Wave effects in upper ocean turbulence models

University Of Washington, Seattle WA

Investigators

Abstract

Wind is the major driving force for the upper ocean. The structure and intensity of turbulent mixing in the surface layer of the ocean vary greatly in the presence of steep wind waves, and existing numerical models are not always very accurate in their predictions. This study seeks to improve the representation of surface wave effects in numerical models of upper-ocean mixing and expand the empirical basis for validating these models against mixed layer turbulence measurements. Existing data from freely drifting instruments that move up and down the well mixed layer of the upper ocean will be analyzed to yield new information about the vertical structure of turbulence under a wide range of wave conditions. Virtual drifters simulated in very high resolution numerical models will be used to develop new analysis methods and to enhance the limited observational data sets. The study will aid comparisons among competing theories on wave-turbulence interaction by expanding and refining the metrics of model-data comparisons. Insights on wave-turbulence interaction will lead to new mixing parameterizations, that can be validated against small-scale turbulence measurements, and ultimately enhance the skill of ocean circulation models. The dissemination of drifter measurements and model code will also benefit other approaches to parameterizing wave effects. The ability of the improved closure models to reproduce the observed anisotropy of mixed layer turbulence, and the homogeneity of momentum and scalar profiles, will be of interest in many physical and biogeochemical oceanographic fields, ranging from climate modeling to coastal sediment transport and oil spill prediction. Implementing the improvements in a widely used modeling framework, the General Ocean Turbulence Model (GOTM) will facilitate usage of these results by a broad range of ocean modelers. This project will promote study at a unique nexus between engineering and geophysics and supports a graduate research associate to work on drifter data analysis and model-data comparisons. The project will contribute new material to the investigators' ongoing individual outreach activities through public school classrooms and science fairs. The second moment closure (SMC) model developments build on recent advances that include Craik-Leibovich (CL) vortex forcing fully in a 'quasi-equilibrium' SMC, adopting a momentum flux closure with a component down the Stokes drift gradient and an inhomogeneous near-surface pressure-strain closure. The CL forcing Reynolds stress terms and new turbulence closures will be applied to modify the larger class of 'weak-equilibrium' SMCs for Langmuir turbulence. These will be implemented and evaluated in the GOTM framework, including the modeled interaction of Langmuir turbulence and surface wave breaking. New analysis of existing high-quality Lagrangian float data will provide profiles of kinetic energy components, turbulent length scales, large-eddy kinetic energy dissipation rates and near-surface Lagrangian statistics that can discriminate between theories for surface -wave forcing of boundary-layer turbulence. New large eddy simulations (LES) will contain embedded virtual floats, and will include forcing by stochastic wave breaking at float- and model-resolved scales. Using this LES, the researchers will develop new data analysis methods and guide SMC modifications for wave forcing. The new near-surface closure for CL forcing will be combined with similar treatments of buoyant production in of turbulent covariance in SMC for convection with nonlocal gradient closures. LES results and float data will be used to verify and tune SMC models. Modified SMC model code and float data test cases will be distributed for public availability via the GOTM framework.