Airborne measurements and large-eddy simulations of small-scale gravity waves at the tropopause inversion layer over Scandinavia

Abstract

Abstract. Coordinated airborne measurements were performed by two research aircraft – Deutsches Zentrum für Luft- und Raumfahrt (DLR) Falcon and High Altitude and Long Range Aircraft (HALO) – in Scandinavia during the GW-LCYCLE II (Investigation of the life cycle of gravity waves) campaign in 2016 to investigate gravity wave processes in the upper troposphere and lower stratosphere (UTLS) region. A mountain wave event was probed over southern Scandinavia on 28 January 2016. The collected dataset constitutes a valuable combination of in situ measurements and horizontal- and altitude-resolved Doppler wind lidar and water vapour measurements with the differential absorption lidar (DIAL). In situ data at different flight altitudes and downward-pointing wind lidar measurements show pronounced changes of the horizontal scales in the vertical velocity field and of the leg-averaged momentum fluxes (MFs) in the UTLS region. The vertical velocity field was dominated by small horizontal scales with a decrease from around 20 to < 10 km in the vicinity of the tropopause inversion layer (TIL). These small scales were also found in the water vapour data and backscatter data of the DIAL. The leg-averaged MF profile determined from the wind lidar data is characterized by a pronounced kink of positive fluxes in the TIL and negative fluxes below. The largest contributions to the MF are from waves with scales > 30 km. The combination of the observations and idealized large-eddy simulations revealed the occurrence of interfacial waves having scales < 10 km on the tropopause inversion during the mountain wave event. The contribution of the interfacial waves to the leg-averaged MF is basically zero due to the phase relationship of their horizontal and vertical velocity perturbations. Interfacial waves have already been observed on boundary-layer inversions but their concept has not been applied to tropopause inversions so far. Our idealized simulations reveal that the TIL affects the vertical trend of leg-averaged MF of mountain waves and that interfacial waves can occur also on tropopause inversions. Our analyses of the horizontal- and altitude-resolved airborne observations confirm that interfacial waves actually do occur in the TIL. As predicted by linear theory, the horizontal scale of those waves is determined by the wind and stability conditions above the inversion. They are found downstream of the main mountain peaks and their MF profile varies around zero and can clearly be distinguished from the MF profile of Kelvin–Helmholtz instability. Further, the idealized large-eddy simulations reveal that the presence of the TIL is crucial in producing this kind of trapped wave at tropopause altitude.

Momentum fluxes from airborne wind measurements in three cumulus cases over land

“…Doppler wind LiDARs (DWLs) are the international standard for wind measurements and have been used for among other things 1) data assimilation experiments (Horányi et al, 2015;Pu et al, 2017;George et al, 2021, e.g.,), 2) to study for instance turbulence, gravity waves, orographic effects (Yuan et al, 2020;Gisinger et al, 2020;Baidar et al, 2020, e.g.,), and 3) to monitor the flow in wind farms (Käsler et al, 2010;Wagner et al, 2017;Zhan et al, 2020;Schneemann et al, 2021, e.g.,). The coherent detection DWL employed in this study has a wavelength of 2022.54 nm (approximately 2 µm), being eye-safe and operating in the Rayleigh scattering regime.…”

Intermittency of gravity wave potential energies and absolute momentum fluxes derived from infrared limb sounding satellite observations

“…Further, it has been shown that mountain waves can be advected over large distances, and in the stratosphere do not necessarily occur over the mountain range where they were excited (e.g., Krisch et al, 2017). Still, mountain waves are often observed near their sources, for example over Scandinavia (e.g., Doernbrack and Leutbecher, 2001;Gisinger et al, 2020). Therefore, to include also a region in the Northern Hemisphere wave sources (Southern Ocean and northern hemisphere polar night jet regions) and PDFs that are more strongly dominated by orographic sources (South America region).…”

Superposition of gravity waves with different propagation characteristics observed by airborne and space-borne infrared sounders

“…4). To study the interaction of the GWs with the tropopause by in situ observations (Gisinger et al, 2020), two flight legs were positioned below (legs 1 and 2) and two flight legs above the tropopause (legs 3 and 4). Both lower legs were performed at 61 • N (leg 1 from point A to point B and leg 2 from point B to point A) and were mainly dedicated to in situ and water vapour observations by BAHAMAS and WALES.…”

“…So far several measurement techniques have been developed to fully characterise gravity waves. For example, in situ measurements of close-to-vertical profiles taken by radiosondes, dropsondes, or falling spheres can be analysed using hodograph analysis, the Stokes method, or a combination of wind and temperature measurements to fully characterise gravity waves (Eckermann and Vincent, 1989;Guest et al, 2000;Wang and Geller, 2003;Zhang et al, 2014), though sampling errors of nearly vertical profiles may introduce biases (e.g. Vosper and Ross, 2020).…”

“…Furthermore, techniques based on horizontal 1-D measurements, e.g. from aeroplanes or super-pressure balloons, have also been used to derive gravity wave characteristics (Hertzog et al, 2008;Fritts et al, 2016;Gisinger et al, 2020). All these methods rely on the polarisation and dispersion relation to infer the wave structure and usually do not show the 3-D distribution and spatial change in wave characteristics.…”