Unusual Trapped Lee Waves in the Stratosphere

Wave activity with maximum amplitude in the stratosphere downstream of major mountain ranges has previously been explained as a result of wave breaking inducing secondary wave generation in the stratosphere (Vadas et. al., 2003) or the vertical propagation of low-amplitude waves from below. However, theoretical results as early as Scorer (1949) and Corby and Wallington (1956) have posited the existence of higher-order trapped wave modes. Although the higher-order modes in these studies were confined to the troposphere due to the studies' theoretical setup, Corby and Wallington (1956) demonstrate that the level of maximum amplitude for higher-order modes in a two-layer atmosphere with constant Scorer parameter in each layer is higher in the vertical than that of lower-order modes. The increased height of the level of maximum amplitude raises the possibility of higher-order modes with significant amplitude in the stratosphere. However, while higher-order trapped wave modes are theoretically predicted to exist, they are not known to have been observed in the real atmosphere or simulated in realistic numerical models.

The DEEPly Propagating Gravity WAVE campaign (DEEPWAVE) was conducted over New Zealand from 29 May 2014 to 27 July 2014. Immediately after the end of the intensive observing period, on 28 July, a strong event occurred. Model simulations of this event revealed unusual wave activity in the stratosphere. These waves were located downstream of the topography, like a trapped wave, but were oriented from south to north, in contrast to the more typical southwest-northeast orientation paralleling the crest of the Southern Alps.

Vertical cross-sections of the unusual waves exhibit a nodal structure consistent with that of a higher-order trapped wave mode. Solutions to the two-dimensional, linear, Boussinesq wave equation for a horizontally homogeneous sounding derived from the 28 July case include higher-order modes supported by the zonal wind which have large amplitude in the stratosphere. These higher-order modes are trapped by very strong westerly winds in the upper stratosphere. In contrast, the cross-mountain wind component is not strong enough in the stratosphere to trap the same wave mode in a crest-parallel orientation. These waves are reproducible in both two- and three-dimensional compressible numerical models with both idealized and realistic terrains, and therefore provide a plausible explanation for the wave activity in the stratosphere.