Wave effects in upper ocean turbulence models

风是上层海洋的主要驱动力。在有陡浪的情况下，海洋表层湍流混合的结构和强度变化很大，现有的数值模式的预测并不总是很准确。本研究旨在改善上层海洋混合数值模型中表面波效应的表现，并扩大根据混合层湍流测量验证这些模型的经验基础。来自自由漂流仪器的现有数据，这些仪器在上层海洋的混合层上下移动，将被分析，以产生关于大范围波浪条件下湍流垂直结构的新信息。以非常高分辨率数值模式模拟的虚拟漂浮物将用于发展新的分析方法和增强有限的观测数据集。该研究将通过扩展和改进模型数据比较的度量来帮助在波浪-湍流相互作用的竞争理论之间进行比较。对波浪-湍流相互作用的深入了解将导致新的混合参数化，这可以通过小尺度湍流测量来验证，并最终提高海洋环流模型的技能。漂移测量和模型代码的传播也将有利于其他参数化波浪效应的方法。改进的闭合模型再现观测到的混合层湍流各向异性以及动量和标量剖面的均匀性的能力，将在许多物理和生物地球化学海洋领域引起兴趣，从气候建模到沿海沉积物运输和溢油预测。通用海洋湍流模式（GOTM）在广泛使用的建模框架中进行了改进，将促进广泛的海洋建模者使用这些结果。该项目将促进工程学和地球物理学之间独特联系的研究，并支持研究生助理从事漂流数据分析和模型数据比较的工作。该项目将为调查人员通过公立学校教室和科学展览进行的个人推广活动提供新的材料。第二个矩闭（SMC）模型的发展建立在最近的进展基础上，包括Craik-Leibovich （CL）涡强迫完全处于“准平衡”SMC中，采用动量通量闭合，其中一个分量沿着Stokes漂移梯度和非均匀近地表压力-应变闭合。CL强迫雷诺应力项和新的湍流闭包将应用于修改更大类别的“弱平衡”SMCs，用于Langmuir湍流。这些将在GOTM框架中实施和评估，包括Langmuir湍流和表面波破碎的模型相互作用。对现有高质量拉格朗日浮子数据的新分析将提供动能分量、湍流长度尺度、大涡动能耗散率和近地表拉格朗日统计数据的剖面，这些数据可以区分边界层湍流的表面波强迫理论。新的大涡模拟（LES）将包含嵌入的虚拟浮子，并将包括在浮子和模型分辨尺度上随机波浪破碎的强迫。利用这种LES，研究人员将开发新的数据分析方法，并指导SMC对波浪强迫的修正。CL强迫的新近地表闭合将与SMC中具有非局部梯度闭合的对流的湍流协方差的浮力产生的类似处理相结合。LES结果和浮动数据将用于验证和调整SMC模型。修改后的SMC模型代码和浮动数据测试用例将通过GOTM框架分发给公众。

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.