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

In order to improve global atmospheric modelling, the trend is towards including specific gravity wave (GW) source processes in general circulation models. Validating such approaches requires to associate observed GWs with source processes. In a case study, we search for the source of a GW observed over Greenland on 10~March~2016 within the framework of a HALO research aircraft campaign. Measurements were taken with the remote sensing GLORIA instrument, which images in the spectral range 780 -- 1400\,\si{cm^{-1}}. The measured infrared limb radiances are converted into a 3D observational temperature field through the use of limited-angle tomography. We observe GWs along a transect through Greenland where the GW packet covers $\approx$ 1/3 of the Greenland mainland. GLORIA observations indicate the GW between 10 and 13\,\si{km} height with a horizontal (vertical) wavelength of 330\,\si{km} (2\,\si{km}). A large temperature amplitude of 4.5\,\si{K} is observed. Slanted phase fronts indicate intrinsic propagation against the jet. Analysis shows the GW packet has a ground-based propagation with the wind and is arrested below a critical layer caused by the upper limit of the jet. Compared to its intrinsic horizontal group velocity (6 -- 38\,\si{ms^{-1}}) the GW packet has a slow vertical group velocity of 0.05 -- 0.2\,\si{ms^{-1}}.To find the GW source, 3D GLORIA observations, GROGRAT raytracer, ERA~5 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 with one having enhanced WKB values within the jet imbalance. The difference between the full and a smooth-topography experiment is investigated to reveal the missing connection between the topography and the jet imbalance. We find that this is synoptic-scale vorticity produced by compression of air above Greenland and the formation of evanescent inertia-GWs. These accelerate and decelerate the jet and cause the geostrophic imbalance, which excites GWs by spontaneous adjustment. We present the first observational evidence of GWs by this topography-jet mechanism to our knowledge. This mechanism could cause quite frequently spontaneous imbalance GWs, which by pure spatial correlation easily could be mixed up with classical mountain waves despite quite different propagation characteristics