This study explores the behavior of the exchange coefficients for wind stress (C_D), sensible heat flux (C_H), and water vapor flux (C_E) as functions of surface wind speed (U₁₀) and atmospheric stability using direct turbulent flux measurements obtained from a platform equipped with fast‐response turbulence sensors in a low‐to‐moderate wind region. Turbulent fluxes are calculated using the eddy‐correlation method with extensive observations. The total numbers of quality‐controlled 30 min flux runs are 12,240, 5813, and 5637 for estimation of C_D, C_H, and C_E, respectively. When adjusted to neutral stability using the Monin‐Obukhov similarity theory (MOST), we found that C_DN, C_HN, and C_EN decrease with neutral‐adjusted wind speed when wind speed is less than 5 m/s. C_DN is constant over the range 5 m/s < U_10N < 12 m/s, then increases with U_10N when U_10N > 12 m/s. In contrast, C_HN and C_EN exhibit no clear dependence on wind speed and are generally constant, with mean values of 0.96 × 10⁻³ and 1.2 × 10⁻³, respectively. This behavior of neutral exchange coefficients is consistent with the findings of previous studies. We also found that C_DN under offshore winds is generally greater than that under onshore wind conditions, which is ascribed to the younger wind waves present due to the shorter fetch in the former case. However, this behavior is not exhibited by C_HN or C_EN. The original C_D, C_H, and C_E values without MOST adjustment are also investigated to develop a new parameterization based on wind speed and stability. Three stability parameters are tested, including the bulk Richardson number, stability as defined in COARE 3.0, and a simplified Richardson number using the Charnock parameter. This new parameterization is free of MOST and the associated self‐correlation. Compared with previous studies and COARE 3.0 results, the new parameterization using the simplified Richardson number performs well, with an increased correlation coefficient and reduction of root‐mean‐square error and bias.

The effect of swell waves on atmospheric boundary layer turbulence under low winds was explored using data from a fixed platform located in the South China Sea. The wind spectra, cospectra, and Ogive curve measured at a height of 8 m above the mean sea surface provided direct evidence that wind stress was affected by swell waves. To interpret such phenomena, an improved approach was derived based on the fact that the total wind stress was the vector sum of turbulent stress and wave‐coherent stress. Different from the approaches of earlier studies, our approach did not align the turbulent stress with the mean wind speed. The influence of swell waves on the magnitude and direction of the total wind stress was analyzed using our approach. The results showed that the wave‐coherent stress derived from our data accounted for 32% of the total wind stress. The magnitude and angle of the wind stress changed by swell waves depended on the relative angle between the turbulent stress and swell direction.

The generation of ocean surface waves by wind has been studied for a century, giving rise to wave forecasting and other crucial applications. However, the reacting force of swell waves on the turbulence in the marine atmospheric boundary layer (ABL) remains unknown partly due to the unclear magnitude and profile of wave-coherent (WC) stress. In this study, the intersection frequency between the energy-containing range and inertial subrange range in the turbulent spectra is identified based on the attached eddy model (AEM), as the intersection modulated by swell wave could help to comprehend the physical process between the ocean surface wave and the marine ABL. Using observations from a fixed platform located in the South China Sea, this study shows that the intersection when the WC stress accounts for a lower proportion of the total wind stress (10%). Considering the unclear magnitude and profile of WC stress, this study derives a new function to depict the WC stress. Significance Statement: Turbulence located in the energy-containing range of the spectra is the primary source of momentum, heat, and water vapor fluxes between the ocean and marine atmospheric boundary layer (ABL). Thus, a better understanding of the turbulent structure within the wave boundary layer can help us accurately parameterize those fluxes. In addition to absorbing energy from the marine ABL, waves, especially swell waves, influence the turbulent structure. However, until now, the turbulent structure is unclear due to the unclear wave-coherent stress size and profile. In this study, a function that can depict the wave-coherent stress is proposed. We found that the wave-coherent stress can modulate the intersection between the energy-containing range and the inertial subrange range in the turbulent spectra.

The turbulent structure within the marine atmospheric boundary layer is investigated based on four levels of observations at a fixed marine platform. During and before a cold front, the ocean surface is dominated by wind sea and swell waves, respectively, affording the opportunity to test the theory recently proposed in laboratory experiments or for flat land surfaces. The results reveal that the velocity spectra follow a k^-1 law within the intermediate wavenumber (k) range immediately below inertial subrange during the cold front. A logarithmic height dependence of the horizontal velocity variances is also observed above the height of 20 m, while the vertical velocity variances increase with increasing height following a power law of 2/3. These features confirm the Attached Eddy Model and the “top-down model” of turbulence over the ocean surface. However, the behavior of velocity variances under swell conditions is much different from those during the cold front, although a remarkable k^-1 law can be observed in the velocity spectra. The quadrant analysis of the momentum flux also shows a significantly different result for the two conditions. The turbulent structure within the marine atmospheric boundary layer is investigated based on four levels of observations at a fixed marine platform. During and before a cold front, the ocean surface is dominated by wind sea and swell waves, respectively, affording the opportunity to test the theory recently proposed in laboratory experiments or for flat land surfaces. The results reveal that the velocity spectra follow a k^-1 law within the intermediate wavenumber (k) range immediately below inertial subrange during the cold front. A logarithmic height dependence of the horizontal velocity variances is also observed above the height of 20 m, while the vertical velocity variances increase with increasing height following a power law of 2/3. These features confirm the Attached Eddy Model and the “top-down model” of turbulence over the ocean surface. However, the behavior of velocity variances under swell conditions is much different from those during the cold front, although a remarkable k^-1 law can be observed in the velocity spectra. The quadrant analysis of the momentum flux also shows a significantly different result for the two conditions.