Oblique propagation of mountain waves to the upwind side of the Andes
observed by GLORIA and ALIMA during the SouthTRAC campaign
Abstract
Gravity waves (GW) carry energy and momentum from troposphere to the
middle atmosphere and have a strong influence on the circulation there.
Global atmospheric models cannot fully resolve GWs, and therefore rely
on highly simplified GW parametrizations that, among other limitations,
account for vertical wave propagation only and neglect refraction. This
is a major source of uncertainty in models, and leads to well-known
problems, such as late break-up of polar vortex due to the “missing”
GW drag around 60°S. To investigate these phenomena, GW observations
over Southern Andes were performed during SouthTRAC aircraft campaign.
This paper presents measurements from a SouthTRAC flight on
21~September 2019, including 3-D tomographic temperature
data of the infrared limb imager GLORIA (8-15 km altitude) and
temperature profiles of the ALIMA lidar (20-80 km altitude). GLORIA
observations revealed multiple overlapping waves of different
wavelengths. 3-D wave vectors were determined from the GLORIA data and
used to initialise a GW ray-tracer. The ray-traced GW parameters were
compared with ALIMA observations, showing good agreement between the
instruments and direct evidence of oblique (partly meridional) GW
propagation. ALIMA data analysis confirmed that most waves at 25-40 km
altitudes were indeed orographic GWs, including waves seemingly upstream
of the Andes. We directly observed horizontal GW refraction, which has
not been achieved before SouthTRAC. Refraction and oblique propagation
caused significant meridional transport of horizontal momentum as well
as horizontal momentum exchange between waves and the background flow
all along the wave paths, not just in wave excitation and breaking
regions.