Abstract
Abstract. It is important to investigate the effects of current on wind waves, called the Doppler shift, at both normal and extremely high wind speeds. Three different types of wind-wave tanks along with a fan and pump are used to demonstrate wind waves and currents in laboratories at Kyoto University, Japan, Kindai University, Japan, and the Institute of Applied Physics, Russian Academy of Sciences, Russia. Profiles of the wind and current velocities and the water-level fluctuation are measured. The wave frequency, wavelength, and phase velocity of the significant waves are calculated, and the water velocities at the water surface and in the bulk of the water are also estimated by the current distribution. The study investigated 27 cases with measurements of winds, waves, and currents at wind speeds ranging from 7 to 67 m s−1. At normal wind speeds under 30 m s−1, wave frequency, wavelength, and phase velocity depend on wind speed and fetch. The effect of the Doppler shift is confirmed at normal wind speeds; i.e., the significant waves are accelerated by the surface current. The phase velocity can be represented as the sum of the surface current and artificial phase velocity, which is estimated by the dispersion relation of the deepwater waves. At extremely high wind speeds over 30 m s−1, a similar Doppler shift is observed as under the conditions of normal wind speeds. This suggests that the Doppler shift is an adequate model for representing the acceleration of wind waves by current, not only for wind waves at normal wind speeds but also for those with intensive breaking at extremely high wind speeds. A weakly nonlinear model of surface waves at a shear flow is developed. It is shown that it describes dispersion properties well not only for small-amplitude waves but also strongly nonlinear and even breaking waves, which are typical for extreme wind conditions (over 30 m s−1).
Highlights
The oceans flow constantly, depending on the rotation of the Earth, tides, topography, and wind shear
Wind waves follow the dispersion relationship and Doppler shift effect at normal wind speeds. These studies were performed at normal wind speeds only, and few studies have been conducted at extremely high wind speeds, for which the threshold velocity is 30–35 m s−1, representing the regime shift of air–sea momentum, heat, and mass transport (Powell et al, 2003; Donelan et al, 2004; Takagaki et al, 2012, 2016; Troitskaya et al, 2012, 2020; Iwano et al, 2013; Krall and Jähne, 2014; Komori et al, 2018; Krall et al, 2019)
The water velocities in the three different wind-wave tanks at Kyoto University, Kindai University, and IAP RAS are separately shown in each panel
Summary
The oceans flow constantly, depending on the rotation of the Earth, tides, topography, and wind shear. Wind waves follow the dispersion relationship and Doppler shift effect at normal wind speeds These studies were performed at normal wind speeds only, and few studies have been conducted at extremely high wind speeds, for which the threshold velocity is 30–35 m s−1, representing the regime shift of air–sea momentum, heat, and mass transport (Powell et al, 2003; Donelan et al, 2004; Takagaki et al, 2012, 2016; Troitskaya et al, 2012, 2020; Iwano et al, 2013; Krall and Jähne, 2014; Komori et al, 2018; Krall et al, 2019). At such extremely high wind speeds, the water surface is intensively broken by strong wind shear, along with the foam layer, dispersed droplets, and entrained bubbles
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