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@ARTICLE{Geldenhuys:894025,
      author       = {Geldenhuys, Markus and Preusse, Peter and Krisch, Isabell
                      and Zülicke, Christoph and Ungermann, Jörn and Ern,
                      Manfred and Friedl-Vallon, Felix and Riese, Martin},
      title        = {{O}rographically induced spontaneous imbalance within the
                      jet causing a large-scale gravity wave event},
      journal      = {Atmospheric chemistry and physics},
      volume       = {21},
      number       = {13},
      issn         = {1680-7324},
      address      = {Katlenburg-Lindau},
      publisher    = {EGU},
      reportid     = {FZJ-2021-02992},
      pages        = {10393 - 10412},
      year         = {2021},
      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 13km altitude with
                      a horizontal wavelength of 330km, a vertical wavelength of
                      2km and a large temperature amplitude of 4.5K. 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
                      -- 72m/s) the GW packet has a slow vertical group velocity
                      of 0.05 -- 0.2m/s. 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.},
      cin          = {IEK-7},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-7-20101013},
      pnm          = {2112 - Climate Feedbacks (POF4-211) / 2A3 - Remote Sensing
                      (CARF - CCA) (POF4-2A3)},
      pid          = {G:(DE-HGF)POF4-2112 / G:(DE-HGF)POF4-2A3},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000672721000003},
      doi          = {10.5194/acp-21-10393-2021},
      url          = {https://juser.fz-juelich.de/record/894025},
}