AUGUST 2019 C H E N E T A L . 2017 Impact of Surface Waves on Wind Stress under Low to Moderate Wind Conditions SHENG CHEN First Institute of Oceanography, Ministry of Natural Resources, and Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China FANGLI QIAO First Institute of Oceanography, Ministry of Natural Resources, and Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology, and Key Laboratory of Marine Sciences and Numerical Modeling, Ministry of Natural Resources, Qingdao, China WENZHENG JIANG,JINGSONG GUO, AND DEJUN DAI First Institute of Oceanography, Ministry of Natural Resources, and Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China (Manuscript received 3 January 2019, in final form 20 May 2019) ABSTRACT The impact of ocean surface waves on wind stress at the air–sea interface under low to moderate wind conditions was systematically investigated based on a simple constant flux model and flux measurements obtained from two coastal towers in the East China Sea and South China Sea. It is first revealed that the swell-induced perturbations can reach a height of nearly 30 m above the mean sea surface, and these per- turbations disturb the overlying airflow under low wind and strong swell conditions. The wind profiles severely depart from the classical logarithmic profiles, and the deviations increase with the peak wave phase speeds. At 2 wind speeds of less than 4 m s 1, an upward momentum transfer from the wave to the atmosphere is predicted, which is consistent with previous studies. A comparison between the observations and model indicates that the wind stress calculated by the model is largely consistent with the observational wind stress when con- sidering the effects of surface waves, which provides a solution for accurately calculating wind stress in ocean and climate models. Furthermore, the surface waves at the air–sea interface invalidate the traditional Monin–Obukhov similarity theory (MOST), and this invalidity decreases as observational height increases. 1. Introduction stress has been widely recognized through field ob- servations (Donelan et al. 1997; Grachev et al. 2003; Wind stress over the ocean is a key parameter for Högström et al. 2015), laboratory experiments (Uz understanding the physical processes in both the atmo- et al. 2003; Makin et al. 2007; Buckley and Veron 2018), sphere and ocean. Determination of the wind stress is and numerical models (Janssen 1989; Makin 2008; Jiang important for coupled ocean–atmosphere modeling, et al. 2016; Babanin et al. 2018). weather forecasting and other applications. The ocean In the presence of surface waves, the total velocity surface is mobile in contrast with rigid land, and surface can be separated into three parts: mean, turbulent, and waves play a significant role in the boundary layer dy- wave-induced components of airflow (Hare et al. 1997). namics between the upper ocean and lower atmosphere In this situation, the total stress t above the surface (Grachev and Fairall 2001; Veron et al. 2009; Babanin tot waves can be expressed as follows: et al. 2012). The influence of surface waves on wind 5 1 1 ttot tturb twave tvisc . (1) Denotes content that is immediately available upon publication as open access. This article is licensed under a Creative Commons Attribution 4.0 license (http://creativecommons.org/ Corresponding author: F. Qiao, qiaofl@fio.org.cn licenses/by/4.0/). DOI: 10.1175/JPO-D-18-0266.1 Ó 2019 American Meteorological Society Unauthenticated | Downloaded 10/02/21 06:19 PM UTC 2018 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 49 Here tturb is the turbulent shear stress, twave is the wave- depth behave differently than surface waves in the open induced stress, and tvisc is the viscous stress that is as- ocean (Huang et al. 1983). Additionally, wind stress sumed to be negligible (Hanley and Belcher 2008). The depends on the surface wave states, which are affected 2 total wind stress can be represented by ttot 5 CDU ac- by depth-induced processes in shallow water areas cording to the bulk formula, CD is the drag coefficient, (Zhao et al. 2015). In the present study, we investigate and U is the wind speed. Within a layer not affected by the influence of surface waves on wind stress under waves, the wave-induced stress equals zero, and the total different sea states and water depth conditions using stress equals the turbulent stress. The wave-induced numerical models and field observations over northern stress is strongly dependent on the wave age (Hanley South China Sea (SCS) and western East China Sea and Belcher 2008). For young wind seas, the wave ex- (ECS) coastal regions. tracts momentum from the wind so that the wave- This study is organized as follows. The field observa- induced stress is positive (twave . 0; Komen 1994)andthe tions from two coastal regions and the datasets used in momentum flux is always directed downward (Semedo the comparisons are described in section 2. A simple et al. 2009); with increasing wave age, twave decreases constant flux model that includes wave-induced stress until reaching zero, and the sign is reversed for old seas, parameterizations is presented in section 3. The water and becomes negative (twave , 0; Smedman et al. 1999; depth factor is considered in the model to indicate the Grachev and Fairall 2001). In the situation of twave , 0and impact of surface waves on the wind stress, and the re- jtwavej . jtturbj, the total stress becomes negative (ttot , 0), sults of comparisons with observations are also de- meaning that upward momentum transfer from the ocean scribed in this section. Finally, our conclusions are to the atmosphere. summarized and discussed in section 4. Mixed wind sea and swell or pure swell conditions are the most common in the ocean. Wind seas are 2. Field observations generated locally and travel more slowly; in contrast, swells are usually generated remotely and travel faster The two finite-depth field measurements used in this than the local wind (Hanley and Belcher 2008). Janssen study were obtained from the BoHe observation tower (1989) found a considerable enhancement of momen- (BHOT) in the northern SCS and from the DongOu tum transfer for young wind seas according to a quasi- observation tower (DOOT) in the western ECS (Fig. 1). linear wind-wave model. In a subsequent study, he The tower is more suitable for flux measurements and proposed that the stress over the surface wave is de- provides less flow distortion and motion influence than a pendent on the wind speed and wave-induced stress conventional shipborne system. An eddy covariance (Janssen 1992). There is some evidence from field ob- system was mounted 17 m above the mean sea surface servations and numerical simulations to support that and 6 m away from the bulky platform, toward 708 north the swell can lead to dramatic changes in wind stress by east. This eddy covariance system was used to ob- and modulate the wind profiles (Semedo et al. 2009; serve the momentum and heat fluxes, and a bottom- Song et al. 2015; Jiang et al. 2016; Zou et al. 2018). supported acoustic wave and current (AWAC) buoy Under low wind conditions, the faster-traveling swell was deployed at a mean water depth of approximately could result in an upward momentum transfer (ttot , 0) 16 m (Fig. 1b) to record the surface displacement at the from the ocean to the atmosphere, which has a con- BoHe site (21826.50N, 111823.50E). Details of this setup siderable effect on the turbulence structure of the can be found in Chen et al. (2018). whole marine atmospheric boundary layer (Drennan The DOOT is located at 278400N, 1218210E, which is et al. 1999; Grachev and Fairall 2001; Semedo et al. approximately 24 km from the coastline (Fig. 1a). The 2009; Nilsson et al. 2012). In addition, swell-induced mean water depth at this site is approximately 28 m. The perturbations have been observed at heights greater site has a 7-m tower at the platform, which has a hollow than 10 m, causing distinct peaks at the dominant swell structural steel frame. To avoid the flow distortion frequency of the turbulent velocity spectra (Rieder and caused by the bulky platform, eddy covariance system Smith 1998; Högströmetal.2015; Chen et al. 2018), and probes, including a three-dimensional ultrasonic and the vertical velocity is more coherent with the swells infrared gas analyzer, were mounted at the northeast than the horizontal velocity (Grare et al. 2013). corner of the tower, approximately 26 m above the While the aforementioned studies have provided new mean sea surface, toward 308 north by east and were insights into these complicated phenomena, the water fixed to a stable boom that extended vertically upward depth factor is not considered in numerical models 2 m from the tower top and horizontally outward ap- (Hanley and Belcher 2008; Semedo et al. 2009; Zou et al. proximately 0.5 m (Fig. 1c). During the observational 2018). Surface waves in the coastal regions with finite periods, an AWAC buoy was also deployed on the Unauthenticated | Downloaded 10/02/21 06:19 PM UTC AUGUST 2019 C H E N E T A L . 2019 FIG. 1. (a) Regional map and locations of the BHOT and DOOT and photos of the (b) BHOT and (c) DOOT. swell seabed to simultaneously measure surface waves near In addition, the swell significant wave height Hs and windsea this tower site. wind sea significant wave height Hs were calculated based on the one-dimensional wave spectrum (Högström a. Basic information et al.