More Like this

1. Wind Turbulence over Misaligned Surface Waves and Air–Sea Momentum Flux. Part II: Waves in Oblique Wind

   https://doi.org/10.1175/JPO-D-21-0044.1  

   Husain, Nyla T. ; Hara, Tetsu ; Sullivan, Peter P. ( January 2022 , Journal of Physical Oceanography)

   The coupled dynamics of turbulent airflow and a spectrum of waves are known to modify air–sea momentum and scalar fluxes. Waves traveling at oblique angles to the wind are common in the open ocean, and their effects may be especially relevant when constraining fluxes in storm and tropical cyclone conditions. In this study, we employ large-eddy simulation for airflow over steep, strongly forced waves following and opposing oblique wind to elucidate its impacts on the wind speed magnitude and direction, drag coefficient, and wave growth/decay rate. We find that oblique wind maintains a signature of airflow separation while introducing a cross-wave component strongly modified by the waves. The directions of mean wind speed and mean wind shear vary significantly with height and are misaligned from the wind stress direction, particularly toward the surface. As the oblique angle increases, the wave form drag remains positive, but the wave impact on the equivalent surface roughness (drag coefficient) rapidly decreases and becomes negative at large angles. Our findings have significant implications for how the sea-state-dependent drag coefficient is parameterized in forecast models. Our results also suggest that wind speed and wind stress measurements performed on a wave-following platform can be strongly contaminated by the platform motion if the instrument is inside the wave boundary layer of dominant waves.

   Surface waves increase friction at the sea surface and modify how wind forces upper-ocean currents and turbulence. Therefore, it is important to include effects of different wave conditions in weather and climate forecasts. We aim to inform more accurate forecasts by investigating wind blowing over waves propagating in oblique directions using large-eddy simulation. We find that waves traveling at a 45° angle or larger to the wind grow as expected, but do not increase or even decrease the surface friction felt by the wind—a surprising result that has significant implications for how oblique wind-waves are represented as a source of surface friction in forecast models.

    

   more » « less
2. Misaligned Wind‐Waves Behind Atmospheric Cold Fronts

   https://doi.org/10.1029/2024JC021162  

   Sauvage, C ; Seo, H ; Barr, BW ; Edson, JB ; Clayson, CA ( September 2024 , Journal of Geophysical Research: Oceans)

   Atmospheric fronts embedded in extratropical cyclones are high‐impact weather phenomena, contributing significantly to mid‐latitude winter precipitation. The three vital characteristics of the atmospheric fronts, high wind speeds, abrupt change in wind direction, and rapid translation, force the induced surface waves to be misaligned with winds exclusively behind the cold fronts. The effects of the misaligned waves under atmospheric cold fronts on air‐sea fluxes remain undocumented. Using the multi‐year in situ near‐surface observations and direct covariance flux measurements from the Pioneer Array off the coast of New England, we find that the majority of the passing cold fronts generate misaligned waves behind the cold front. Once generated, the waves remain misaligned, on average, for about 8 hr. The parameterized effect of misaligned waves in a fully coupled model significantly increases the roughness length (185%), drag coefficient (19%), and air‐sea momentum flux (11%). The increased surface drag reduces the wind speeds in the surface layer. The upward turbulent heat flux is weakly decreased by the misaligned waves because of the decrease in temperature and humidity scaling parameters being greater than the increase in friction velocity. The misaligned wave effect is not accurately represented in a commonly used wave‐based bulk flux algorithm. Yet, considering this effect in the current formulation improves the overall accuracy of parameterized momentum flux estimates. The results imply that better representing a directional wind‐wave coupling in the bulk formula of the numerical models may help improve the air‐sea interaction simulations under the passing atmospheric fronts in the mid‐latitudes.

    

   more » « less
3. Wind Turbulence over Misaligned Surface Waves and Air–Sea Momentum Flux. Part I: Waves Following and Opposing Wind

   https://doi.org/10.1175/JPO-D-21-0043.1  

   Husain, Nyla T. ; Hara, Tetsu ; Sullivan, Peter P. ( January 2022 , Journal of Physical Oceanography)

   Air–sea momentum and scalar fluxes are strongly influenced by the coupling dynamics between turbulent winds and a spectrum of waves. Because direct field observations are difficult, particularly in high winds, many modeling and laboratory studies have aimed to elucidate the impacts of the sea state and other surface wave features on momentum and energy fluxes between wind and waves as well as on the mean wind profile and drag coefficient. Opposing wind is common under transient winds, for example, under tropical cyclones, but few studies have examined its impacts on air–sea fluxes. In this study, we employ a large-eddy simulation for wind blowing over steep sinusoidal waves of varying phase speeds, both following and opposing wind, to investigate impacts on the mean wind profile, drag coefficient, and wave growth/decay rates. The airflow dynamics and impacts rapidly change as the wave age increases for waves following wind. However, there is a rather smooth transition from the slowest waves following wind to the fastest waves opposing wind, with gradual enhancement of a flow perturbation identified by a strong vorticity layer detached from the crest despite the absence of apparent airflow separation. The vorticity layer appears to increase the effective surface roughness and wave form drag (wave attenuation rate) substantially for faster waves opposing wind.

   Surface waves increase friction at the sea surface and modify how wind forces upper-ocean currents and turbulence. Therefore, it is important to include effects of different wave conditions in weather and climate forecasts. We aim to inform more accurate forecasts by investigating wind blowing over waves propagating in the opposite direction using large-eddy simulation. We find that when waves oppose wind, they decay as expected, but also increase the surface friction much more drastically than when waves follow wind. This finding has important implications for how waves opposing wind are represented as a source of surface friction in forecast models.

    

   more » « less
4. Drag Coefficient and Its Sea State Dependence under Tropical Cyclones

   https://doi.org/10.1175/JPO-D-21-0246.1  

   Zhou, Xiaohui ; Hara, Tetsu ; Ginis, Isaac ; D’Asaro, Eric ; Hsu, Je-Yuan ; Reichl, Brandon G. ( July 2022 , Journal of Physical Oceanography)

   The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper-ocean current observations [using electromagnetic autonomous profiling explorer (EM-APEX) floats deployed under five tropical cyclones] and a coupled ocean–wave (Modular Ocean Model 6–WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2–3 × 10⁻³for wind speeds of 25–55 m s⁻¹. While the drag coefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is significantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 45°, a feature that is underestimated by current models of sea state–dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left-front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state–dependent parameterizations of the drag coefficient in tropical cyclone conditions.

    

   more » « less
5. Evidence of Langmuir Mixing Effects in the Upper Ocean Layer During Tropical Cyclones Using Observations and a Coupled Wave‐Ocean Model

   https://doi.org/10.1029/2023JC020062  

   Zhou, Xiaohui ; Hara, Tetsu ; Ginis, Isaac ; D’Asaro, Eric ; Reichl, Brandon G. ( October 2023 , Journal of Geophysical Research: Oceans)

   Mixing of the ocean beneath tropical cyclones (TC) cools the surface temperature thereby modifying the storm intensity. Modeling studies predict that surface wave forcing through Langmuir turbulence (LT) increases the mixing and cooling and decreases near‐surface vertical velocity shear. However, there are very few quantitative observational validations of these model predictions, and the validation efforts are often limited by uncertainties in the drag coefficient (C_d). We combine EM‐APEX and Lagrangian float measurements of temperature, salinity, velocity, and vertical turbulent kinetic energy (VKE) from five TCs with a coupled ocean‐wave model (Modular Ocean Model 6—WAVEWATCH III) forced by the drag coefficientC_ddirectly constrained for these storms. On the right‐hand of the storms in the northern hemisphere, where wind and waves are nearly aligned, the measured VKE is consistent with predictions of models including LT and 2–3 times higher than predictions without LT. Similarly, vertical shear in the upper 20 m is small, consistent with predictions of LT models and inconsistent with the large shears predicted by models without LT. On the left‐hand of the storms, where wind and waves are misaligned, the observed VKE and cooling are reduced compared to those on the right‐hand, consistent with the measured decrease inC_d. These results confirm the importance of surface waves for ocean cooling and thus TC intensity, through bothC_dand LT effects. However, the model predictions, even with the LT parameterization, underestimate the upper ocean cooling and mixed layer deepening by 20%–30%, suggesting possible deficiency of the existing LT parameterization.

    

   more » « less