Collaborative Research: Breaking Wave Effects on Wave Boundary Layer and Air-Sea Momentum Flux

0526318海气通量参数化是海洋、大气和海洋大气耦合模式的重要组成部分。最近的飓风预报模型研究表明，准确的通量参数在大风条件下尤为重要。然而，目前在业务模式中的海气通量参数化是基于简单的整体公式，显然远远不能令人满意。越来越多的观测证据表明，海气通量强烈依赖于海况。最近，这位首席研究员和他的同事们根据边界层能量和动量守恒的基本原理，开发了一种新的波边界层模型(直接受表面波影响的大气边界层的下部)。该模式已与面波模式耦合，用于预报成熟、成长和复杂海面上的当量表面粗糙度和阻力系数。特别是，该模型预测了飓风条件下大风时阻力系数的降低水平，这与最近的现场和实验室观察一致。本文提出的波浪边界层和平衡波谱模型将基于以下假设：(1)在强风强迫作用下，破碎波显著改变了波浪边界层的结构和由此产生的海气动量通量。(2)在已知表面波谱和破碎波统计数据的情况下，如果将现有的波浪边界层模式扩展到包含破碎波峰的形式阻力效应和破碎波峰后的气流分离效应，从而有效地保护短波免受直接风的强迫(空间遮挡效应)，则可以准确地量化破碎波对海气动量通量的影响。(3)在平衡范围内，表面波谱和破碎波统计也可与海气动量通量一起预报。该模型将明确包括形状阻力和气流分离效应，并将用于预测大风条件下的阻力系数。接下来，将根据迈阿密大学进行的新的实验室观察来验证该模型的每个组成部分。该试验是专门为模型验证而设计的，并提供了总风应力、表面粘性应力、表面波谱和破碎波统计的同时测量。拟议中的开发将与英国雷丁大学的斯蒂芬·贝尔彻密切合作完成。更广泛的影响：拟议的海浪边界层和平衡波谱研究将是朝着我们的最终目标迈出的重要和必要的一步，我们的最终目标是开发一套新的海气通量参数化方案，该方案将适用于整个风速和表面波条件，并可用作海洋、大气和海洋/大气耦合系统的高分辨率数值模式的改进边界条件，包括热带气旋预报模式。因此，这项研究解决了改善天气预报(特别是热带气旋预报)和减少极端风浪条件造成的自然灾害的社会需求。该项目包括开发一种新的实验技术来估计破碎波的统计数据。海面破碎波事件的量化对于海-气相互作用研究的各个方面都是极其重要的。该项目还涉及一名研究生的教育和培训。学生将学习实验室实验技术和理论/数值模拟方法，以研究破碎波对海-气相互作用过程的影响。最后，将促进现有的国际合作。

0526318Intellectual Merit: Air-sea flux parameterizations are important components of ocean, atmosphere, and coupled ocean atmosphere models. Accurate flux parameterizations are particularly important at high wind conditions as demonstrated by recent hurricane prediction model studies. Yet, ther present air-sea flux parameterizations in the operational models are based on simple bulk formulas and are clearly far from satisfactory. Increasing observational evidences suggest that air-sea fluxes strongly depend on sea states. Recently, the lead investigator and his colleagues have developed a new model of the wave boundary layer (the lower part of the atmospheric boundary layer that is directly influenced by surface waves) based on the fundamental principles of conservation of energy and momentum across the boundary layer. The model has been coupled with surface wave models to predict the equivalent surface roughness and the drag coefficient over mature, growing, and complex seas. In particular, the model predicts reduced levels of the drag coefficient at very high winds under hurricane conditions, consistent with recent field and laboratory observations. One shortcoming of the present model is that it does not include the effect of surface breaking waves on the wave boundary layer.The proposed model of the wave boundary layer and the equilibrium wave spectrum will be based on the following hypotheses: (1) Breaking waves significantly modify the structure of the wave boundary layer and the resulting air-sea momentum flux under strong wind forcing. (2) Breaking wave effects on the air-sea momentum flux can be quantified accurately if the existing wave boundary layer model is extended to incorporate the effect of the form drag due to breaking wave crests and the effect of airflow separation behind breaking wave crests that effectively shelters shorter waves from direct wind forcing (spatial sheltering effect), provided the surface wave spectrum and the breaking wave statistics are known. (3) In the equilibrium range, the surface wave spectrum and the breaking wave statistics may also be predicted together with the air-sea momentum flux. The model will explicitly include the form drag and airflow separation effects and will be used to predict the drag coefficient under high wind conditions. Next, each component of the model will be validated against new laboratory observations carried out at the University of Miami. The experiment is specifically designed for the model validation and provides simultaneous measurements of the total wind stress, the surface viscous stress, the surface wave spectrum, and the breaking wave statistics. The proposed development will be done in close collaboration with Stephen Belcher at the university of Reading, UK. Broader Impacts: The proposed study of the wave boundary layer and the equilibrium wave spectrum will be an important and essential step towards our ultimate goal of developing a new set of parameterizations of air-sea fluxes, which will be valid for the whole range of wind speeds and surface wave conditions and can be used as improved boundary conditions for high-resolution numerical models of ocean, atmosphere, and coupled ocean/atmosphere systems, including the tropical cyclone prediction models. Therefore, this research addresses social needs to improve weather forecast (in particular, tropical cyclone forecast) and to reduce natural hazards caused by extreme wind and wave conditions. The project includes the development of a new experimental technique to estimate the breaking wave statistics. Quantifying surface breaking wave events is extremely important for all aspects of the air-sea interaction studies. The project also involves the education and training of one graduate student. The student will learn both laboratory experimental techniques and theoretical/numerical modeling approaches to study breaking wave effects on air-sea interaction processes. Finally, an existing international collaboration will be fostered.