Generation of sea sprays and their impact on near surface turbulence and air-sea momentum flux

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。智力优势：海气动量和热通量的准确预测/参数化很重要，因为它们为大气、海洋和耦合海洋大气模式提供了边界条件。特别是在大风条件下，热带气旋模式预测对海气通量参数化非常敏感。最近的野外观测表明，在非常高的风速下，动量通量（阻力）系数往往比现有的整体参数化要低得多。阻力减少的一个可能原因是大风中海面破浪产生的大量海雾的影响。研究人员推测：(1)在高风速下，海雾对近地面大气湍流和海气动量和热通量具有重要的调节作用；(2)对浪花产生的准确预测必须考虑到在一定尺度上的破碎波和个别破碎波事件的浪花产生过程的统计；(3)准确模拟海洋喷雾与湍流相互作用需要一种新的数值方法，该方法可以明确地解决单个喷雾液滴或流体包与破碎波改变的湍流气流相互作用的问题。为了验证这些假设，研究人员将继续进行一项研究计划，其长期目标是开发新的混合数值模拟方法-结合目前最好的工具中的模型组件-研究海浪产生过程和海浪与近地面湍流之间的相互作用，并预测海浪对大风条件下海气动量和热通量的影响。作为实现这些长期目标的第一步，重点将放在两个主要的具体目标上，以开发一种方法来模拟：(1)使用混合方法结合全非线性势波求解器和两相流体动力学的晶格-玻尔兹曼方法，从单个破波事件产生喷雾过程；(2)分解后的喷雾液滴（或分解后的小空气包）与湍流剪切流之间的相互作用，采用两相流体动力学的格点-玻尔兹曼方法。利用这种方法，我们将研究海喷对海气动量通量的潜在影响。这项工作将由两位博士合作完成。Tetsu Hara， URI的Stephan Grilli和德国布伦瑞克工业大学的Manfred Krafczyk。此外，将与特拉华大学的Fabrice Veron博士合作，根据实验室观察验证喷雾生成模型。更广泛的影响：项目的更广泛的影响体现在五个方面。首先，拟议的工作为理解海喷如何影响海气通量以及这种改变的通量如何最终影响强风下的大气和海洋预测提供了基础。海面通量的估算是海洋和大气模式（包括热带气旋和风暴潮模式）中最关键的问题之一。事实上，通量参数化的主要不确定因素之一是海雾的影响。他的研究涉及社会？美国需要减轻和预防极端风浪可能造成的自然灾害。这项研究将产生更准确的风暴和飓风预报，从而导致更长的预警时间，从而增加准备时间，以减轻这种危险，并有望减少死亡和财产损失。结合不同数值方法的新混合方法的发展不仅有利于海洋/大气模拟领域，而且有利于流体动力学领域的广泛应用。第四，该项目将资助两名研究生，因此将有助于教育和培训新一代的科学家/工程师，他们完全精通最先进的环境建模。最后，拟议的工作促进了研究和教育方面的国际合作，因为它涉及美国和德国科学家的密切合作。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Intellectual Merit: Accurate predictions/parameterizations of air-sea momentum and heat fluxes are important because they provide boundary conditions for atmospheric, oceanic and coupled ocean atmosphere models. In particular, tropical cyclone model prediction is very sensitive to air-sea flux parameterizations under high wind conditions. Recent field observations suggest that the momentum flux (drag) coefficient tends to be much lower than the existing bulk parameterizations at very high wind speeds. One possible reason for this drag reduction is the effect of sea sprays generated in great amount by surface breaking waves in high winds. The investigators hypothesize that: (1) sea sprays play an important role in modifying the near surface atmospheric turbulence and air-sea fluxes of momentum and heat at high wind speeds; (2) accurate predictions of sea spray generation must account for statistics of breaking waves and spray generation processes from individual breaking wave events over a range of scales; and (3) accurate modeling of sea spray-turbulence interactions requires a new numerical methodology that explicitly resolves individual spray droplets or fluid parcels interacting with the turbulent air flow that is modified by breaking waves. To test these hypotheses the investigators will pursue a research program, whose long term objectives are to develop new hybrid numerical simulation methodologies - combining model components that are among the best currently available tools - to study sea spray generation processes and interactions between sprays and near surface turbulence, and to predict sea spray effects on air-sea momentum and heat fluxes in high wind conditions. As a first step towards meeting these long term objectives, the focus will be on two main specific objectives in developing a methodology to simulate: (1) spray generation processes from one single breaking wave event, using a hybrid approach combining a fully nonlinear potential wave solver and a Lattice-Boltzmann method for two-phase fluid dynamics; and (2) interactions between resolved spray droplets (or resolved small air parcels) and a turbulent shear flow, using the Lattice-Boltzmann method for two-phase fluid dynamics. Using this methodology we will investigate the potential effects of sea sprays on the air-sea momentum flux. The work will be performed as a collaboration between Drs. Tetsu Hara, Stephan Grilli, of URI, and Manfred Krafczyk, Technical University of Braunschweig, Germany. In addition the spray generation model will be validated against laboratory observations, in collaboration with Dr. Fabrice Veron, University of Delaware.Broader impacts: The broader impacts of the project are evident in five areas. First, the proposed work provides a basis for understanding how sea sprays affect air-sea fluxes and how such modified fluxes consequently impact atmospheric and ocean predictions at high winds. Estimation of fluxes over the sea surface is one of the most crucial issues in ocean and atmospheric modeling, including tropical cyclone and storm surge modeling. In fact, one of the main uncertainties regarding the flux parameterizations is the effect of sea sprays. his research addresses society?s need to mitigate and prevent possible natural hazards caused by extreme winds and waves. The more accurate storm and hurricane forecasts that will result from this research will thus lead to a longer advance warning and hence increased preparation time, for mitigating such hazard and hopefully diminishing fatalities and property damage. The development of a new hybrid approach combining different numerical methods will benefit not only the field of oceanographic/atmospheric modeling but also a wide range of applied fluid dynamics community. Fourth, the project will support two graduate students, and hence will contribute to the education and training of a new generation of scientists/engineers, fully fluent with state-of-the-art environmental modeling. Finally, the proposed work promotes international cooperation in research and education, since it involves a close collaboration of US and German scientists.