Exploration of Anisotropy and Inhomogeneity of Ocean Boundary Layer Turbulence

University Of Washington, Seattle WA

Investigators

Abstract

Ocean Surface Boundary Layer is a turbulent region that governs the interaction between air-sea fluxes and the ocean's deeper layers. Since the regional and global numerical models cannot fully resolve OSBL processes, simplified parameterization schemes are used to approximate them. Recent evidence suggests that discrepancies between different parameterizations and other errors may result from the limited and idealized representation of physics, such as the assumption that small-scale turbulence is uniform by direction (isotropy) and location (homogeneity). OSBL turbulence is anticipated to deviate from this idealized model due to the influences of shear, stratification, and waves. This project will provide the first direct observational study of anisotropy and inhomogeneity of OSBL turbulence with measurements from acoustic instruments that drift with the water flow. These custom instruments will have acoustic beams pointing horizontally, vertically, and diagonally to carry out direct and simultaneous observations of turbulence statistics in multiple directions, encompassing scales from centimeters to tens of meters, thereby resolving their anisotropy and inhomogeneity. The observations from this project will produce both an empirical insight and a comprehensive dataset that could be used for development, refinement, and benchmarking of simulations and turbulence closures. Thus, this project will contribute to the evolution of our capacity to forecast the dynamics of the Earth's system across scales ranging from local phenomena to the broader spectrum of global climate dynamics. Understanding the inhomogeneity and anisotropy of ocean turbulence will clarify the errors, biases, and limitations associated with various techniques used to characterize turbulence and lead to the development of corrections for existing observations and better future observational practices. This project will support a number of complementary Applied Physics Laboratory outreach efforts aimed at general broadening of public awareness and participation in science. It will provide dedicated line-item support for APL STEM Development Program Coordinator to develop ongoing and future outreach projects at APL, facilitate connections to local schools as well as undergraduate students attending UW, surrounding community colleges, and technical schools. This project will deploy two Lagrangian floats, each equipped with a pair of customized Acoustic Doppler Current Profilers (ADCPs) providing a total of 10 acoustic Doppler beams on each float pointing horizontally, vertically, and diagonally. These beams, used in combinations, will gather multiple directional second- and third-order statistics of velocity fluctuations, thereby estimating how turbulence energy and energy flux is partitioned across scales and directions. The observations will be conducted in a turbulent tidal channel and at the Ocean Climate Station P site (North Pacific) providing a variety of forcing conditions. The primary impact of the project will be an enhanced understanding of the extent to which ocean boundary layer turbulence deviates from the idealized model of homogeneity and isotropy. A more comprehensive description of ocean turbulence will lead to a deeper understanding of the complex processes that drive and govern turbulence in the ocean. It will help clarify the mechanisms responsible for turbulence generation, dissipation, and energy transfer, leading to a more comprehensive understanding of the dynamics of oceanic flows. The project will specifically address a number of outstanding scientific questions, such as: What is the extent of the isotropic inertial subrange in OSBL turbulence? What is the magnitude and distribution of anisotropy? Can anisotropy be predicted and parameterized? How does anisotropy affect the measurements and parameterization of ocean turbulence? This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.