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A note on a critical wind speed for air–sea boundary processes

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Abstract

Wind and wind-generated waves were measured in a wind-wave tank. A clear transition was found in the relation between the wind speed U 10 and the wind friction velocity u * near u * = 0.2 m/s, where U 10 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 U 10 = 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.

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Notes

  1. Results of the microwave backscattering are not included in the present paper to focus on the fluid dynamical problem.

  2. Selection of these spectral components of high frequency waves is due to the study on backscattering of microwaves of 9.6 GHz (wavelength λ = 3.12 cm) from wind wave surface.

  3. The present author once measured the wind profile over the mechanically generated water waves with smooth surface, where the generation of wind waves was suppressed by using a soluble surfactant. It was found that the wind profiles were little affected by the water waves with smooth surface, and the drag coefficient over the water waves was almost the same as that over the smooth and flat water surface. The result is not published yet.

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Acknowledgments

The author would like to express sincere thanks to Steve Thorpe, Professor Emeritus of Southampton University, for his critical reading of the manuscript and valuable suggestions. Professor Masuda provided insightful comments on the revised manuscript, Professor Mizuno helped to prepare the figures in the revised manuscript, and Dr. Sugihara provided valuable comments on an early draft of the paper. The author is grateful to two anonymous referees for their careful reading of the manuscript.

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  1. Kyushu University, Miwadai 4-16-12, Higashiku, Fukuoka, 811-0212, Japan

    Hisashi Mitsuyasu

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  1. Hisashi Mitsuyasu
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Correspondence to Hisashi Mitsuyasu.

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Mitsuyasu, H. A note on a critical wind speed for air–sea boundary processes. J Oceanogr 73, 169–180 (2017). https://doi.org/10.1007/s10872-016-0394-9

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