Abstract
Horizontal gravity wave (GW) refraction was observed around the Andes
and Drake Pas- sage during the SouthTRAC campaign. GWs interact with the
background wind through refraction and dissipation. This interaction
helps to drive mid-atmospheric circulations and slows down the polar
vortex by taking GW momentum flux from one location to an- other. The
SouthTRAC campaign was composed to gain improved understanding of the
propagation and dissipation of GWs. This study uses observational data
from this cam- paign collected by the German research aircraft on 12
September 2019. During the cam- paign a minor sudden stratospheric
warming in the Southern Hemisphere occurred, which heavily influenced GW
propagation and refraction and thus also the location and amount of GW
momentum flux deposition. Observations include, amongst others,
measurements from below the aircraft by GLORIA (Gimballed Limb Observer
for Radiance Imaging of the Atmosphere), and above the aircraft by ALIMA
(Airborne Lidar for the Middle Atmosphere). Refraction is identified in
two different GW packets as low as ≈4 km and as high as 58 km. One GW
packet of orographic origin and one of non-orographic ori- gin is used
to investigate refraction. Observations are supplemented by the
Gravity-wave Regional Or Global Ray Tracer (GROGRAT), a simplified
mountain wave model, ERA5 data and high-resolution (3 km) WRF data.
Contrary to some previous studies we find that refraction makes a
noteworthy contribution in the amount and the location of GW momentum
flux deposition. This case study highlights the importance of refraction
and provides compelling arguments that models should account for this.