Collaborative Research: Airflow separations over wind waves and their impact on air-sea momentum flux

合作研究：风浪上的气流分离及其对海气动量通量的影响

• 批准号: 1458984
• 负责人: Tetsu Hara
• 金额: $ 35.52万
• 依托单位: University of Rhode Island
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-03-01 至 2020-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1458984&HistoricalAwards=false
• 关键词: Collaborative; Research; Airflow; separations; over

When a surface ocean wave field is not in equilibrium with local wind forcing, which is a common occurrence, the wind stress may deviate significantly from the bulk parameterization and require sea-state dependent parameterization. Recent modeling studies suggest that one of the most significant causes of the sea-state dependence is the enhanced form drag due to airflow separation over breaking waves, particularly at higher wind speeds. However, the uncertainties in these model results remain large because our understanding of airflow separation is limited. While it has been generally accepted that airflow separation only occurs over breaking waves, recent laboratory observations and Large Eddy Simulations (LES?s) show that transient separation-like flows (quasi-separations), characterized by high vorticity layers detached from the wave surface and separation bubbles below, are ubiquitous and may occur over steep but non-breaking waves. The proposed laboratory study and its extension to the open ocean conditions will contribute to development of accurate parameterizations of sea state dependent air-sea momentum flux, which may be immediately incorporated in the ongoing coupling efforts for atmosphere wave-ocean tropical cyclone and climate models. This study will improve heat and humidity flux parameterizations as well, because airflow separation/quasi-separation events play an important role in dispersion of sea spray droplets. A series of outreach activities aimed at three key audiences: graduate students, high school and undergraduate science educators, and the general public will be facilitated by education and outreach staff at the Inner Space Center at the University of Rhode Island. At the University of Delaware, a summer undergraduate student will participate in the laboratory experiments that will highlighted in a series of open house events and laboratory visits for the general public.In this project, laboratory observations and LES are closely combined to study airflow separation/ quasi-separation events and their impact on air-sea momentum flux. The main hypothesis of the project is that: (a) airflow separation/quasi-separation significantly modifies the wave form drag, the near surface turbulence, and the air-sea momentum flux, and that (b) LES can reproduce realistic airflow fields provided the wave shape/speed, the surface velocity field, and the surface roughness distribution are accurately specified based on observations. To test these hypotheses, combined laboratory observations and LES of wind over a finite amplitude wave train will be carried, providing accurate air-water interface boundary conditions to the LES based on observations, and validating the LES results of airflow turbulence against observations. Then, the occurrence of airflow separation/quasi-separation and the resulting impact on wave form drag and air-sea momentum flux will be quantified. Once the LES methodology of wind over waves is validated against laboratory observations in this study, the LES can be extended to open ocean conditions, with multiwave components (short crested waves), and with misaligned wind and waves. The results will help meet a growing interest by modeling and prediction centers in coupling ocean surface wave models with atmospheric and ocean models from global/climate scales to regional scales.

当表面海浪场与局部风强迫不平衡时，这是一个常见的情况，风应力可能会显着偏离大量参数化，并且需要海态依赖性参数化。最近的建模研究表明，海态依赖性最重要的原因之一是由于在爆破波（尤其是在较高的风速下）的气流分离而引起的增强形式阻力。但是，这些模型结果中的不确定性仍然很大，因为我们对气流分离的理解是有限的。虽然人们普遍认为，气流分离仅发生在断断续续的情况下发生，但最近的实验室观察和大型涡流模拟（LES？s）表明，这种短暂分离样流（准分离）的特征是，高涡度层与下面的波形表面和分离气泡脱离的高涡度层，并且可能在陡峭的波浪中出现泛滥，但可能发生了陡峭的波浪。拟议的实验室研究及其扩展到公开海洋条件将有助于开发依赖海洋状态的空气海洋动量通量的准确参数化，这可以立即将其纳入正在进行的耦合工作中，以促进大气波西海热带气旋和气候模型。这项研究也将改善热量通量参数化，因为气流分离/准排序事件在海洋喷雾液滴的分散体中起着重要作用。一系列针对三个关键受众的外展活动：研究生，高中和本科科学教育工作者以及公众将由罗德岛大学内部空间中心的教育和外展员工促进。 在特拉华大学，夏季的本科生将参加实验室实验，这些实验将在一系列开放式公众活动和实验室访问中强调。在该项目中，实验室观察和LES已密切相结合，以研究气流分离/准分离事件及其对空中势头的影响。该项目的主要假设是：（a）气流分离/准分离显着修改了波形拖动，近表面湍流和空气动量通量，并且（b）LES可以繁殖现实的空气流场为波形/速度提供了波浪形状，表面速度磁场和表面粗糙度的分布精确地是基于表面粗糙度分布的。为了检验这些假设，将携带实验室的联合观察和风能通过有限幅度波序列的风，根据观测值为LES提供准确的空气水界面边界条件，并验证针对观察结果的气流湍流的LES结果。然后，将量化气流分离/准分离的出现以及对波形拖动和空气动量通量的产生影响。一旦在这项研究中对实验室观察的验证了波浪的LES方法，可以将LES扩展到开放海洋条件，并具有多波组件（短冠波），并具有未对准的风和波浪。该结果将通过建模和预测中心在将海面波模型与大气和海洋模型结合到从全球/气候尺度到区域尺度的耦合中，有助于满足日益增长的兴趣。