Capsule summaryThe SOUTHTRAC-GW airborne mission explored the dynamics of gravity waves in the region of the Southern Andes and Antarctic Peninsula during the extraordinary southern hemisphere SSW of September 2019.

Abstract. To better understand the impact of gravity waves (GWs) on the middle atmosphere in the current and future climate, it is essential to understand their excitation mechanisms and to quantify their basic properties. Here a new process for GW excitation by orography–jet interaction is discussed. In a case study, we identify the source of a GW observed over Greenland on 10 March 2016 during the POLSTRACC (POLar STRAtosphere in a Changing Climate) aircraft campaign. Measurements were taken with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument deployed on the High Altitude Long Range (HALO) German research aircraft. The measured infrared limb radiances are converted into a 3D observational temperature field through the use of inverse modelling and limited-angle tomography. We observe GWs along a transect through Greenland where the GW packet covers ≈ 1/3 of the Greenland mainland. GLORIA observations indicate GWs between 10 and 13 km of altitude with a horizontal wavelength of 330 km, a vertical wavelength of 2 km and a large temperature amplitude of 4.5 K. Slanted phase fronts indicate intrinsic propagation against the wind, while the ground-based propagation is with the wind. The GWs are arrested below a critical layer above the tropospheric jet. Compared to its intrinsic horizontal group velocity (25–72 m s−1) the GW packet has a slow vertical group velocity of 0.05–0.2 m s−1. This causes the GW packet to propagate long distances while spreading over a large area and remaining constrained to a narrow vertical layer. A plausible source is not only orography, but also out-of-balance winds in a jet exit region and wind shear. To identify the GW source, 3D GLORIA observations are combined with a gravity wave ray tracer, ERA5 reanalysis and high-resolution numerical experiments. In a numerical experiment with a smoothed orography, GW activity is quite weak, indicating that the GWs in the realistic orography experiment are due to orography. However, analysis shows that these GWs are not mountain waves. A favourable area for spontaneous GW emission is identified in the jet by the cross-stream ageostrophic wind, which indicates when the flow is out of geostrophic balance. Backwards ray-tracing experiments trace into the jet and regions where the Coriolis and the pressure gradient forces are out of balance. The difference between the full and a smooth-orography experiment is investigated to reveal the missing connection between orography and the out-of-balance jet. We find that this is flow over a broad area of elevated terrain which causes compression of air above Greenland. The orography modifies the wind flow over large horizontal and vertical scales, resulting in out-of-balance geostrophic components. The out-of-balance jet then excites GWs in order to bring the flow back into balance. This is the first observational evidence of GW generation by such an orography–jet mechanism.

A fatal light aircraft crash, within the Kareedouw mountains, highlighted the need to equip forecasters with the knowledge of the turbulence produced when wind flow encounters complex small-scale terrain near a cold front. With this in mind, an experiment was designed, according to the 6 W's (why, who, where, when, what, whereby) of design, to measure and address the unanswered questions regarding blocking, gap flow and mountain waves. Experiments were conducted with party balloons, dronesondes, dropsondes, thethersondes, parasails, simultaneous ascents and smoke grenades within the Kareedouw Pass. The topographical extent of the pass is the smallest where such a field experiment have ever been conducted. Data were collected during six radiosonde field experiments and from an installed Automatic Weather Station network. Numerous parameters were calculated, where features were successfully characterised by the; Burger-, Froude number, Froude derived height scale and thermal wind equation. The upwind blocking region rendered the Bernoulli equation, which governs gap flow, unusable. Gap flow was identified by pressure-and temperature gradients during non-blocking and weak synoptic conditions. ACKNOWLEDGEMENT I would like to express my sincere gratitude to:  God for providing me with a sound mind and ability to have completed this study.  Eshanè Geldenhuys, my loving wife, for all her support, encouragement and all the proof reading into the late hours of the evening. Without your support, this would not have been possible!  Dr LL Dyson, my Academic supervisor, for all her guidance and support during the project.  Mr D van der Mescht, my field work supervisor, for all his assistance, guidance and patience under trying conditions during the field campaign. Climbing the gruelling Africa Peak multiple times in extreme weather conditions; ranging from 40°C in temperature to-7°C apparent temperature and 160km/h winds.  Mr E Engelbrecht, for always availing himself during field campaigns and for not complaining to climb Africa Peak during 160km/h winds and-7°C apparent temperature.  The late Prof. J van Heerden, for teaching me everything I know with regards to experiment design. We will miss him dearly.  Ms A Demertzis and Ms K Oxley, the South African Weather Service Library staff, for all the assistance in digging up all the un-digitised reading material that formed the basis of this dissertation.  Ms J Savy for the compilation of Figure 6.1.  Mr H van Niekerk and the South African Weather Service Eastern Cape forecasting team, without your support and standing in for my shifts, this project would have been a failure.  South African Weather Service for the time you provided me with and the study bursary.

Refraction in the horizontal is the process whereby a GW phase front changes in orientation. Such changes in orientation are linked with changes in the wavelength in the x-direction and y-direction (Durran, 2009), which has been shown to have important implications for GW propagation (e.g., Dunkerton & Butchart, 1984;Ehard et al., 2017;Sato et al., 2009). A literature survey shows a very small amount of papers on GW refraction compared to GW dissipation and GW breaking. This indicates that a large portion of the academic effort does not include refraction. This article uses high-resolution observational data from the lower troposphere to the lower mesosphere to quantify refraction and show the importance there-of for wave-mean flow interaction.