Transient Tropopause Waves

Horizontally short gravity waves are often observed near the tropopause. Here, airborne observations of mountain waves over southern Scandinavia are used to characterize these waves and to detect non-stationary modes. A series of two-dimensional numerical simulations is used to explain the generation of these transient wave modes that are trapped in the lowermost stratosphere and propagate horizontally downstream along the tropopause inversion layer. The numerical results reveal on which external parameters the properties of the short waves depend on. It turns out that the interaction of wave breaking aloft in the middle atmosphere and the tropospheric flow is the essential process explaining the generation of these non-stationary modes. Their characteristics is controlled by the sharpness of the tropopause inversion layer, the strength of the orographic forcing and, partially, by the spectra of the underlying orography exciting the vertically propagating mountain waves.