CAREER: Internal Waves, Turbulence and Diapycnal Mixing in Oceanic Flows

Colorado State University, Fort Collins CO

Investigators

Abstract

The oceanic internal wave field is widely accepted to be one of the most dominant sources of energy for the sustenance of small-scale ocean turbulence. The advancement of the current state of knowledge on internal waves, turbulence and diapycnal (cross-isopycnal) mixing in oceanic flows is critical for the accurate parameterization of the processes that control the dynamics of these flows. A key example concerns the parameterization of mixing processes in ocean general circulation models, where long term predictions are highly sensitive to mixing parameterizations that can result in uncertain predictions of the ocean's role in climate change. A key related question is how should the small-scale mixing effects be incorporated into models of mixing and dispersion in stably stratified turbulence? These are fundamental research questions that the oceanographic community need to address in order to enhance the predictive capability of large scale circulation models. The subject of this CAREER project is on dissipation of internal waves linked closely to turbulence and diapycnal mixing. The primary theme and vision of this proposal is to improve understanding and modeling of geophysical flows that will lead to new generalized formulations for turbulent fluxes of momentum and scalars. A holistic approach that integrates basic and applied multi-disciplinary research with a strong educational program is proposed. In particular, the key research objectives of this CAREER project are to obtain new fundamental insights into the details of the turbulent mixing processes in geophysical stratified flows that will translate into simple effective parameterizations for use in large scale numerical models and to bridge the gap between parameterizations/models for turbulent mixing to be developed (and already developed) by the PI from fundamental numerical simulations and laboratory experiments to geophysical scale models by addressing up-scaling issues via novel model-data comparisons. To this end, high-resolution direct numerical simulations (DNS) and large-eddy simulations (LES) coupled with particle tracking of forced stably stratified turbulence and internal wave induced turbulence in Knight Inlet sill will be performed in order to gain insight into small-scale processes and develop/extend models for mixing. Second, model-data comparisons will be performed using relevant turbulence and wave measurements in the ocean that will be available through collaborations forged with field scale oceanographers. A third aspect of this research is to formulate turbulence closure schemes for use in Reynold-Averaged Navier-Stokes (RANS) models. The intellectual merit of this proposal is based on the generation of new knowledge and concepts for describing mixing and dispersion in stratified geophysical flows. State of the art turbulence simulations coupled with particle tracking will be used to build on the principal investigator's prior innovative steps to identify new ways for elucidating mixing mechanisms using a combination of Eulerian and Lagrangian analysis of fluid motion. In particular, new turbulence diagnostic tools for the separation and quantification of irreversible momentum and scalar fluxes associated with the energetics of these flows will be developed. The proposed work will seek to formulate a seamless approach to parameterize the intermingled dynamics resulting from the co-existence of internal waves and turbulence in stratified geophysical flows. The insights gained from this study will significantly improve our understanding of the key dynamical features of stably stratified flows which are ubiquitous in the ocean. The insights gained in concert with theoretical ideas will form the basis to develop better parameterizations for mixing that will be tested in non-homogeneous turbulent flows in large-scale water column models. Another novel aspect of the research methodology proposed is the use of "real world" field-scale case studies via model-data comparisons to test and provide feedback on the efficacy of mixing models thus allowing for ongoing refinement of the models. The broader impacts of this research come from the broad applicability of improved mixing parameterizations in oceanography, atmospheric science and engineering, especially concerning climate change, environmental sustainability, renewable energy and national security. Broader impact also comes from the strong integrated educational and outreach program. The principal investigator will establish an environmental fluid dynamics program at Colorado State University (CSU) with an emphasis on empowering students with skill sets required to solve complex geophysics and engineering related problems in multi-disciplinary settings. Research and education are integrated through two new graduate courses in environmental fluid mechanics. The principal investigator will recruit and train underrepresented minority and female students in the field of environmental fluid mechanics. This project will support the training of two PhD students (at least one of whom will be a female student). Outreach efforts include the principal investigator's involvement in summer camps/programs coordinated by the Women and Minorities in Engineering Program (WMEP) at CSU as well as through outreach to a local high school. The results of this project will be widely disseminated through premier international journal articles in physical oceanography and fluid mechanics as well as through seminars and presentations at scientific conferences and a dedicated website.