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

EAGER：2015 年秋季哈特拉斯角 35-40m 深度处的朗缪尔湍流测量

• 批准号: 1540648
• 负责人: Dana Savidge
• 金额: $ 20万
• 依托单位: University of Georgia Research Foundation Inc
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-05-01 至 2018-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1540648&HistoricalAwards=false
• 关键词: EAGER; Langmuir; Turbulence; Measurements; 35

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.

朗缪尔环流是上层海洋对风和海浪强迫的众所周知的反应，通常可以看到一系列的风降，其中泡沫和其他漂浮物质通过汇聚的表面气流聚集在一起。环流的主要感觉是在垂直于这些行的垂直平原上，水在这些行的下面循环，在一定深度向两边循环，然后再次向上循环。风强迫倾向于增长、合并和加深朗缪尔环流，而密度层结或下方较致密层的存在往往限制其向下的范围。在适当的条件下，可以观察到超级细胞在整个水柱的深度。这些电池非常有效地将水垂直混合，并将沉积物和其他物质从底部提升到水柱中。该项目将部署一个专门设计的声学多普勒海流计(VADCP)，作为由海军研究办公室单独资助、由海军研究实验室(NRL)运行的现有实验的补充。这些详细的多普勒测量将被用来研究朗缪尔环流的发展和演变，该位置比以前的任何测量都要深得多，并在更大范围的层结下。有关朗缪尔环流的数据将有助于更好地解释广泛的NRL测量，而这些测量为朗缪尔动力学的分析提供了空间背景。部署VADCP最关键的要素是垂直轴的精确对齐。部署将在35-40米处进行，处于潜水员安全部署的边缘，但还将开发和测试其他部署手段，可能导致未来更深层次的部署。NRL试验是2015年10月在哈特拉斯角附近进行的为期30天的密集行动。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，以及海气动量和热量通量，来自波浪浮标的定向表面波谱，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，使用带有湍流微结构探测器的滑翔机测量混合层的次表层轮廓，由飞机测量多色染料释放，以及一架带有光学/微结构探测器的自航器。该项目将在8月至10月期间在35-40米深、36度至36.5度北纬之间增加一条VADCP和张力线电导率-温度线，以测量大西洋中部夏季跃层被摧毁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单元格水平尺度的范围；单元格是否如预期的那样随风拉长和对齐，以及极值海洋数据中心记录的时间变异性是否代表统计冰冻场海洋反应的空间变化，由静止的极值数据中心记录为单元格漂移。地面通量、风场和波浪场的水平空间结构将告诉我们，随着风浪强迫的增加，强迫体制如何在跨陆架方向演变，与不断变化的浮力通量竞争。滑翔机微结构测量可以用VADCP的大涡方法交叉验证滑翔机的TKE耗散率，就像以前对自由落体探头和潮流中的VADCP所做的那样。VADCP速度场将显著提高NRL组光学数据的解释能力。尽管如此，最引人注目的动机是该项目有可能拓宽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出现在大陆架上。这一部署将把现有的VADCP测量扩展到15米和27米深。如果朗缪尔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认识到的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在风/浪强迫下的海水和沉积物通量的理解和建模。