The Structure and Dynamics of the Surfzone Eddy Field

该项目的目标是提高对长度尺度大于水深的二维涡流的理解。美国各地的海滩长期受到水质差的影响，使游泳者生病并影响沿海经济。径流污染通常直接排入地表，分散和稀释污染或其他示踪剂的机制尚不清楚。最近的观察暗示了示踪剂分散和稀释的表面二维湍流漩涡。海面带2D湍流是由沿岸流的剪切不稳定性、单个波浪的有限波峰长度断裂或波群强迫中的沿岸梯度产生的。这些涡度源都有不同的性质，而表面涡场对这些涡度源的依赖性尚不清楚。此外，跨岸可变（单调和条形）海面测深对二维湍流涡旋场的影响尚不清楚。知识价值：第一组建议的目标是在模型和现有的实地观察中检查表层湍流漩涡。在两种不同的波平均（有和没有波群强迫）和一种波分解模型（有限波峰长度破波）中，海面湍流涡流的大小、时间和空间尺度将与欧拉测量（如频率波数谱）和拉格朗日观测（如相对色散）进行比较。并对具有不同涡度源的3种模式进行了比较。对于每一个涡度模型，强迫和由此产生的涡流的长度尺度，以及存在逆能量或熵级联区域的长度尺度，都将被检查。粒子分离统计（例如，相对扩散系数）将被估计为每个模型，以确定湍流涡旋长度尺度。分析每个模式中的位势涡量和位势熵收支将阐明地表涡旋演化的动力过程。对影响涡度强迫的因素，如波浪的方向传播和沿岸流的大小，将进行变化，以揭示海面涡场如何依赖于各种强迫机制。第二组目标包括了解单调或条状地形如何影响经典二维湍流。这将提高对表层二维湍流涡的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。可变水深对涡旋空间分布和各向同性的影响将被检查。在自由衰减湍流中，将考察衰减涡流向深水或浅水的优先迁移。此外，还将确定跨岸变水深对二维湍流其他特性的影响，如随时间变化的涡数和大小，以及逆能量和熵级联。对于强迫/耗散湍流，将研究潜在涡度和熵预算如何依赖于跨岸位置。更广泛的影响：在美国的海滩，2008年有超过20,000个关闭和咨询日，7%的海滩水样超过了国家健康标准。严重的健康风险与细菌污染有关，通常来自直接排入海滩的径流源。为了做出明智的管理决策，需要对表层污染物混合的定量了解，这就需要对表层湍流涡流场有更好的了解。作为教育推广工作的一部分，pi将与海洋研究所共同主办约100名高中生参加的两次青少年表面科学会议。这将是以前成功的由PI共同主办的青少年冲浪科学会议的延续。此外，pi将通过Preuss学校向社会经济地位较低的高中学生介绍关于冲浪区/海滩科学的讲座，并将编写课程材料。

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.