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

During the DEEPWAVE campaign, large-amplitude mesospheric temperaturefluctuations were observed by the ground-based Rayleigh lidar at Lauder,NZ on July 4 2014. For the same day, coordinated aircraft observations ofthe DLR Falcon and the NSF/NCAR GV report one of the largest wave eventsmeasured by both aircraft with vertical velocity amplitudes in excess of6 m/s around the tropopause level at around 11 km altitude.Mesoscale numerical simulations indicate the existence of breakinghydrostatic gravity waves between about 15 km to 25 km altitudeabove the southern island. In most cases, the breaking regionsare linked with individual orographic peaks. As time evolves, theygenerate a stratospheric turbulent layer extending over the wholeisland. Nevertheless, the wave attenuation in this layer cannot preventthe penetration of mountain waves to higher altitudes and the simulationsreveal a boost of wave amplitudes above about 30 km altitude. Furthermore,the mesoscale numerical model simulates the excitation of secondarygravity waves from the turbulent layer.Obviously, the small-scale dynamics in this stratospheric turbulencelayer influences the momentum deposition and the spatial scales of thewaves. Therefore, this layer plays a central role in defining the spectrumof gravity waves penetrating to the mesosphere. By a combination of amultitude of ground-based and airborne measurements, mesoscale (ECMWF'sIFS, WRF and Unified Model of UK MetOffice) as well as idealized numericalsimulations we investigate the role of the breaking region on the deeppropagation of mountain waves over New Zealand.