Wind Turbulence over Misaligned Surface Waves and Air–Sea Momentum Flux. Part II: Waves in Oblique Wind
- 1 January 2022
- journal article
- research article
- Published by American Meteorological Society in Journal of Physical Oceanography
- Vol. 52 (1), 141-159
- https://doi.org/10.1175/jpo-d-21-0044.1
Abstract
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. Therefore, 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. 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. Therefore, 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.Keywords
Funding Information
- National Science Foundation (1458984)
This publication has 47 references indexed in Scilit:
- Swell and Slanting-Fetch Effects on Wind Wave GrowthJournal of Physical Oceanography, 2007
- Wind Profile and Drag Coefficient over Mature Ocean Surface Wave SpectraJournal of Physical Oceanography, 2004
- On the limiting aerodynamic roughness of the ocean in very strong windsGeophysical Research Letters, 2004
- Wind Stress Vector over Ocean WavesJournal of Physical Oceanography, 2003
- Numerical Simulation of Sea Surface Directional Wave Spectra under Hurricane Wind ForcingJournal of Physical Oceanography, 2003
- Wind forcing in the equilibrium range of wind-wave spectraJournal of Fluid Mechanics, 2002
- The Influence of Swell on the DragPublished by Cambridge University Press (CUP) ,2001
- Evidence for the Effects of Swell and Unsteady Winds on Marine Wind StressJournal of Physical Oceanography, 1999
- Coupled sea surface‐atmosphere model: 1. Wind over waves couplingPublished by American Geophysical Union (AGU) ,1999
- On the Dependence of Sea Surface Roughness on Wave DevelopmentJournal of Physical Oceanography, 1993