Author(s): Wenyi Li; Yuji Sugihara; Michio Sanjou

Linked Author(s):

Keywords: Erodynamic roughness length; Air-water interface; Drag coefficient; Friction velocity; Wind wave; Wind-wave tank

Abstract: The momentum transfer across the wind-driven air-water interface is an important phenomenon for understanding of the local air-sea interaction. The aerodynamic roughness length at the water surface z0 is considered to be a key parameter for characterizing the mechanism of the momentum transfer across the air-water interface. There exist many studies which attempted to describe universally the dimensionless roughness length such as gz0/u*2 and z0/Hs by using various types of wave-wind parameters, where g, u* and Hs indicate the gravitational acceleration, the friction velocity and the significant wave height, respectively, but no sufficiently established conclusion has been provided regarding such universal descriptions. The object of this study is to investigate experimentally wind-wave-dependent properties of the aerodynamic roughness length using a large-scale wind-wave tank. The aerodynamic roughness length was systematically examined under the conditions of long fetch and wide range of wind speeds. The present results show the fetch relations and spectral properties of wind waves to be consistent with the results from previous studies. In addition, our results indicate that the dimensionless aerodynamic roughness length depends strongly on a dimensionless wind-wave parameter such as ωpu*/g called the wave-wind coefficient or the inverse wave age, where ωp denotes the angular frequency of the spectral peak wave. The drag coefficient CD is found to be less sensitive to wave-field properties compared to the dimensionless aerodynamic roughness length, increasing linearly with wind speed under low-to-moderate conditions but saturates at higher wind speeds. This provides a valuable insight for improving the parameterization of the aerodynamic roughness length. The findings in this study are also useful for describing the carbon dioxide exchange process at the air-sea interface and the blue carbon stocks, being one of important methods for climate change mitigation.

DOI: https://doi.org/10.64697/978-90-835589-7-4_41WC-P1977-cd

Year: 2025