A note on a critical wind speed for air–sea boundary processes
A note on a critical wind speed for air–sea boundary processes
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关于海-气边界过程临界风速的注解
DOI：
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发表时间：
2017
影响因子：
2.3
通讯作者：
H. Mitsuyasu
中科院分区：
文献类型：
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作者：
H. Mitsuyasu
在风浪水槽中测量了风浪和风浪。风速U_(10)与摩擦风速u~(**)在u~*=100.2m/S附近有明显的转换关系，其中U~(10)是由对数层风廓线推算的10米高度风速，u~*_(10)=00.2m/S大致相当于本次测量的U~(10)=10.8m/S。高频风浪的谱密度与u~*之间的关系也有类似的转变。这些结果表明，海-气边界过程的临界风速是存在的，该临界风速是由Munk(J Marine Res 6：203-218,1947)在半个多世纪前提出的。他关于临界风速的最初想法是基于白帽、风应力和蒸发等现象的不连续性，这些现象通常出现在风速接近7m/S的地方。在我们目前和以前研究结果的基础上，我们讨论了与海-气边界过程的临界风速有关的现象。结论是：临界风速是存在的，临界风速不是由Kelvin-Helmholtz不稳定性引起的，而是由海浪破裂开始引起的，但海气边界过程在某一特定风速下并不是不连续的；由于破碎海浪的随机性，这种变化在一定风速范围内发生。详细讨论了与临界风速相关的动力学过程，如风致海面粗糙度变化和高频波谱。然而，还需要进一步的研究来定量地阐明动力学过程。特别是，需要进一步研究风浪破碎模式随着风速的增加而逐渐变化，以及与风浪结构相关的变化，如风浪峰顶气流的分离、湍流应力和波浪诱导应力。还需要对高频波谱的动力学结构进行研究。
Wind and wind-generated waves were measured in a wind-wave tank. A clear transition was found in the relation between the wind speed U10 and the wind friction velocity u* near u* = 0.2 m/s, where U10 is the wind speed at 10 m height extrapolated from the measured wind profile in a logarithmic layer, and u* = 0.2 m/s corresponds roughly to U10 = 8 m/s in the present measurement. Quite a similar transition was found in the relation between the spectral density of high frequency wind waves and u*. These results suggest the existence of the critical wind speed for air–sea boundary processes, which was proposed by Munk (J Marine Res 6:203–218, 1947) more than half a century ago. His original idea of the critical wind speed was based on the discontinuities in such phenomena as white caps, wind stress, and evaporation, which commonly appear at a wind speed near 7 m/s. On the basis of the results of our present study and those of earlier studies, we discuss the phenomena which are relevant to the critical wind speed for the air–sea boundary processes. The conclusion is that the critical wind speed exists and it is attributed to the start of wave breaking rather than the Kelvin–Helmholtz instability, but the air–sea boundary processes are not discontinuous at a particular wind speed; because of the stochastic nature of breaking waves, the changes occur over a range of wind speeds. Detailed discussions are presented on the dynamical processes associated with the critical wind speed such as wind-induced change of sea surface roughness and high frequency wave spectrum. Future studies are required, however, to clarify the dynamical processes quantitatively. In particular, there is a need to further examine the gradual change of breaking patterns of wind waves with the increase of wind speed, and the associated change of the structure of the wind over wind waves, such as separation of the airflow at the crest of wind waves, the turbulent stress, and wave-induced stress. Studies on the dynamical structure of the high frequency wave spectrum are also needed.