PROJECT SUMMARY REPORT

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Project Title	Mean flow generation/modification in beta-plane geostrophic turbulence
Name of Group Leader	Dr Peter L Read
Home Laboratory	Dept. of Physics, Clarendon Laboratory, University of Oxford, UK
E-mail address	p.read1@physics.ox.ac.uk
Telephone	+44 1865 272082

1. Project objectives (no more than 10 lines)

The experiments were aimed at the quantitative investigation of the generation and/or modification of zonal flows in continuously forced geostrophic turbulence on a topographic beta-plane in cylindrical geometry. The intention was to seek evidence for spatial anisotropies promoted by the presence of a large-scale radially-sloping bottom under conditions where (U/beta)^1/2 was small compared with the radial dimensions of the tank. Such conditions are difficult to achieve in small tanks without requiring extremely rapid rotation or being limited to relatively small Reynolds numbers. A new conical sloping boundary was to be constructed covering the entire area of the Coriolis tank. Experiments were to be run using near-continuous (salt-generated) convective forcing for sufficiently long to establish an equilibrium turbulence field, and to measure the resulting mean flow profile and eddy statistics (rms velocity, Reynolds stress etc.).

2. Main achievements and difficulties encountered (no more than 20 lines)

All main objectives were achieved, and a detailed series of measurements of velocity fields and density profiles obtained over a reasonable range of conditions. A topographic beta-plane was successfully constructed using a PVC fabric sheet stretched between rigid frames at the outer wall of the tank and attached to the central post. This novel method proved feasible as a means of providing a large-scale topographic slope, despite some difficulties with reliability of the attachment of the outer frame to the walls of the tank. Convective forcing was also achieved via a novel technique, in which a uniform flux of dense, salty water was sprayed continuously along a radius arm, which was rotated around the tank every 40-70s. Uniformity of flux (+/- 10%) was obtained by careful spacing of the nozzles in radius, and the method generally worked well for periods of up to 6 hours continuous running, though some difficulties were encountered with occasional blockages and excitation of surface waves and/or inertial wave resonances. A series of experiments were completed covering a range of at least 50 in buoyancy flux, two slopes (6 degrees and zero), and two tank rotation periods (40 and 80s). Horizontal velocity fields were obtained at a single level for all these cases, using both wide-angle (6.5 x 4m) and narrow-angle (2.5 x 2.5m) cameras. Some cases at 5 levels were also measured, which demonstrated the barotropic nature of the induced flow. The convective excitation was also investigated using dye injection and CIV measurements in a vertical plane. Preliminary results indicate that the topographic beta-effect did induce significant zonal anisotropies and the generation of weak mean zonal flows on the (U/beta)^1/2 scale, with the appearance of several 'jets' across the tank. Without a slope, the flow was dominated by a small number of large discrete vortices, which may be on a scale comparable with the external deformation radius.