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

当海面波浪场与局地风强迫不平衡时（这种情况很常见），风应力可能明显偏离体参数化，需要依赖于海况的参数化。最近的模型研究表明，海况依赖性的最重要原因之一是破碎波上气流分离导致的形式阻力增强，特别是在较高的风速下。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只发生在破碎波上，但最近的实验室观察和大涡模拟(LES？S)表明瞬态类分离流动（准分离），其特征是高涡度层与波面分离，下面是分离泡，普遍存在，并且可能发生在陡峭但不破碎的波浪上。拟议的实验室研究及其扩展到开放海洋条件将有助于发展依赖海况的海气动量通量的精确参数化，这可能立即纳入正在进行的大气波-海洋热带气旋和气候模式的耦合工作。由于气流分离/准分离事件在海雾液滴的分散过程中起着重要的作用，因此该研究也将改进热湿通量的参数化。罗得岛大学内太空中心的教育和外展工作人员将为针对三个主要受众：研究生、高中和本科科学教育者以及普通公众的一系列外展活动提供便利。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得到强调。本项目将实验室观测与LES紧密结合，研究气流分离/准分离事件及其对海气动量通量的影响。项目的主要假设是：(a)气流分离/准分离显著地改变了波浪阻力、近地面湍流和海气动量通量；(b)在观测的基础上，如果能准确地确定波形/速度、表面速度场和表面粗糙度分布，LES可以再现真实的气流场。为了验证这些假设，将结合实验室观测和有限振幅波列上风的LES，在观测的基础上为LES提供准确的空气-水界面边界条件，并根据观测验证气流湍流的LES结果。然后，量化气流分离/准分离的发生及其对波浪阻力和海气动量通量的影响。一旦在本研究中风过浪的LES方法通过实验室观测得到验证，LES就可以扩展到开放海洋条件下，具有多波分量（短波峰波），以及风和波的不对齐。这些结果将有助于满足模拟和预测中心对从全球/气候尺度到区域尺度将海洋表面波模式与大气和海洋模式耦合的日益增长的兴趣。

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.