Gravity Waves in the Presence of Shear during DEEPWAVE

In this study, we utilize the DEEP propagating gravity WAVE program (DEEPWAVE) observations and the nonhydrostatic COAMPS configured at high resolution (2 km) with a deep domain (60-80 km) to explore the effects of horizontal wind shear on gravity wave propagation and wave characteristics. DEEPWAVE is a comprehensive, airborne and ground-based measurement and modeling program centered on New Zealand. The goal of DEEPWAVE is to advance our understanding of gravity wave dynamics and impacts from the troposphere through the mesosphere and lower thermosphere. The NSF/NCAR GV (NGV) research aircraft was the primary observational platform and deployed from a base in New Zealand during the 6-week field measurement campaign in June-July 2014.

Real-data simulations have been conducted for several DEEPWAVE intensive observing periods (IOPs). The results suggest that horizontal shear associated with the stratospheric polar night jet refracts the gravity waves and leads to propagation of waves significantly downwind of the South Island of New Zealand. These waves have been referred to as “trailing gravity waves”, since they are found predominantly downwind of the orography of the South Island and the wave crests rotate nearly normal to the mountain crest. Observations from the G-V, remote sensing instruments (AMTM, Rayleigh lidar), and the AIRS satellite confirm the presence of gravity waves downwind of the orography in numerous events during DEEPWAVE. The horizontal propagation in the stratosphere can be explained by group velocity arguments for three-dimensional gravity waves in which the wave energy is advected downwind by the component of the flow normal to the horizontal wavevector. We further explore the impact of the shear on gravity wave propagation in COAMPS configured in an idealized mode 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.