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

基本信息

批准号：
1458977
负责人：
Fabrice Veron
金额：
$ 36.96万
依托单位：
University of Delaware
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 2020-08-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1458977&HistoricalAwards=false
关键词：
Collaborative Research Airflow separations over

项目摘要

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力强迫不平衡时（这种情况很常见），风应力可能会显着偏离体参数化，并且需要依赖于海况的参数化。最近的建模研究表明，海况依赖性的最重要原因之一是由于破浪时气流分离而导致的形状阻力增强，特别是在较高风速下。然而，由于我们对气流分离的理解有限，这些模型结果的不确定性仍然很大。虽然人们普遍认为气流分离仅发生在破碎波上，但最近的实验室观察和大涡模拟 (LES) 表明，瞬态分离状流动（准分离）的特征是与波表面分离的高涡度层和下面的分离气泡无处不在，并且可能出现在陡峭但不间断的波浪上。拟议的实验室研究及其对开放海洋条件的扩展将有助于开发与海况相关的空气-海洋动量通量的精确参数化，这可能会立即纳入正在进行的大气波-海洋热带气旋和气候模型的耦合工作中。这项研究还将改进热和湿度通量参数化，因为气流分离/准分离事件在海浪喷雾液滴的分散中发挥着重要作用。罗德岛大学内部太空中心的教育和外展工作人员将协助针对三个主要受众（研究生、高中和本科生科学教育工作者以及公众）开展一系列外展活动。在特拉华大学，一名暑期本科生将参加实验室实验，这些实验将在一系列面向公众的开放日活动和实验室参观中重点展示。在这个项目中，实验室观察和 LES 紧密结合起来，研究气流分离/准分离事件及其对海气动量通量的影响。该项目的主要假设是：（a）气流分离/准分离显着改变了波形阻力、近地表湍流和海气动量通量，并且（b）LES可以再现真实的气流场根据观察准确指定波形/速度、表面速度场和表面粗糙度分布。为了检验这些假设，将结合实验室观测和有限振幅波列风的 LES，根据观测结果为 LES 提供准确的空气-水界面边界条件，并根据观测结果验证气流湍流的 LES 结果。然后，气流分离/准分离的发生及其对波形阻力和海气动量通量的影响将被量化。一旦根据本研究中的实验室观察验证了风对波浪的 LES 方法，LES 就可以扩展到具有多波分量（短波峰）以及未对准的风和波浪的开放海洋条件。这些结果将有助于满足建模和预测中心日益增长的兴趣，将海洋表面波浪模型与从全球/气候尺度到区域尺度的大气和海洋模型耦合起来。