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

Abstract:

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.