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

这项研究的主要目的是描述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)温度和界面上微溶气体交换速率的影响。当风吹过最初静止的海气界面时，它首先产生短毛细波，这些短毛细波最终作为宽广波谱的一部分与较长的波共存。风生波与切变流的相互作用导致了以朗缪尔环流(LC)为特征的朗缪尔湍流，朗缪尔环流是由大致对准风向的反向旋转涡旋组成的。典型的涡旋长度范围从最初出现短毛细管波时的几厘米到波谱扩大时的几十米。厘米尺度的Lc随着风场中的阵风迅速产生(也很快消失)，从而在间歇和重复的阵风事件中在地面提供非常强烈的湍流爆发，这可能很好地主导平均地面更新过程。这些更新事件对于我们理解海-气标量通量至关重要。到目前为止，还没有对小尺度液晶及其演化进行直接的数值模拟，也没有对表面更新时间尺度和受这些相干结构影响的其他参数进行测量。因此，这项研究将基于精细的DNS计算和高分辨率的实验室实验，捕捉到LC的生长阶段，并在波老化过程中过渡到朗缪尔湍流。通过实验验证的模拟将解决以下主要问题：1.当风的切变大于波的轨道速度时，小尺度LC发展初期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.小尺度LC和微破碎对海面分子层的累积作用是什么？4.随着LC和表面波谱的展宽，LC发展的后期阶段LC的结构是什么，LC对分子层的影响是什么？实验将集中在小尺度LC的产生和演化上。他们的目标是：1.评估小尺度朗缪尔环流的结构、演变和稳定性。2.评估这些结构对凉爽皮肤的影响，以及对海-气热量和气体通量的潜在影响。3.收集水柱内高质量的动力学测量数据，并与计算结果进行比较。预计LC将通过加强表面更新机制来影响冷表面温度和气体浓度分子层，该机制通常被调用在冷表面和气体传输速度(跨海气界面的气体传输效率的度量)的参数化中。因此，要使用的DNS解算器配备了界面捕获技术，以产生可解析的表面分子层，并准确地解析剧烈的自由表面运动。这些实验将包括PIV和主动红外辐射测量，产生对地下和表面运动学的直接估计。这种数值/实验相结合的方法可能会导致对控制热和气体的海-气分子通量的精细尺度过程的新的物理见解。更广泛的影响：将通过加强对细尺度物理的基本了解来产生更广泛的影响，这些物理特征描述了海洋凉爽的表层，以及在LC存在的情况下通过海-气界面的气体转移。LC在海洋表面的出现和消失非常迅速，在全球海气标量通量中起着重要的作用。了解LC的影响将使科学家能够开发改进的全球海洋通量吸收二氧化碳等温室气体的参数。此外，所获得的结果将有助于科学家基于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。私人投资机构将继续努力，通过实验室访问和向公众提供信息的科学讲座，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.