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

当海面波浪场与当地风强迫不平衡时，这是一种常见的情况，风应力可能会显着偏离整体参数化，并需要依赖于海况的参数化。最近的模拟研究表明，海况依赖性的最重要的原因之一是由于破碎波的气流分离，特别是在较高的风速下，增强的形状阻力。然而，由于我们对气流分离的了解有限，这些模型结果的不确定性仍然很大。虽然它已被普遍接受，气流分离只发生在破碎波，最近的实验室观察和大涡模拟（LES？s）表明，瞬态分离流（准分离），其特征是高涡度层脱离波表面和分离气泡下面，是普遍存在的，可能会发生在陡峭的，但非破碎波。拟议的实验室研究及其扩展到开阔海洋条件将有助于发展与海况有关的海气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式的耦合工作。这项研究将提高热量和湿度通量参数化，以及，因为气流分离/准分离事件发挥了重要作用，在分散的海喷雾液滴。罗得岛大学内部空间中心的教育和外联工作人员将为针对三个主要受众的一系列外联活动提供便利：研究生、高中和本科科学教育工作者以及公众。 在特拉华州大学，一名暑期本科生将参加一系列开放日活动和公众参观实验室的实验室实验。在这个项目中，实验室观测和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。该项目的主要假设是：（a）气流分离/准分离显著地改变了波形阻力、近表面湍流和海气动量通量，并且（B）LES可以再现真实的气流场，前提是基于观测准确地指定波形/速度、表面速度场和表面粗糙度分布。为了检验这些假设，将进行有限振幅波列上的风的实验室观测和LES，根据观测结果为LES提供精确的空气-水界面边界条件，并验证气流湍流的LES结果与观测结果。然后，将量化气流分离/准分离的发生及其对波形阻力和海气动量通量的影响。一旦LES方法的风在波浪中进行验证，在这项研究中的实验室观测，LES可以扩展到开放的海洋条件下，多波组件（短峰波），与错位的风和波浪。这些结果将有助于满足建模和预测中心日益增长的兴趣，将海洋表面波模型与大气和海洋模型从全球/气候尺度耦合到区域尺度。

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.