Oblique propagation and refraction of mountain waves over the Andes observed by GLORIA and ALIMA during the SouthTRAC campaign

The SouthTRAC measurement campaign was held in Argentina from September to November 2019 using the HALO research aircraft. One of the main goals of the campaign was gravity wave (GW) study in the region of Southern Andes: a global hotspot for GW activity. The measurements included air temperature data from the IR limb imaging spectrometer GLORIA jointly developed by FZJ and KIT, as well as ALIMA, an upward looking lidar developed by DLR, and in situ instruments. GLORIA's viewing direction can be panned between 45° and 135° with respect to the flight direction. Combining this capability with flight paths that encircle the observed atmospheric region, multiple measurements of the same air mass can be performed, allowing for 3D tomography of the atmosphere with a vertical resolution down to 250 m and horizontal resolution of around 25 km. GLORIA provides data below the flight altitude of the HALO aircraft (up to 15 km), while ALIMA observations cover the altitude range between 20 and about 60 km. GLORIA is the airborne demonstrator of the satellite based infrared limb imager CAIRT proposed as the Earth Explorer 11 candidate.  

During a research flight on 20-21 September, a large amplitude mountain wave was observed over the Andes. GLORIA 3D data showed a complex temperature structure with several overlapping gravity wave families at altitudes of 9 to 14 km above the mountain ridges. The amplitudes and 3D wave vectors for each of those families were determined by performing a least-squares fit of harmonic disturbances to the GLORIA temperature data. These wave parameters were then used to initialise a ray-tracer (GROGRAT ray-tracing code was used) and follow the path of the waves as they propagated upwards and away from the mountain range. The results of this study could be summarised as follows:

• Many of the waves observed by GLORIA in the 9-14 km altitude range propagated into the regions observed by ALIMA in 25-40 km altitudes. There was very good agreement between ALIMA data and GLORIA data-initialised ray tracing results: GWs observed by GLORIA were shown to propagate into the same regions where waves were seen by ALIMA and their spectral characteristics also closely matched ALIMA observations.
• Oblique GW propagation was directly observed, including propagation of some GWs toward the upwind side of the Andes mountain range. Oblique propagation also resulted in significant meridional transport of zonal gravity wave momentum flux (GWMF).
• We observed strong horizontal GW refraction, with some wave vectors turning by more than 50°. This resulted in significant momentum exchange between waves and the background flow outside of wave generation and breaking regions. 
• We also use the GLORIA data to study the time dependence (as a result of changing winds) of the mountain wave pattern over the Andes.