Effects of Sea-Surface Waves and Ocean Spray on Air–Sea Momentum Fluxes
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The effects of sea-surface waves and ocean spray on the marine atmospheric boundary layer (MABL) at different wind speeds and wave ages were investigated. An MABL model was developed that introduces a wave-induced component and spray force to the total surface stress. The theoretical model solution was determined assuming the eddy viscosity coefficient varied linearly with height above the sea surface. The wave-induced component was evaluated using a directional wave spectrum and growth rate. Spray force was described using interactions between ocean-spray droplets and wind-velocity shear. Wind profiles and sea-surface drag coefficients were calculated for low to high wind speeds for wind-generated sea at different wave ages to examine surface-wave and ocean-spray effects on MABL momentum distribution. The theoretical solutions were compared with model solutions neglecting wave-induced stress and/or spray stress. Surface waves strongly affected near-surface wind profiles and sea-surface drag coefficients at low to moderate wind speeds. Drag coefficients and near-surface wind speeds were lower for young than for old waves. At high wind speeds, ocean-spray droplets produced by wind-tearing breaking-wave crests affected the MABL strongly in comparison with surface waves, implying that wave age affects the MABL only negligibly. Low drag coefficients at high wind caused by ocean-spray production increased turbulent stress in the sea-spray generation layer, accelerating near-sea-surface wind. Comparing the analytical drag coefficient values with laboratory measurements and field observations indicated that surface waves and ocean spray significantly affect the MABL at different wind speeds and wave ages.
Key wordsdrag coefficient marine atmospheric boundary layer ocean spray droplets surface waves
本文主要研究了在不同风速和波龄下, 海表面波浪和海洋飞沫对海-气边界层的影响. 首先, 通过在海表面总界面应力中引入波致应力和飞沫应力, 修正了海-气边界层动量模型. 在考虑涡流粘性系数随海表面高度线性变化的前提下, 得到了模型的解析解. 波致应力的计算通过海浪方向谱和海浪增长函数获得, 飞沫应力的计算则是通过参数化描述海洋飞沫液滴与风速剪切相互作用的物理过程. 另外, 计算了在不同海浪成长状态和固定海表面高度处不同风速下, 近海表面风廓线和海表面拖曳系数的值. 同时, 风廓线与拖曳系数的值也与之前没有引进波浪与飞沫作用的模型的解析解进行了对比. 结果表明: 在中低海表面风速下, 海表面波浪对近海风廓线和海表面拖曳系数有影响, 低波龄海浪较高波龄海浪, 对风廓线与拖曳系数的影响更为明显, 而飞沫的影响基本忽略. 然而在高风速下, 海浪大量破碎, 风从破碎波脊吹落的飞沫液滴, 对海-气边界层的动量通量有很强的影响, 进而波龄对动量通量的影响被忽略. 大量飞沫液滴使近海飞沫生成层的湍应力增加, 使得大气向海洋中输送的动能减小, 进而拖曳系数减小. 最后, 拖曳系数的理论值与实验室风波槽所得数据和外海观测数据的对比表明, 不同波龄的海表面波浪和海洋飞沫对海-气边界层动量通量有很重要的影响, 进一步证明模型的准确性.
关键词海-气边界层 海表面波浪 海洋飞沫 拖曳系数
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The research presented in this paper was supported by the National Natural Science Foundation of China (Grant No. 41576013) and the National Key Research and Development Program of China (Grant No. 2016YFC1401404). The research also was supported by the National Natural Science Foundation of China (Grant No. 41476021 and 41621064) and the Indo-Pacific Ocean Environment Variation and Air–Sea Interaction project (GASI-IPOVAI-04).
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