Orographically induced spontaneous imbalance within the jet causing a large-scale gravity wave event

Abstract

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.

On gravity wave parameterisation in vicinity of low‐level blocking

“…Meaning a GW will form with a larger amplitude and more drag. Literature indicates that broad ridges can result in indirect GWs by compressing the air above (Geldenhuys et al, 2021; Trüb & Davies, 1995). This is another GW production mechanism that models will miss.…”

“…Trüb and Davies (1995) provide a good overview of GWs that forms over a broad ridge (classified as having a halfwidth of up to a few hundred kilometres) for wind flow of differing Rossby numbers. Geldenhuys et al (2021) introduces a new mechanism where a broad ridge (halfwidth ≈ 1500 km) pushes the tropospheric jet out of geostrophic balance as an indirect source for GWs. Depending on the model resolution, some of these GWs may be resolved, but that would require the blocking layer and the ridge to be adequately resolved.…”

On gravity wave parameterisation in vicinity of low-level blocking...

“…Meaning a GW will form with a larger amplitude and more drag. Literature indicates that broad ridges can result in indirect GWs by compressing the air above (Geldenhuys et al, 2021;Trüb & Davies, 1995). This is another GW production mechanism that models will miss.…”

“…Trüb and Davies (1995) provide a good overview of GWs that forms over a broad ridge (classified as having a halfwidth of up to a few hundred kilometres) for wind flow of differing Rossby numbers. Geldenhuys et al (2021) introduces a new mechanism where a broad ridge (halfwidth ≈ 1500 km) pushes the tropospheric jet out of geostrophic balance as an indirect source for GWs. Depending on the model resolution, some of these GWs may be resolved, but that would require the blocking layer and the ridge to be adequately resolved.…”

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

“…Many gravity wave sources are located in the troposphere and lower stratosphere. Some of the most relevant gravity wave sources are atmospheric flow over topography (e.g., McFarlane, 1987;Lott and Miller, 1997;Eckermann and Preusse, 1999;Kruse et al, 2022), deep convection (e.g., Fovell et al, 1992;Pfister et al, 1993;Piani et al, 2000;Song and Chun, 2005;Stephan et al, 2019a, b;Ern et al, 2022), and processes related to strong wind jets and fronts (e.g., Charron and Manzini, 2002;Zhang, 2004;Zülicke and Peters, 2006;Plougonven and Zhang, 2014;Kim et al, 2016;Wei et al, 2016;Geldenhuys et al, 2021). According to their sources, these waves are also called mountain waves (or orographic gravity waves), convective gravity waves, and jet-or front-generated gravity waves, respectively.…”