Air-Sea Momentum Flux in Tropical Cyclones

热带气旋的强度受到海气界面两个相互竞争的物理过程的显著影响。它通过吸收下层温暖海洋的热能而增强，但由于粗糙的海洋表面的阻力而减弱。当风速超过30米/秒时，这些过程会发生显著变化。目前，人们对飓风时的海气动量通量知之甚少，严重影响了热带气旋路径和强度的预报能力。海洋表面波浪无疑起着核心作用，但海浪的作用越来越重要。该项目通过分析三个热带气旋的现场测量（表面波谱、水下环境声、风等）和利用海洋动量预算计算的应力，解决了这一具有挑战性的问题。分析由最先进的UWIN-CM（迈阿密大学统一波界面耦合大气-波-海洋模型）模拟指导。通过与UWIN-CM模拟结果的比较以及对模拟波耗散函数的分析，将研究波浪应力和破碎率随风速和定向波特性的变化。在这个项目中获得的知识将提高对飓风的认识和预测，并将直接使公众受益。此外，对飓风了解不足的部分原因是在恶劣的飓风条件下缺乏可靠的测量技术。该项目将发展水下拉格朗日和声学技术，能够从水下监测海气界面。将制作一个用户界面设计的电影，展示科学家如何使用不同的技术，如卫星、飞机和水下仪器来观察飓风。这部电影将在科学讲座、当地学校和互联网上播放给来访的孩子们听。通过结合独特的野外测量和最先进的UWIN-CM模拟，我们将回答以下科学问题：(1)平衡距离波在控制热带气旋的海气动量通量方面有多重要？我们假设，当风速大于30 m/s时，海面上的应力大于平衡范围的破波应力。(2)破波率随风速和复杂表面波场的变化规律？在中等风速下，破波率随风速的增加而增加。这种情况在极端大风中还会继续吗？(3)我们能否探测到大风时海浪的声学特征？对热带气旋中浪花的测量是非常罕见的。我们将寻找水滴撞击海洋表面的声学特征。(4)控制海气动量通量的过程是什么？我们将使用耦合大气-波浪-海洋模型来指导现场测量的解释，评估其预测能力，并使用内部诊断来评估各种过程的重要性。通过回答这些问题所获得的知识将提高对海气动量通量的认识和建模，并提高预报热带气旋的能力。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The intensity of a tropical cyclone is significantly influenced by two competing physical processes at the air-sea interface. It strengthens by drawing thermal energy from the underlying warm ocean but weakens due to the drag of rough ocean surface. These processes change dramatically as the wind speed increases above 30 m/s. The air-sea momentum flux at hurricane winds is still poorly understood, which severely affects the capability of forecasting the path and intensity of tropical cyclones. Ocean surface waves undoubtedly play a central role, but sea spray plays an increasingly important role. This project addresses this challenging problem by analyzing field measurements (surface wave spectra, underwater ambient sound, wind, etc.) in three tropical cyclones and stress calculated using an ocean momentum budget. The analyses are guided by the state-of-the-art UWIN-CM (the University of Miami Unified Wave INterface Coupled Atmosphere-Wave-Ocean Model) simulations. Variations in wave stress and breaking rate as a function of wind speed and directional wave properties will be investigated by comparisons with the UWIN-CM simulations and by analyses of the modeled wave dissipation function. Knowledge acquired in this project will improve the understanding and forecasting of hurricanes and will directly benefit the public. In addition, poor understanding of hurricanes is partly due to the lack of reliable measuring techniques in the harsh hurricane condition. This project will develop the underwater Lagrangian and acoustic techniques, which are able to monitor the air-sea interface from the beneath. A user-interface designed movie will be created to display how scientists observe hurricanes using different techniques, such as satellites, aircrafts, and underwater instruments. The movie will be played to visiting children, in science talks, in local schools and on the Internet.By combining the unique field measurements and state-of-the-art UWIN-CM simulations, we will answer the following scientific questions: (1) How important are equilibrium-range waves in controlling the air-sea momentum flux in tropical cyclones? We hypothesize that for wind speeds higher than 30 m/s the stress on the ocean surface is larger than the equilibrium-range wave breaking stress. (2) How does the wave breaking rate vary with wind speed and the complex surface wave field? At moderate wind speeds the wave breaking rate increases with increasing speed. Does this continue at extreme high winds? (3) Can we detect acoustic signatures of sea spray at high winds? Measurements of sea spray in tropical cyclones are very rare. We will seek for the acoustic signatures of spray droplets impacting the ocean surface. (4) What are the processes controlling the air-sea momentum flux? We will use the coupled atmosphere-wave-ocean model to guide the interpretation of the field measurements, assessing its ability to predict and using internal diagnostics to assess the importance of various processes. Knowledge gained by answering these questions will improve the understanding and modeling of the air-sea momentum flux and improve the capability of forecasting tropical cyclones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.