.[1] Momentum transfer across the wind-driven breaking air-water interface under strong wind conditions was experimentally investigated using a high-speed wind-wave tank together with field measurements at normal wind speeds. An eddy correlation method was utilized to measure roughness length and drag coefficient from wind velocity components measured by laser Doppler and phase Doppler anemometers. As a result, a new model for the roughness length and drag coefficient was proposed for predicting momentum transfer across the sea surface under both normal and strong wind conditions using the universal relationship between energy and significant frequency of wind waves normalized by the roughness length. The model shows that the roughness length and drag coefficient are uniquely determined at all wind speeds by energy and significant frequency of wind waves, and they can be given against U 10 only from the measurements of the wave parameters and one-point mean air velocity in the logarithmic law region. Citation: Takagaki, N., S. Komori, N. Suzuki, K. Iwano, T. Kuramoto, S. Shimada, R. Kurose, and K. Takahashi (2012), Strong correlation between the drag coefficient and the shape of the wind sea spectrum over a broad range of wind speeds, Geophys.

Heat and momentum transfer across the wind-driven breaking air–water interface at extremely high wind speeds was experimentally investigated using a high-speed wind-wave tank. An original multi-heat-balance method was utilized to directly measure latent and sensible heat transfer coefficients. The results show that both heat transfer coefficients level off at low and normal wind speeds but increase sharply at extremely high wind speeds. The coefficients have a similar shape for wind speeds at a height of 10 m. Therefore, the wind speed dependence on the latent and sensible heat transfer coefficients can be represented by that of the enthalpy coefficient even in the extremely high-speed region. To show how significantly the drag and enthalpy coefficients affect the intensity of tropical cyclones, the coefficients were applied to Emanuel’s analytic model. The analytic model shows that the difference between the present laboratory and conventional correlations significantly affects the maximum storm intensity predictions, and the present laboratory enthalpy and drag coefficients have the remarkable effect on intensity promotion at extremely high wind speeds. In addition, the simulations of strong tropical cyclones using the Weather Research and Forecasting (WRF) Model with the present and conventional correlations are shown for reference in the appendix. The results obtained from the models suggest that it is of great importance to propose more reliable correlations, verified not only by laboratory but also by field experiments at extremely high wind speeds.

Previous studies have demonstrated the saturation of drag coefficients at strong wind speeds. But the mechanism behind this saturation has not yet been fully clarified. In this study, at normal and strong wind speeds, we use a wind‐wave tank for investigating the peak enhancement factor of the wind‐sea spectrum, which is an appropriate wave parameter for representing interfacial flatness. We measured the water‐level fluctuation using wave gauges. At strong wind speeds, the result shows that the peak enhancement factor of the wind‐sea spectrum decreases with decreasing inverse wave age and with increasing wind speed. This suggests that the distinctive wind‐wave breaking occurs at strong wind speeds. It also suggests that this distinctive breaking of wind waves causes the saturation of drag coefficients at strong wind speeds.

The effects of turbulent eddies and Langmuir circulations in liquid flow on scalar transfer across a sheared wind-driven gas-liquid interface are investigated by means of a direct numerical simulation of a gas-liquid two-phase turbulent flow with a wind-driven nonbreaking wavy interface. The wind-driven wavy gas-liquid interface is captured using an arbitrary Lagrangian-Eulerian method with boundary-fitted coordinates on moving grids. The results show that Langmuir circulations are generated on the liquid side below the sheared wind-driven gas-liquid interface. The marker particles on the gas-liquid interface, the turbulent eddies in the form of streamwise vortices on the liquid side (i.e., the typical horseshoe vortices associated with bursting motions), and the low scalar flux lines on the gas-liquid interface induced by the turbulent eddies on the liquid side tend to locally concentrate in the regions along the downward flows caused by the Langmuir circulations. It is suggested that the turbulent eddies on the liquid side mainly control the scalar transfer across the sheared wind-driven gas-liquid interface, and the effect of the Langmuir circulations is relatively small.

A B S T R A C T Mass transfer velocity k L across the wind-driven airÁwater interface was estimated at extremely high wind speeds (up to U 10 070 m s(1 ) in a high-speed wind-wave tank by measuring changes in CO 2 concentration in the water. In addition, the volume flux of dispersing droplets lost from the tank and the wave height were measured. k L increases drastically with wind speed at extremely high wind speeds. The volume flux of dispersing droplets begins to increase drastically and the mean height of significant waves changes its rate of increase at almost the same wind speed as that at which the rate of increase of k L changed. These results suggest that intense wave breaking occurs at extremely high wind speeds and it has significant effects on mass transfer. k L is well correlated with the free-stream wind speed for both present laboratory and previous field measurements in the low and moderate wind speed regions. Present k L agrees well with the conventional correlation curves proposed by Wanninkhof (1992), Wanninkhof and McGillis (1999)