Stratospheric Trailing Mountain Waves

Trailing waves are spectacular long wave beams frequently observed in the stratosphere over mid- to high latitude topography. Two trailing wave events documented over New Zealand during the DEEPWAVE experiment are examined using numerical simulations and theoretical analysis to better understand the trailing wave characteristics and formation mechanisms. The DEEP propagating gravity WAVE program (DEEPWAVE) is a comprehensive, airborne and ground-based measurement and modeling program centered on New Zealand and focused on gravity wave dynamics and impacts from the troposphere through the mesosphere and lower thermosphere. This program employed the NSF/NCAR GV (NGV) research aircraft from a base in New Zealand in a 6-week field measurement campaign in June-July 2014. During the field phase, the NGV was equipped with new Rayleigh and sodium resonance lidars and an advanced mesospheric temperature mapper (AMTM), a microwave temperature profiler (MTP), as well as dropwindsondes and a full suite of flight level instruments providing measurements spanning altitudes from immediately above the NGV flight altitude (~13 km) to ~100 km.

We utilized numerical simulations using the nonhydrostatic COAMPS and AIRS satellite observations to explore the dynamics of trailing waves. We find that trailing waves over New Zealand are orographically generated, and the formation of trailing waves is regulated by several aspects including the interaction between terrain and mountaintop winds, critical level absorption, wave reflection, and refraction. Among them, the interaction between topography and low-level winds determines the perturbation energy distribution over scales and directions near the wave source and it follows that trailing waves are sensitive to terrain features and low-level winds. Terrain induced perturbations are filtered by critical level absorptions associated with directional wind shear and when the wave intrinsic frequency approaches the Coriolis coefficient. The former plays a role in limiting the wave beam orientation, and the latter sets an upper limit for the permissible wavelength for trailing waves. Once entering into the stratosphere, the orographic waves are subject to refraction associated with the meridional shear of the stratospheric westerlies, which tends to refract waves toward stronger winds. This effect stretches out the wave fronts pointing toward stronger winds, resulted in elongated trailing wave beams, and in the meantime, shortens the wave fronts pointing toward weaker winds. We further explore the dynamics of trailing waves using idealized simulations initialized with a zonally balanced stratospheric jet. The idealized results confirm the importance of horizontal wind shear for the refraction of the waves. Furthermore, the zonal momentum flux minimum is shown to bend or refract into the jet in the stratosphere as a consequence of the wind shear.