Transient Tropopause Waves

Flight-level airborne observations have often detected gravity waves with horizontal wavelengths λx ≲ 10 km near the tropopause. Here, in-situ and remote sensing aircraft data of these short gravity waves trapped along tropopause inversion layer and collected during a mountain wave event over southern Scandinavia are analyzed to quantify their spectral energy and energy fluxes and to identify non-stationary modes. A series of three-dimensional numerical simulations are performed to explain the origin of these transient wave modes and to investigate the parameters on which they depend. It turns out that mountain wave breaking in the middle atmosphere and the subsequent modification of the stratospheric flow are the key factors for the occurrence of trapped modes with λx ≲ 10 km. In particular, the intermittent and periodic breaking of mountain waves in the lower stratosphere forms a wave duct directly above the tropopause, in which the short gravity waves are trapped. The characteristics of the trapped, downstream-propagating waves are mainly controlled by the sharpness of the tropopause inversion layer. It could be demonstrated that different settings for optimizing the numerical solver have a significantly smaller influence on the solutions.