Thomas von Larcher

An experimental study of regime transitions in a differentially heated baroclinic annulus with flat and sloping bottom topographies

Abstract. A series of laboratory experiments has been carried out in a thermally driven rotating annulus to study the onset of baroclinic instability, using horizontal and uniformly sloping bottom topographies. Different wave flow regimes have been identified and their phase boundaries – expressed in terms of appropriate non-dimensional parameters – have been compared to the recent numerical linear stability analysis of von Larcher et al. (2013). In the flat bottom case, the numerically predicted alignment of the boundary between the axisymmetric and the regular wave flow regime was found to be consistent with the experimental results. However, once the sloping bottom end wall was introduced, the detected behaviour was qualitatively different from that of the simulations. This disagreement is thought to be the consequence of nonlinear wave–wave interactions that could not be resolved in the framework of the numerical study. This argument is supported by the observed development of interference vacillation in the runs with sloping bottom, a mixed flow state in which baroclinic wave modes exhibiting different drift rates and amplitudes can co-exist.

Results from the German Priority Program SPP 1276 “Multiple Scales in Fluid Mechanics and Meteorology” (MetStröm), Part II.

The special issue 6/2014 of the Meteorologische Zeitschrift is dedicated to contributions from the German Priority Program SPP 1276 “MetStröm”. Information about this program was already provided in the editorial by Klein et al. (2014) prefacing the nine papers published in the first special issue on a variety of topics investigated within MetStröm earlier this year. In this issue, a second set of six papers is presented with four of them focusing on research activities around the benchmark experiments in the “Rotating Annulus”, which were performed at the Brandenburg Technical University at Cottbus. These are the contributions by Borchert et al. (2014): Finite volume models with implicit subgrid-scale parameterisations for the heated rotating annulus, by Hoff et al. (2014): Empirical singular vectors of baroclinic flows deduced from experimental data of a differentially heated rotating annulus, by von Larcher and Dörnbrack (2014): Numerical simulations of baroclinic driven flows in a thermally driven annulus using the immersed boundary method and by Vincze et al. (2014): Benchmark in a rotating annulus: a comparative experimental and numerical study of baroclinic wave dynamics. The paper by Queck et al. (2014): TurbEFA: an interdisciplinary effort to investigate the turbulent flow across a forest clearing is related to the field experiment on turbulence within a forest canopy. Finally, the article by Dietze et al. (2014): Controlling entrainment in the smoke cloud using level set-based front tracking is concerned with the subtopic “Two-Phase Flows/Convection”, which is one of the three sub-topics in MetStröm as mentioned in Klein et al. (2014). The last papers of this MetStröm series will appear as part of the next regular issue of Meteorologische Zeitschrift and will contain topics on semi-convection, turbulence effects in clouds, tropical cyclones and subgrid scale modelling.

Dynamics of baroclinic wave pattern in transition zones between different flow regimes

Baroclinic waves, both steady and time-dependent, are studied experimentally in a differentially heated rotating cylindrical gap with a free surface, cooled from within. Water is used as working fluid. We focus especially on transition zones between different flow regimes, where complex flow pattern like mixed-mode states are found. The transition from steady wave regime to irregular flow is also of particular interest. The surface flow is observed with visualisation techniques. Velocity time series are measured with the optical laser-Doppler-velocimetry technique. Thermographic measurements are applied for temperature fleld visualisations.