferring Stratospheric Mountain Wave Breaking through Observations at the Tropopause

Mountain wave breaking and secondary wave generation in models has been documented by Lane and Sharman (2006), Holton and Alexander (1999), Satomura and Sato (1999), and Bacmeister and Schoeberl (1989). Secondary waves were first observed using in situ aircraft data from the Terrain-Induced Rotor Experiment (T-REX) by Smith et al. (2008) and Woods and Smith (2010). To verify our interpretation of these secondary wave observations in T-REX data, we use the WRF model to simulate secondary wave generation. Adopting the initial condition of Satomura and Sato (1999) we simulate mountain wave breaking in both 2D and 3D in the middle stratosphere using a typical mid latitude Northern Hemisphere winter wind profile.

Secondary waves are found propagating from the wave breaking region and partially reflecting up from the tropopause. Wavelet filtering techniques are employed to separate the primary and secondary wave packets and their associated Eliassen - Palm (EP) fluxes. Secondary waves are shown to be unsteady and non-stationary. Despite this, both upgoing and downgoing secondary waves are found on the same line of an EP flux scatter diagram like that shown in Smith et al. (2008), suggesting that they are quasi-stationary. Above the wave breaking region, EP fluxes are reduced by an order of magnitude.

Simple idealized 2D simulations consistent with Satomura and Sato (1999) are shown to reproduce secondary wave patterns that bare striking resemblance to those observed in T-REX. This suggests that despite the inherently 3D nature of the wave breaking process, the onset of breaking and propagation patterns can be predicted on a basic level using relatively computationally inexpensive 2D simulations. With this conclusion in hand, our simulations are extended for typical wintertime conditions over the southern Andes in preparation for the Southern Andes - Antarctic Gravity Wave Initiative (SAANGRIA). Wave breaking and secondary generation are predicted much higher near the stratopause.