Influence of a stratospheric turbulence layer on the penetration of mountain waves into the middle atmosphere

During the DEEPWAVE campaign, large-amplitude mesospheric temperature fluctuations were observed by the ground-based Rayleigh lidar at Lauder, NZ on July 4 2014. For the same day, coordinated aircraft observations of the DLR Falcon and the NSF/NCAR GV report one of the largest wave events measured by both aircraft with vertical velocity amplitudes in excess of 6 m/s around the tropopause level at around 11 km altitude. Mesoscale numerical simulations indicate the existence of breaking hydrostatic gravity waves between about 15 km to 25 km altitude above the southern island. In most cases, the breaking regions are linked with individual orographic peaks. As time evolves, they generate a stratospheric turbulent layer extending over the whole island. Nevertheless, the wave attenuation in this layer cannot prevent the penetration of mountain waves to higher altitudes and the simulations reveal a boost of wave amplitudes above about 30 km altitude. Furthermore, the mesoscale numerical model simulates the excitation of secondary gravity waves from the turbulent layer. Obviously, the small-scale dynamics in this stratospheric turbulence layer influences the momentum deposition and the spatial scales of the waves. Therefore, this layer plays a central role in defining the spectrum of gravity waves penetrating to the mesosphere. By a combination of a multitude of ground-based and airborne measurements, mesoscale (ECMWF's IFS, WRF and Unified Model of UK MetOffice) as well as idealized numerical simulations we investigate the role of the breaking region on the deep propagation of mountain waves over New Zealand