A new atmospheric-like differentially heated rotating annulus configuration to study gravity wave emission from jets and fronts
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Significant inertia-gravity wave activity has been frequently observed in the vicinity of jet and front systems in the atmosphere. Although many studies have established the importance of these non-orographic sources, the mechanisms responsible for spontaneous wave emissions are still not fully understood. The complexity of the three-dimensional flow pattern and distribution of the sources over large areas point towards the need for laboratory experiments and idealised numerical simulations. These will help understand the correct interpretation of the fundamental dynamical processes in a simplified, but yet realistic flow. In this study, we emphasise the importance of using set-ups of the differentially heated rotating annulus experiment with a ratio between the buoyancy frequency N, and the Coriolis parameter f larger than one to investigate atmosphere-like emission of gravity waves from baroclinic jets. Indeed, in the atmosphere \(N/f\sim 100\), but for table-top size experiments this ratio is smaller than one, resulting in an unfavourable condition for the propagation of gravity waves. For this reason, we offer a newly built laboratory experiment supported by numerical simulations that allow \(N/f>1\). The conditions for gravity wave emission in this new configuration are examined in detail, and the first evidence of IGWs is reported. Moreover, we compare numerical simulations and experimental data focusing on the variations of the temperature T, and its effects on the buoyancy frequency N. It becomes clear, that despite the fact the global structure and baroclinic instability characteristics are very similar, the model and experiment show deviations in N with implications for gravity wave emission. Due to the complex horizontal structure of N, where the largest values occur along the baroclinic jet axis, the inertia-gravity waves in the experiment are observed to be trapped.
C.R. and U.H. thank Ludwig Stapelfeld, Robin Stöbel, Vilko Ruoff for technical support. Moreover, we thank the Spontaneous Imbalance group of MS-GWaves, (Lena Schoon, Christoph Zülicke) for support and fruitful discussions. C.R. and U.H. thank the German Research Foundation (DFG) for the support through the Spontaneous Imbalance project (HA 2932/8-1 and HA 2932/8-2), which is part of the research unit FOR 1898 Multiscale Dynamics of Gravity Waves (MS-GWaves). S.H. and U.A. thank the DFG for partial support through the research unit FOR 1898 MS-GWaves and through grants AC 71/8-1, AC 71/9-1, AC 71/10-1, AC 71/9-2, and AC 71/10-2. The authors are also grateful to the anonymous reviewers for their suggestions that helped to improve the paper.
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