The Structure and Dynamics of the Surfzone Eddy Field

该项目的目标是改进对长度尺度大于水深的二维涡的理解。美国各地的海滩长期受到水质差的影响，导致游泳者生病，并影响沿海经济。径流污染往往直接流入冲浪区，分散和稀释污染或其他示踪剂的机制尚不清楚。最近的观测表明，在示踪剂扩散和稀释过程中存在着表面带2D湍流旋涡。表面波带二维湍流是由近岸水流的切变不稳定、单个波浪的有限波峰长度破裂或波群强迫下的近岸梯度产生的。这些涡量源都具有不同的性质，而表面带涡量场对这些涡度源的依赖关系尚不清楚。此外，跨岸可变(单调和条形)表面带水深测量对二维湍流涡场的影响尚不清楚。智力优势：第一组提议的目标是检查两个模型和现有的现场观测中的表面带湍流涡流。将两种不同的平均波浪模式(有无波群强迫)和一个波浪分辨模式(有限波峰长度的波浪破碎)中的表面带湍流涡旋的大小、时间和空间尺度与欧拉测量(如频率-波数谱)和拉格朗日观测(如相对色散)进行比较。3个不同涡度来源的模式也将进行相互比较。对于每个涡度模式，将检查强迫和由此产生的涡旋的长度尺度，以及存在逆能量或涡旋级联区的长度尺度。将为每个模型估计颗粒分离统计数据(例如，相对扩散系数)，以确定湍流涡长尺度。对每个模式中的位涡和位能能预算的分析将阐明表面带涡旋演变的动力过程。影响涡度强迫的因素，如波的方向扩散和沿岸流的大小，将会有所不同，以揭示表面带涡场如何依赖于不同的强迫机制。第二组目标包括发展对单调或条状地形如何影响经典二维湍流的理解。这将加深对SsurfZone 2D湍流涡旋演化和动力学的理解。将研究两种情况：1)自由衰减湍流和2)强迫/耗散湍流。我们将研究可变水深对涡旋的空间分布和各向同性的影响。在自由衰变湍流中，将考察衰变涡优先向深水或浅水的迁移。此外，还将确定跨岸变水深对二维湍流的其他性质的影响，如随时间变化的涡旋数量和大小，以及逆能量和拟能叶栅。对于强迫/消散湍流，将审查潜在涡度和涡度变位预算如何取决于跨岸位置。广泛影响：在美国海滩，2008年有20,000多个关闭和咨询日，7%的海滩水样超过国家卫生标准。严重的健康风险与细菌污染有关，通常来自径流来源，直接排到海滩上。对冲浪区污染物混合的定量知识是做出明智的管理决策所必需的，这需要对冲浪区湍流涡流场有更好的理解。作为教育外展工作的一部分，PIS将与海洋研究所共同主办两次青少年冲浪科学会议，约有100名高中生参加。这将是由国际冲浪协会联合主办的前几次成功的青少年冲浪科学会议的延续。此外，PIS将通过普赖斯学校为社会经济地位较低的高中生举办关于冲浪区/海滩科学的讲座，并将编制课程材料。

The project goal is an improved understanding of two dimensional eddies with length-scale greater than the water depth. Beaches throughout the United States are chronically impacted by poor water quality, making swimmers sick and affecting coastal economies. Run-off pollution often drains directly into the surfzone and the mechanisms dispersing and diluting pollution or other tracers are not clear. Recent observations implicate surfzone 2D turbulent eddies in tracer dispersion and dilution. Surfzone 2D turbulence is generated either from a shear instability of the alongshore current, from finite-crest length breaking of individual waves, or from alongshore gradients in wave-group forcing. These vorticity sources all have distinct properties and the dependence of the surfzone eddy field upon these vorticity sources is not understood. In addition, the effects of cross-shore variable (both monotonic and barred) surfzone bathymetry on the 2D turbulence eddy field is unclear.Intellectual Merit: The first set of proposal objectives is to examine surfzone turbulent eddies in both models and existing field observations. The magnitude, time- and spatial scales of surfzone turbulent eddies in two different wave-averaged (with and without wave-group forcing) and one wave-resolving models (finite crest length wave-breaking) will be compared to Eulerian measures such as frequency-wavenumber spectra and Lagrangian observations like relative dispersion. The 3 models with distinct vorticity sources will also be inter-compared. For each vorticity model, the length-scales of forcing and of the resulting eddies will be examined, as will the length-scales where inverse-energy or enstrophy cascade regions are present. Particle separation statistics (e.g., relative diffusivities) will be estimated for each model to identify turbulent eddy length-scales. Analysis of the potential-vorticity and potential-enstrophy budgets within each model will illuminate the dynamical processes underlying the evolution of surfzone eddies. The factors that affect the vorticity forcing, such as wave directional-spread and the alongshore current magnitude, will be varied to reveal how the surfzone eddy field depends upon the various forcing mechanisms.The second set of objectives includes developing an understanding of how monotonic or barred topography affects classical 2D turbulence. This will improve the understanding of the evolution and dynamics of surfzone 2D turbulent eddies. Two scenarios will be examined: 1) freely decaying turbulence and 2) forced/dissipated turbulence. The effects of variable bathymetry upon the spatial distribution and isotropy of the eddies will be examined. In freely decaying turbulence, the preferential migration to deep or shallow water of decaying eddies will be examined. Furthermore, the effect of cross-shore variable bathymetry upon other properties of 2D turbulence, such as the time-dependent vortex number and size, and the inverse-energy and enstrophy cascades, will be determined. For forced/dissipated turbulence, how the potential vorticity and enstrophy budgets depend upon cross-shore position will be examined.Broader Impacts: At United States beaches, there were over 20,000 closure and advisory days in 2008, and 7% of beach water samples exceed national health standards. Significant health risks are associated with bacterial pollution typically from run-off sources that drains directly onto the beach. Quantitative knowledge of the mixing of surfzone pollutants, needed for informed management decisions, requires an improved understanding of the surfzone turbulent eddy field. As part of educational outreach efforts, the PIs will co-host with the Ocean Institute two Teen SurfScience Conferences for about 100 high-school students. This will be a continuation of previous successful Teen SurfScience conferences co-hosted by the PI. In addition, PIs will present talks on surfzone/beach science to high school students of low socio-economic status through the Preuss School and curriculum materials will be developed.