Tower-based observation of air-sea momentum flux: comparisons between onshore and offshore winds
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Tower-based observation of air-sea momentum flux: comparisons between onshore and offshore winds Maosheng Ye1, 2, Shuang Li1*, Zhongshui Zou1*, Jinbao Song1, Hailun He2, Jian Huang3, Hongwei Yang4 1 Ocean College, Zhejiang University, Zhoushan 316021, China 2 State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of
Natural Resources, Hangzhou 310012, China 3 Guangzhou Institute of Tropical and Marine Meteorology, China Meteorological Administration, Guangzhou
510640, China 4 College of Mathematics and System Science, Shandong University of Science and Technology, Qingdao 266590,
China Received 4 July 2019; accepted 26 July 2019 © Chinese Society for Oceanography and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract
We investigate the air-sea momentum flux in the marine atmospheric boundary layer using a tower-based direct measurement method. First, we compare the collected data with previous observations, and the results are roughly consistent. Next, in the low-to-moderate winds, the exchange coefficients (or drag coefficients) deviate between onshore and offshore winds, which exhibits the influence of surface wave on the momentum flux. Furthermore, we use a surface-wave-involved parameterization scheme to explain the dependence of momentum flux on surface wave. The results consolidate the influence of surface wave on momentum flux on the one hand, and validate the surface-wave-involved parameterization scheme on the other hand. Key words: air-sea momentum flux, direct measurement method, marine atmospheric boundary layer Citation: Ye Maosheng, Li Shuang, Zou Zhongshui, Song Jinbao, He Hailun, Huang Jian, Yang Hongwei. 2020. Tower-based observation of air-sea momentum flux: comparisons between onshore and offshore winds. Acta Oceanologica Sinica, 39(7): 61–68, doi: 10.1007/s13131020-1626-9
1 Introduction Air-sea momentum flux (also termed wind stress) in the marine atmospheric boundary layer (henceforth MABL) represents the friction between ocean and atmosphere and serves as core content in air-sea interaction (Hanley et al., 2010; Sun et al., 2015; He and Xu, 2016; He et al., 2018b). In the atmosphere, wind stress exports mechanical energy and takes part in the turbulent exchange of heat and moisture. At the same time, from the oceanic side, wind stress drives ocean movement. The physics and parameterization of wind stress have undoubted scientific significance and practical value (Peng et al., 2007). Specifically, air-sea momentum flux is the key process towards multiscale ocean dynamics. On the sea surface, air-sea momentum flux determines the momentum and energy flux from wind to waves, and therefore influences the surface wave state (Young, 1999; Xu et al., 2017; He et al., 2018a). For the nearsurface current, the magnitude is mainly controlled by the air-sea momentum fluxes. The misalignment of near-surface currents and surface winds represents the Ekman spiral of the upper ocean current, which provokes the water mass and leads to subsu
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