We combine lidar temperature observations onboard a research aircraft with numerical simulations in the framework of the SOUTHTRAC (Southern Hemisphere Transport, Dynamics, and Chemistry) Campaign. Deep propagation of gravity waves (GW) from the troposphere to the lower mesosphere is studied above the Southern Andes during two flights in September 2019. We use the Weather Research and Forecasting (WRF) model with a configuration for the simulations that has been validated in a previous study of this campaign. Strong orographic GW were detected during both flights, that were conceived for different latitudes. The observational and numerical data reveal the presence of significant GW attenuation, breaking and secondary wave generation above the stratopause due to the development of convective and dynamic instability as well as conditions for wave evanescence. The GW generated by topography were not able to alter the stable structure of the stratosphere, but the scenario was quite different in the lower mesosphere. The disturbed zones in that layer were produced by the combined effect on lapse rate of the background temperature variation and the perturbations associated with GW, which together may induce large vertical gradients. As a consequence, areas of reduced stability (with low or even negative buoyancy parameter) emerge above the stratopause. The existence of these GW self-induced attenuation layers in the mesosphere where temperature perturbations produce large negative gradients may lead to an amplitude growth control mechanism for the upward propagating waves.

Gravity waves (GW) transport momentum flux (MF) and energy across the lower, middle and upper atmosphere. Global Positioning System (GPS) radio occultation (RO) is one of the measuring techniques used onboard satellites to provide vertical temperature profiles with global and permanent coverage. These retrievals may be applied in the study of GW. Some usual analysis methods provide absolute GWMF, but are missing its net direction. This happens because the procedures cannot deduce the propagation sense of GW and hence the direction of MF. We therefore recall here a recently presented method that allows the net calculation with four close in space and time RO soundings (quartets). We use about 10,000 daily retrievals from March 1, 2022 to February 28, 2023 to study the seasonal and latitudinal global characteristics of net GWMF in the height interval from 20 to 35 km. About 600 quartets were found.

We use observations from one of the SOUTHTRAC (Southern Hemisphere Transport, Dynamics, and Chemistry) Campaign flights in Patagonia and the Antarctic Peninsula during September 2019 to analyze possible sources of gravity wave (GW) in this hotspot during austral late winter and early spring. Data from two of the instruments onboard the German High Altitude and Long Range Research Aircraft (HALO) are employed: the Airborne Lidar for Middle Atmosphere research (ALIMA) and the Basic HALO Measurement and Sensor System (BAHAMAS). The former provides vertical temperature profiles along the trajectory while the latter gives the three components of velocity and temperature at the flight position. GW induced perturbations are obtained from these observations. We include numerical simulations from the Weather Research and Forecast (WRF) model to place a four-dimensional context for the GW observed during the flight and in order to present possible interpretations of the measurements, as for example the orientation or eventual propagation sense of the waves may not be inferred using only data obtained onboard. We first evaluate agreements and discrepancies between the model outcomes and the observations. This allowed us an assessment of the WRF performance in the generation, propagation and eventual dissipation of diverse types of GW through the troposphere, stratosphere and lower mesosphere. We then analyze the coexistence and interplay of mountain waves (MW) and non-orographic (NO) GW. The MW dominate above topographic areas and in direction of the so-called GW belt whereas the latter waves are mainly relevant above oceanic zones.

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.

To understand the main orographic and non-orographic sources of gravity waves (GWs) over South America during an Experiment (Rapp et al, 2021, https://doi.org/10.1175/BAMS-D-20-0034.1), we propose the application of a rotational spectral analysis based on methods originally developed for oceanographic studies. This approach is deployed in a complex scenario of large-amplitude GWs by applying it to reanalysis data. We divide the atmospheric region of interest into two height intervals. The simulations are compared with lidar measurements during one of the flights. From the degree of polarization and the total energy of the GWs, the contribution of the upward and downward wave packets is described as a function of their vertical wavenumbers. At low levels, a larger downward energy flux is observed in a few significant harmonics, suggesting inertial GWs radiated at polar night jet levels, and below, near to a cold front. In contrast, the upward GW energy flux, per unit area, is larger than the downward flux, as expected over mountainous areas. The main sub-regions of upward GW energy flux are located above Patagonia, the Antarctic Peninsula and only some oceanic sectors. Above the sea, there are alternating sub-regions dominated by linearly polarized GWs and sectors of downward GWs. At the upper levels, the total available GW energy per unit mass is higher than at the lower levels. Regions with different degrees of polarization are distributed in elongated bands. A satisfactory comparison is made with an analysis based on the phase difference between temperature and vertical wind disturbances.