Baroclinic Geostrophic Turbulence and Jets in the Laboratory

摘要

Baroclinic, geostrophic turbulence is random, chaotic flow characterized by significant vertical gradients in density (Bu " 1) in which rotation plays a major role (Ro " 1). In the presence of a large-scale background gradient of potential vorticity (a beta-effect ), a symmetry-breaking occurs which admits anisotropy in the system. These conditions form the fundamental dynamical basis of many natural geophysical flows on a planetary scale (Rh " 1), and even fairly simple models of these phenomena can exhibit quite complex behavior. One such aspect that is common to these flows is that of multiple, zonal jets. These are spontaneous flow structures characterized by fast East-West (azimuthal) motion.;This thesis describes the creation of multiple jets in the laboratory within a fully-stratified, baroclinically-forced fluid subject to rotation and a beta-effect. By carefully controlling the forcing parameters, we observe the transition between a single "classical" baroclinic wave and regimes that closely resemble conditions of natural planetary flows. Observing this transition in the laboratory shows that the proposed scaling arguments are valid and have predictive power in the case of multiple zonal jets in a baroclinic fluid. Spontaneous eddy forcing of the mean flow is shown to be the ultimate driving force of the jets, whose evolution is observed in time. A long-duration drift in the jet structure is observed and quantified to be an order of magnitude less than the Rossby wave phase speed.;A detailed quantitative analysis of the structure the flow field sheds further light. This is done through both a standpoint of the Eulerian flow fields, and the raw data associated with the (Lagrangian) tracks of neutrally buoyant particles within the flow. There is a significant transition of the power law scaling of Fourier spectra between dynamically significant scales. As the flow changes between experiments, the power law of the Eulerian spectra can change over particular scale ranges, which is direct evidence of a regime change. Access to raw Lagrangian tracks of the fluid allow a direct characterization of the flow field that is independent of the Eulerian. The structure function technique is introduced and shows a fundamental change in behavior between dynamic scales, and between experiments, in a way consistent with theory.;A preliminary analysis is carried out of an experiment studying the competing mechanisms of buoyancy and wind forcing present on a single zonal jet. This is simulated in a rotating annulus of fluid by imposing a radial temperature gradient across the annulus gap, while applying mechanical forcing at the surface through the differential rotation of a rigid lid in contact with a surface layer of oil. A radially-sloping bottom creates a fluid depth gradient and simple topography in the form of five regularly spaced meridional ridges creates azimuthally varying f/h contours that steer the first-order flow. By varying the strength of wind and thermal forcing on the fluid, several regimes of flow are produced. Analysis of the Eulerian field shows the response of zonal transport and eddy kinetic energy to these different forcing regimes.;The thesis concludes with a description of the development of an apparatus to push the observations into a more turbulent dynamical range. This includes information about the spin-up and maintenance of a large-scale sloping thermal gradient in the apparatus, as well as some preliminary results.