This project seeks to understand the fundamental dynamics of the turbulent motions commonly observed in atmospheric and oceanic flow, or more precisely in geophysical flows over a broad range of periodicities induced by stratification and rotation. The scales of motion found in fluid flow, identified by the amount of energy in fluid motions in each spatial scale, are strongly determined by the kinematic quantities which are conserved following the motion. This project uses computationally intensive integrations of Direct Numerical Simulation (DNS) models to elucidate the fundamental dynamics which determine the power spectrum of geophysical flows and the transfer of energy between scales.
A first-principles theory of the spectral distribution of energy in turbulent fluids has been a grand challenge in fluid dynamics since the inception of the topic. Progress towards such a theory could be transformative for the development of atmospheric and oceanic models, particularly for the development of closure schemes and turbulence parameterizations used in these models. In addition, the work will have a strong educational component, through the education of a graduate student and the mentoring of two postdoctoral researchers. In addition, a workshop is planned for the summer of 2012 on geophysical turbulence. Outreach will be conducted through the Cooperative program for Operational Meteorology, Education, and Training (COMET, at the National Center for Atmospheric Research), through the development of "learning objects" on turbulent flows, to be posted on the COMET website in English, French, and Spanish.
With increased computing resources available to model complex flows on geophysical (climate) scales, it becomes necessary to revisit simplifying assumptions. In particular the so-called 'quasi-geostrophic' (QG) approximation is currently the basis for modeling flows on account of its averaging over the small scale, fast motions which then would not need to be explicitly computed in a model. To test and demonstrate the limitations of this assumption, we performed a series of high resolution simulations of flows with three variable parameters that are also of fundamental interest to climate and geophysical flows, namely, rotation, stratification and aspect-ratio. Rotation effects naturally arise from the earth's rotation on its own axis. Stratification arises due to dense or cooler fluid in the ocean or atmosphere settling below lighter or warmer fluid. Aspect-ratio is the term used to describe how deep a body of fluid is relative to its horizontal extent. We take a step away from the QG assumption by retaining all small scale motions in a suite of computer simulations of flows which spanned a broad range in the three-parameter space described above. We showed that the small scale fast motions played a critical role in the modification of flow structures relative to QG predictions. Our results indicate that future models for geophysical flows must either compute the small scales explicitly or must parameterize their behavior so that their impact on flow evolution over geophysical timescales is accurately captured.
