Generation of mountain wave-induced mean flows and turbulence near the tropopause

Gravity waves interacting with the tropopause are investigated using linear and nonlinear numerical simulations. The tropopause is modelled as the interface between two layers of constant Brunt-Väisälä frequency. The simulations are two-dimensional with uniform horizontal flow, the background rotation is ignored, and the gravity waves are generated by flow over an idealized isolated obstacle shape at the surface. The nonlinear simulation results show a horizontal wave-induced mean flow at the tropopause similar to previous results treating horizontally periodic internal waves impinging on a density-gradient interface. The mean flow created by the impinging gravity waves is increased over the background wind below the tropopause and decreased above the tropopause. This effect is not present in the linear simulations. The nonlinear effect is felt more strongly for cases with higher mountain heights and larger values of the stability in the upper layer. The final steady mountain-wave flow appears to permanently retain this mean-flow change. The deceleration region above the tropopause results in a patch of slow-moving fluid near the interface which induces local regions of reduced Richardson number and may help explain some observational results of higher turbulence intensities near the tropopause over mountainous regions.