Momentum transfer over the coastal zone

Abstract

Spatial variations of surface stress over the coastal shoaling zone are studied offshore of Duck, North Carolina, by the LongEZ research aircraft, equipped to measure both atmospheric turbulence and oceanic waves. We find that the spatial variation of the friction velocity with offshore distance is much larger with offshore flow than with onshore flow. In general, the mean square slope of the short waves (wavelength shorter than 2 m) decreases with offshore distance, while the mean square slope of the long waves (wavelength longer than 2 m) increases with offshore distance. With onshore flow the friction velocity is strongly correlated with surface waves. In addition, the variation of the neutral drag coefficient is well correlated with the atmospheric bulk Richardson number. With offshore flow the observed momentum flux significantly decreases with offshore distance. Within the first few kilometer offshore, the relationship between the friction velocity and the mean square slope of the short waves and the relationship between the neutral drag coefficient and the atmospheric bulk Richardson number are obscured by the direct influence of the upstream land surface on the measured turbulence. These relationships for offshore flow agree well with those for onshore flow if the fetch is beyond the immediate influence of the land surface. The results in this study suggests that the effects of the strong turbulence advected from over the nearby land surface in offshore flow may lead to ambiguous physical interpretation of the correlation between the momentum flux and the wave state.