Collaborative Research: DNS and high resolution measurements of scalar transfer across an air-water interface during inception and growth of Langmuir circulation

本研究的首要目标是表征小尺度朗缪尔湍流对海气界面下常见的冷热分子边界层（即冷皮肤）温度的影响，以及对界面上微可溶性气体交换率的影响。当风吹过最初静止的海气界面时，它首先产生短的毛细波，这些毛细波在时间上与较长的波共存，作为广谱波的一部分。风致波与切变流之间的相互作用导致了Langmuir湍流，其特征是Langmuir环流（LC），由大致与风方向对齐的反向旋转涡组成。典型的涡旋长度范围从最初出现短毛细波时的几厘米到波谱变宽时的几十米。厘米尺度的LC随着风场的阵风很快产生（也很快消失），从而在间歇性和重复的阵风事件中，在地表提供非常强烈的湍流爆发，这可能很好地主导了平均的地表更新过程。这些更新事件对我们理解海气标量通量至关重要。迄今为止，还没有对小尺度LC及其演变进行直接数值模拟，也没有对这些相干结构影响的地表更新时间尺度和其他参数进行测量。因此，本研究将基于精细尺度的DNS计算和高分辨率的实验室实验，捕捉波浪老化过程中LC的生长阶段和向Langmuir湍流过渡的阶段。通过实验验证的模拟将解决以下主要问题：1。在气旋发展初期，当风切变比波轨道速度占优势时，小尺度气旋对标量海气通量的影响是什么？2. 微破碎波对小尺度LC结构的影响是什么？3. 小型LC和微型破碎机对海面分子层的累积效应是什么？4. 在LC发展的后期，随着LC和表面波谱的扩大，LC的结构是什么？ LC对分子层的影响是什么？实验将重点关注小尺度LC的产生和演化。他们将瞄准1。评估小尺度朗缪尔环流的结构、演变和稳定性。2. 评估这些结构对冷皮肤的影响以及对空气-海洋热量和气体通量的潜在影响。3. 在水柱内收集高质量的动态测量数据，与计算结果进行比较。预计LC将通过增强表面更新机制来影响冷表层温度和气体浓度分子层，这种机制通常在冷表层和气体传递速度（一种衡量空气-海洋界面气体传递效率的指标）的参数化中被调用。因此，所使用的DNS求解器配备了界面捕获技术，可以产生可分辨的表面分子层，并精确分辨剧烈的自由表面运动。实验将包括PIV和主动红外辐射测量，产生地下和地表运动学的直接估计。这种数值/实验相结合的方法可能会对控制热和气体的空气-海洋分子通量的精细尺度过程产生新的物理见解。更广泛的影响：广泛的影响将通过加强对细尺度物理特性的基本理解来实现，这些特性描述了在LC存在下海洋冷层和气体在海气界面上的传输。LC在海洋表面快速出现和消失，在全球海气标量通量中起着重要作用。了解LC的影响将使科学家能够改进全球海洋通量对二氧化碳等温室气体吸收的参数化。此外，获得的结果将有利于科学家根据卫星红外测量来估计整体海洋温度，同时必须考虑到凉爽的皮肤。这项研究的结果将在期刊出版物和会议上传播，并酌情在更受欢迎的出版途径上传播。在地方一级，通过这项研究工作获得的知识将纳入对学生的教育和培训。pi将继续努力通过实验室参观和向公众提供信息科学讲座，向更广泛的受众（K-12和公众）推广科学和研究。

The overarching goal of this study is to characterize the effect of small scale Langmuir turbulence on the temperature of the commonly occurring cool thermal molecular boundary layer (i.e. the cool skin) beneath the air-sea interface and on slightly soluble gas exchange rate across the interface. When wind blows over an initially quiescent air-sea interface, it first generates short capillary waves which in time coexist with longer waves as part of a broad spectrum of waves. The interaction between the wind-driven waves and shear current leads to Langmuir turbulence characterized by Langmuir circulation (LC) consisting of counter rotating vortices roughly aligned in the direction of the wind. The typical length scale of the vortices ranges from several centimeters when short capillary waves first appear up to tens of meters when the spectrum of waves broadens. The centimeter-scale LC are generated very quickly with the gustiness in the wind field (and disappear very quickly as well), thereby providing, over intermittent and repeated gust events, very intense turbulent bursts at the surface which may very well dominate the average surface renewal processes. These renewal events are critical to our understanding of air-sea scalar fluxes. To date, direct numerical simulations (DNS) of small scale LC and its evolution has not been made and measurements of surface renewal time scales and other parameters influenced by these coherent structures have not been performed. Accordingly, this study, based on fine-scale DNS computations along with high resolution laboratory experiments, will capture the growth stages of LC and transition to Langmuir turbulence during the wave aging process. Simulations validated with the experiments will address the following major questions: 1. What is the influence of small scale LC on scalar air-sea fluxes during the early stages of LC development when wind shear is dominant over wave orbital velocities? 2. What is the impact of micro-breaking waves on the structure of small scale LC? 3. What is the cumulative effect of small scale LC and micro-breakers on sea surface molecular layers? 4. What is the structure of LC and the effect of LC on molecular layers during later stages of LC development as LC and surface wave spectra broaden? The experiments will focus on the generation and evolution of small scale LC. They will aim at 1. Assessing the structure, evolution, and stability of small scale Langmuir circulations. 2. Evaluating the impact of these structures on the cool skin with potential influence on the air-sea heat and gas flux. 3. Collecting high quality dynamics measurements within the water column for comparison with the computations. It is anticipated that the LC will impact the cool skin temperature and gas concentration molecular layer by enhancing the surface renewal mechanism, often invoked in parameterizations of the cool skin and gas transfer velocity (a measure of gas transfer efficiency across the air-sea interface). Accordingly, the DNS solver to be used is equipped with an interface capturing technique yielding resolved surface molecular layers, and accurate resolution of violent free-surface motions. The experiments will include PIV and active infrared radiometry yielding direct estimates of sub-surface and surface kinematics. This combined numerical/experimental approach will likely lead to new physical insights into the fine scale processes which control the air-sea molecular fluxes of heat and gas. Broader Impacts: Broad impacts will be made through enhanced fundamental understanding of the fine-scale physics characterizing the ocean cool skin and gas transfer across the air-sea interface in the presence of LC. LC is known to appear and disappear quickly at the ocean surface and plays an important role in global air-sea scalar fluxes. Understanding the influence of LC would allow scientists to develop improved parameterizations of global ocean flux uptake of greenhouse gases such as CO2. Furthermore, results obtained would benefit scientists making estimates of bulk ocean temperatures based on satellite infrared measurements while having to account for the cool skin. Results from this research will be disseminated in journal publications and conferences, and, where appropriate, more popular avenues of publication. At a local level, knowledge gained through this research effort will be incorporated into the education and training of students. The PIs will continue their efforts to promote science and research to a broader audience (K-12 and public) through laboratory visits and informational scientific talks to the public.