Gravity-Wave Emission by Spontaneous Imbalance of Baroclinic Waves in the Continuously Stratified Rotating Annulus

We use a finite-volume model of the classic differentially heated rotating annulus experiment to study the spontaneous emission of gravity waves (GWs) from jet stream imbalances, which is a major source of these waves in the atmosphere for which no satisfactory parameterization exists. Atmospheric observations are the main tool for the testing and verification of theoretical concepts, but given their specific potential for yielding reproducible data and for studying process dependence on external system parameters, laboratory experiments are an invaluable complementary tool. Experiments with a rotating annulus exhibiting a jet modulated by large-scale waves due to baroclinic instability have already been used to study GWs. These have, however, either considered a two-layer set up or the identified GWs were found to be due to a boundary-layer instability. We here suggest the investigation of GWs in a wide and shallow, continuously stratified, annulus with relatively large temperature difference between inner and outer cylinder walls. This configuration is more atmosphere-like than the classic set up. In contrast to there the Brunt-Vaisala frequency is larger than the inertial frequency so that the qualitative dependence of the GW frequency on the phase-propagation direction is reproduced. Various analyses of our simulations suggest a distinct gravity wave activity. To identify regions of GW emission we decompose the flow into the geostrophic and ageostrophic part through the inversion of the quasi-geostrophic potential vorticity. The analysis of the geostrophic sources of the ageostrophic flow indicates that, in addition to boundary layer instabilities, spontaneous imbalance in the jet region acts as an important source mechanism.