The Quest for the Gravity Wave Source

Apart from orography, no specific sources of parametrized gravity waves are considered in most global circulation models. This is an inadequate generalization in the long run. In 2002, Charron and Manzini stated that no global gravity wave source climatology exist, and in the meantime, little has been done to address this observationally. A single observational case over Greenland is used to illustrate how difficult it is to disentangle the source of a gravity wave. The observation was made during the POLar STRAtosphere in a Changing Climate (POLSTRACC) campaign on 10 March 2016. The campaign was based in Kiruna, Sweden and investigated polar air during the polar vortex breakdown in spring. The gravity wave was observed between 10 and 15 km, with a horizontal wavelength of around 300 km and a vertical wavelength of around 2 km. Several plausible source mechanisms were present at the time of observation, a breaking Rossby wave, jet exit region, cold front, strong wind shear, and topography. POLSTRACC observations, ECMWF high resolution analysis and ERA 5 reanalysis were used to find the gravity wave source. Observations were obtained by the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument based on the High Altitude Long Range (HALO) German research aircraft. GLORIA is an infrared spectrometer that measures many trace gasses in the mid-infrared frequencies. Radiance is changed by inverse-modelling to temperatures. The temperature structure of the gravity wave was tomographically reconstructed. The 3D observations correlate well with the model data. ERA 5 data and the Gravity-wave Regional Or Global Ray Tracer (GROGRAT) was used to determine the exact location of where the gravity wave was emitted. Evidence exists that the gravity wave could have been released by the geostrophic imbalance in the jet. In contrast, dedicated ECMWF model runs suggest that the origin of the gravity wave is in fact orography. However, the Scorer parameter suggests no gravity wave can propagate from the surface upwards. Two ECMWF model runs were used to prove this, one normal operational model run, and the second, with a ‘flat’ orography. This work indicates that care needs to be exercised to diagnose the source of gravity waves, especially without a full informational analysis. Our study illustrates that a better parametrization scheme of gravity wave sources should be included in models for a more realistic representation.