Numerical investigation and validation of stratified up-side down Langmuir turbulence

分层倒置朗缪尔湍流的数值研究和验证

• 批准号: 2124611
• 负责人: Ramsey Harcourt
• 金额: $ 43.67万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2124611&HistoricalAwards=false
• 关键词: Numerical; investigation; validation; stratified; up

This project will investigate wind-aligned circulation features in the ocean upper layer during conditions of low winds and strong (typically rain-induced) stratification. Under these conditions, such features do not conform to expectations for either typical Langmuir turbulence or wind-driven viscous sheer induced structures. The hypothesis is that under appropriate forcing conditions, stratified shear flow at the base of a surface-trapped jet becomes unstable to convergence and upward displacement in the presence of surface-intensified Stokes shear. This drives upwelling of slower, denser water towards the surface, and recirculation. Numerical modeling and further analysis of existing data will be used to test the hypothesis that the observed streaks in question arise from this process of ‘stratified upside-down Langmuir turbulence’ (SUDL-T). The proposed activity will lead to better parameterizations of mixing at the ocean surface, with broad relevance for air-sea fluxes of heat, gas, and momentum. The project will support a PhD graduate student, contribute to local public outreach and educational programs, and have broad dissemination through presentations at conferences, publication in peer-reviewed journals, and public availability of validated numerical simulations of near-surface mixing. Infrared imagery of the sea surface during periods of modest winds and strong near-surface salinity stratification from rain shows wind streaks at spacings that suggest they are generated by a different mechanism than classic Langmuir circulations or wind-driven surface stress. The project objectives are to investigate this potentially newly observed mechanism by: 1) characterizing the vertical scale, kinematics, and energetics of the secondary circulation associated with the wind streaks as a function of environmental conditions; 2) determining the mechanism responsible for the observed elevated crosswind length scales at low wind speeds and the balance of forcing associated with transition to smaller scales and Langmuir turbulence in moderate wind; and 3) evaluating the impact of the secondary circulation at low winds on near-surface vertical mixing. To do this the project will use a combination idealized and realistic Large Eddy Simulation modeling, coupled to further data analysis and direct model-data comparison. Modeling will include suites of runs from spin-up, or increasing forcing from low to moderate ranges, and spin-down, the relaxation of forcing through similar forcing levels. Reanalysis of the observations will focus on determining the surface forcing and subsurface hydrography and shear during the observations, in order to form a more stable basis for validating the project hypothesis with a more realistic modeling suite and direct model-data comparison.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.

该项目将研究低风和强烈（通常是降雨引起的）分层条件下海洋上层的风向环流特征。在这些条件下，这些特征不符合典型朗缪尔湍流或风驱动粘性剪切诱发结构的预期。假设是，在适当的强迫条件下，在表面强化斯托克斯剪切的情况下，表面截留射流底部的分层剪切流变得不稳定，无法收敛和向上位移。这推动了速度较慢、密度较大的水向地表上升并进行再循环。数值模型和对现有数据的进一步分析将用于检验以下假设：所观察到的条纹是由“分层颠倒朗缪尔湍流”（SUDL-T）过程产生的。拟议的活动将导致海洋表面混合的更好参数化，与热、气体和动量的海气通量具有广泛的相关性。该项目将支持一名博士研究生，为当地公共宣传和教育项目做出贡献，并通过在会议上的演讲、在同行评审期刊上发表文章以及向公众公开经过验证的近地表混合数值模拟来进行广泛传播。在微风和雨水造成的强烈近地表盐度分层期间，海面的红外图像显示出风条纹的间隔，这表明它们是由与经典朗缪尔环流或风驱动的表面应力不同的机制产生的。该项目的目标是通过以下方式研究这一潜在的新观察到的机制：1）描述与风纹相关的二次环流的垂直尺度、运动学和能量学作为环境条件的函数； 2) 确定低风速时观测到的侧风长度尺度升高的机制，以及与向较小尺度过渡和中风时朗缪尔湍流相关的强迫平衡； 3) 评估低风时二次环流对近地表垂直混合的影响。为此，该项目将结合使用理想化和现实的大涡模拟建模，并结合进一步的数据分析和直接模型数据比较。建模将包括从自旋向上（或从低到中等范围增加强迫）和自旋向下（通过类似强迫水平放松强迫）的一系列运行。对观测结果的重新分析将侧重于确定观测过程中的地表强迫、地下水文学和剪切力，以便通过更现实的建模套件和直接的模型数据比较来为验证项目假设奠定更稳定的基础。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。