Characterising ‘Zonostrophic flow’ in b-plane Geostrophic Turbulence
| Project acronym: | |
| Name of Group Leader: | P. Read, Atmospheric, Oceanic & Planetary Physics, Clarendon Laboratory, Oxford, UK, p.read1-at-physics.ox.ac.uk |
| User-Project Title: | Characterising ‘Zonostrophic flow’ in b-plane Geostrophic Turbulence |
| Facility: | Coriolis platform |
| Proceedings TA Project: | Eddy-driven jets and vortices in convectively forced geostrophic turbulence on a topographic beta-plane |
| Data Management Report: | Data Management Reports Page You will need to login to view this page |
Summary:
This project was a direct follow-on to an earlier project, carried out in 2002, to investigate the dynamics of convectively-driven geostrophic turbulence on a topographic b-plane, particularly concerning the possible formation of banded zonal jets through anisotropic nonlinear interactions with eddies. Such processes are believed to underlie the formation of the banded circulation of the outer gas giant planets, and possibly also banded zonal features in the terrestrial global oceans.
The main objectives of both projects were
- To establish the occurrence and robustness of eddy-driven jet structures,
- To quantify the parametric dependence of the equilibrated flow structures and energy spectra, including the energy injection process, and
- To understand the impact of scaling effects of various parameters on the zonal flow.
- To investigate details of the rotation-modified convection and eddy formation processes.
In the current project, convective turbulence was to be obtained via electrical heating at the conically-sloping lower boundary, and to carry out a series of long duration experiments to measure the equilibrated flow structures and associated turbulence statistics.
Achievements
The data acquisition phase of the project was completed in late December 2007, and detailed analysis of the substantial volume of data is still ongoing at the time of writing. During the experimental period (November-December 2007), the experimental configuration was successfully commissioned and tested, including the implementation of an 11kW heating element placed under the sloping bottom surface – the first time a thermal convection experiment has been conducted at Coriolis. This facility worked well for most of the acquisition period, though the transformer developed a fault in week 3 and had to be replaced.
In addition to a short run without rotation, five long experimental sequences were successfully carried out during the acquisition phase, each maintaining steady convection and rotation for a continuous period of at least 4-5 days. This enabled us to cover a wide range of parameter space, in particular for values of bL2/U from 40 to 400 (where L = channel width = 4.5m and U = rms convective velocity 2.3 mm s-1). Long sequences of high resolution CIV measurements of horizontal velocity fields were obtained in each case, within which clear indications of multiple parallel jet formation were apparent with a radial scale that decreased systematically with increasing values of bL2/U. Evidence for baroclinic instability was also apparent in the inner (unheated) part of the tank, and some further velocity fields of this region were also acquired for further study. Vertical temperature profiles were typically measured in two locations across the tank, and a sequence of combined temperature and velocity measurements were made in one experiment in an attempt to quantify horizontal heat transport. Some additional flow visualizations were also made in one experiment using a thermographic camera, which revealed the thermal signatures of evolving eddies at the water surface. Some velocity measurements were made in vertical slices to study the vertical structure of the convection, though the flow was found to be strongly barotropic on large scales. Dye visualizations revealed the presence of highly intermittent, compact convective vortices, rising from the heated bottom, that constitute the main mechanism for energizing the turbulence. Analysis is now under way to determine the energetics and vorticity dynamics of the eddies and zonal jets, and to quantify the upscale energy transfers and potential vorticity exchanges within the flow.
Finally, it is noteworthy that a film crew also visited Coriolis in December to record views of this experiment as part of a forthcoming production (Naked Science: Saturn) for the National Geographic TV Channel, provisionally scheduled for release in Autumn 2008. Click here for further information about this project on the CNRS website.