Preprints
https://doi.org/10.5194/acp-2020-1289
https://doi.org/10.5194/acp-2020-1289

  21 Jan 2021

21 Jan 2021

Review status: this preprint is currently under review for the journal ACP.

Orographically-Induced Spontaneous Imbalance within the Jet Causing a Large Scale Gravity Wave Event

Markus Geldenhuys1,2, Peter Preusse1, Isabell Krisch3, Christoph Zülicke4, Jörn Ungermann1, Manfred Ern1, Felix Friedl-Vallon5, and Martin Riese1 Markus Geldenhuys et al.
  • 1Forschungszentrum Jülich, Institute of Energy- and Climate Research, Stratosphere (IEK-7), Jülich, Germany
  • 2South African Weather Service, Private Bag X097, Pretoria 0001, South Africa
  • 3Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 4Leibniz Institute of Atmospheric Physics, University of Rostock, Kühlungsborn, Germany
  • 5Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research - Atmospheric Trace Gases and Remote Sensing (IMK-ASF), Karlsruhe, Germany

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 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 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 ms−1) the GW packet has a slow vertical group velocity of 0.05–0.2 ms−1. This causes the GW packet to propagate long distances while spreading over a large area while remaining constrained to a narrow vertical layer. Not only orography is a plausible source, but also out of balanced winds in a jet exit region and wind shear. To identify the GW source, 3D GLORIA observations are combined with a gravity wave raytracer, 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 raytracing 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.

Markus Geldenhuys et al.

Status: open (until 18 Mar 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2020-1289', Anonymous Referee #1, 16 Feb 2021 reply

Markus Geldenhuys et al.

Markus Geldenhuys et al.

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Short summary
Gravity waves are a very important driver for circulations in the middle atmosphere influencing surface weather and climate studies. The gravity waves proposed in this manuscript comes from a new jet-topography mechanism. The topography is observed to force the jet out of balance, which then releases the gravity waves through spontaneous adjustment. This topography-jet interaction was not previously considered by the community, rendering the impact of the gravity waves largely unaccounted for.
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