EGU21-5591, updated on 04 Mar 2021
https://doi.org/10.5194/egusphere-egu21-5591
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

A new mechanism for spontaneous imbalance exciting large-area gravity waves

Markus Geldenhuys1,2,3, Peter Preusse1, Isabell Krisch4, Christoph Zülicke5, Jörn Ungermann1, Manfred Ern1, Felix Friedl-Vallon6, and Martin Riese1
Markus Geldenhuys et al.
  • 1Forschungszentrum Jülich, Institute of Energy- and Climate Research, Stratosphere (IEK-7), Jülich, Germany (m.geldenhuys@fz-juelich.de)
  • 2South African Weather Service, Port Elizabeth Forecasting Office, Port Elizabeth, South Africa (markusgeld@gmail.com)
  • 3Bergische Universität Wuppertal, Atmospheric Physics, Wuppertal, Germany
  • 4Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 5Leibniz Institute of Atmospheric Physics, University of Rostock, Kühlungsborn, Germany
  • 6Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research - Atmospheric Trace Gases and Remote Sensing (IMK-ASF), Karlsruhe, Germany

In order to improve global atmospheric modelling, the trend is towards including source-specific gravity waves (GWs) in general circulation models. In a case study, we search for the source of a GW observed over Greenland on 10 March 2016 using the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) onboard the German research aircraft HALO. GLORIA is a remote sensing instrument where the measured infrared radiances are converted into a 3D temperature field through tomography. 
We observe a GW packet between 10 and 13km that covers ∼1/3 of the Greenland mainland. GLORIA observations indicate a horizontal (vertical) wavelength of 330km (2km) and a temperature amplitude of 4.5K. Slanted phase fronts indicate intrinsic propagation against the jet but the GW packet propagates (ground-based) with the wind. To find the GW source, 3D GLORIA observations, GROGRAT raytracer, ERA5 data, and an ECMWF numerical experiment are used. The numerical experiment with a smoothed topography indicates virtually no GWs suggesting that the GW field in the full model is caused by the orography. However, these are not mountain waves. A favourable area for spontaneous GW emission is identified within the jet exit region by the cross-stream ageostrophic wind speed, which indicates when the flow is not in geostrophic balance. Backtracing experiments (using GROGRAT) trace into the jet and imbalance regions. The difference between the full and the smooth-topography experiment is the change in wind components by the compression of air above Greenland. These accelerations and decelerations in the jet cause the jet to become out of geostrophic balance, which excites GWs by spontaneous adjustment. We present, to the best of our knowledge, the first observational evidence of GWs by this topography-jet mechanism.

How to cite: Geldenhuys, M., Preusse, P., Krisch, I., Zülicke, C., Ungermann, J., Ern, M., Friedl-Vallon, F., and Riese, M.: A new mechanism for spontaneous imbalance exciting large-area gravity waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5591, https://doi.org/10.5194/egusphere-egu21-5591, 2021.