EAGER: Langmuir Turbulence Measurements at 35-40m Depth off Cape Hatteras in Fall 2015

朗缪尔环流是一种众所周知的上层海洋对风和波浪强迫的反应，它通常以一系列窗口的形式出现，在这些窗口中，泡沫和其他漂浮物通过会聚的表面流聚集在一起。循环的主要意义是在垂直于这些行的垂直平原上，在那里水在行下面循环，在某一深度向两边流出，然后再回来。风强迫倾向于生长、合并和加深朗缪尔细胞，而密度分层或下面密度更大的层的存在往往限制了它们向下的范围。在适当的条件下，可以观察到超级细胞到达水柱的整个深度。这些细胞非常有效地垂直混合水，并将沉积物和其他物质从底部提升到水柱中。该项目将部署一个声学多普勒电流计（VADCP），专门设计用于精确解析垂直运动，作为现有实验的补充，该实验由海军研究办公室单独资助，由海军研究实验室（NRL）运行。这些详细的多普勒测量将用于研究Langmuir细胞的发育和进化，其位置比以往任何测量都要深得多，分层范围也更广。关于Langmuir细胞的数据将有助于更好地解释广泛的NRL测量结果，而这些测量结果为分析Langmuir动力学提供了空间背景。部署VADCP的最关键因素是垂直轴的精确对准。部署将在35-40米的深度，这是潜水员安全部署的边缘，但其他部署方式也将被开发和测试，未来可能会导致更深的部署。NRL的实验是2015年10月在哈特拉斯角附近进行的为期30天的密集活动。它包括两架飞机，携带多通道合成孔径雷达，红外和可见光高光谱成像仪，动量和热量的海气通量，波浪浮标的定向表面波光谱，使用波浪滑翔机和漂流浮标对温度、湿度和风的基本海气测量，使用湍流微观结构探针从滑翔机获得混合层的亚表面剖面，以及由飞机测量的多色染料释放。以及带有光学/微结构探头的AUV。该项目将在8月至10月在35-40米深度，360度至36.5度之间增加VADCP和绷紧线电导率-温度串，以测量强烈的中大西洋夏季斜倾角破坏期间和之后的风暴事件。飞机和系泊/滑翔机的空间覆盖将揭示细胞水平尺度的范围；细胞是否如预期的那样被风拉长和风排列，以及VADCP记录中的时间变异性是否代表了静止VADCP记录的细胞漂移在统计上的冰冻场海洋响应中的空间变化。地表通量、风和波场的水平空间结构将告诉我们，随着风和波力的增加，与不断变化的浮力通量竞争，强迫机制如何在跨大陆架方向演变。滑翔机的微观结构测量可以用来交叉验证滑翔机的TKE耗散率与VADCP的大涡接近，就像之前在潮汐流中对自由落体探针和VADCP所做的那样。VADCP速度场将显著提高对NRL小组光学数据的解释。尽管如此，最令人信服的动机是该项目有可能扩大我们对沿海海洋湍流的看法。越来越多的证据表明，全深度超级单体经常发生在大陆架上。该部署将扩展现有的15米和27米深度的VADCP测量。如果Langmuir超级细胞可以通过直接测量来验证，那么它们的影响将需要在比以前更广泛的货架设置范围内进行考虑。这将需要重新评估目前对沿海海洋风/波强迫下海水和沉积物通量的理解和模拟。

Langmuir circulation is a well-known response of the upper ocean to wind and wave forcing and is often visible as a series of windrows where foam and other floating material is aggregated by converging surface flow. The main sense of circulation is on a vertical plain perpendicular to these rows, where the water circulates down under the rows, out to either side at some depth and back up again. Wind forcing tends to grow, merge and deepen the Langmuir Cells, while density stratification or the presence of denser layers below tends to limit their downward extent. Under the right conditions, supercells are observed going the full depth of the water column. These cells are very efficient at mixing the water vertically and lifting sediments and other material from the bottom into the water column. This project will deploy an acoustic Doppler current meter (VADCP) specially designed to accurately resolve the vertical motion as an addition to an existing experiment separately funded by Office of Naval Research and run by the Naval Research Laboratory (NRL). These detailed Doppler measurements will be used to study the development and evolution of the Langmuir Cells at a location significantly deeper than any of the previous measurements and under a wider range of stratification. The data about Langmuir Cells will help better interpret the extensive NRL measurements, while those measurements provide a spatial context for the analysis of the Langmuir dynamics. The most critical element in the deployment of the VADCP is the precise alignment of the vertical axis. The deployment will be at 35-40m, which is at the edge of safe deployment with divers, but other means of deployment will also be developed and tested, possibly leading to deeper deployments in the future.The NRL experiment is an intensive 30-day campaign in October 2015 near Cape Hatteras. It includes two aircraft carrying a multichannel synthetic aperture radar, infrared and visible hyperspectral imagers, and air-sea fluxes of momentum and heat, directional surface wave spectra from wave buoys, basic air-sea measurements of temperature, humidity, and wind using wave gliders and drifting buoys, sub-surface profiles of the mixed layer from gliders with turbulence microstructure probes, and multi-color dye releases surveyed by aircraft, and an AUV with optical/microstructure probes. This project will add a VADCP and taut-line conductivity-temperature string at 35-40m depth, between 36o and 36.5oN in August through October, to measure storm events during and after destruction of the strong Mid-Atlantic Bight summertime pycnocline. Aircraft and mooring/glider spatial coverage will reveal the range of cell horizontal scales; whether cells are wind-elongate and wind-aligned as expected, and whether temporal variability in the VADCP records represents spatial variation in a statistically frozen-field ocean response, recorded as cells drift by the stationary VADCP. Horizontal spatial structure of surface fluxes, winds and wave fields will inform how the forcing regimes may evolve in the cross-shelf direction with increasing wind and wave forcing, competing with evolving buoyancy fluxes. Glider microstructure measurements can be used to cross-validate TKE dissipation rates from the gliders with the large eddy approach from the VADCP, as done previously for freefall probes and VADCP in tidal flows. VADCP velocity fields will significantly enhance the interpretation of the NRL group's optical data. Still, the most compelling motivation is the project's potential to broaden our view of turbulence in the coastal ocean. There is growing evidence the full depth supercells occur often over the continental shelf. This deployment will extend existing VADCP measurements at 15m and 27m depth. If Langmuir supercells can be verified with direct measurements, their effects will need to be considered over a much wider range of shelf settings than appreciated before. This will require reassessment of current understanding and modeling of seawater and sediment fluxes under wind/wave forcing in the coastal ocean.