Momentum transfer over the coastal zone

Spatial variations of surface stress over the coastal shoaling zone are studied off-shore of Duck, NC, by the research aircraft, LongEZ, equipped to measure both atmospheric turbulence and oceanic waves. We find that the existence of the shoaling zone is clearly defined by the decrease of the mean square slope of the short waves (wavelength shorter than 2m) and the increase of the mean square slope of the long waves (wavelength longer than 2m) with the off-shore distance. However, the spatial variation of the friction velocity with off-shore distance is much larger with off-shore flow than with on-shore flow.

With on-shore flow, the spatial variation of the stress in the coastal zone is small. The friction velocity is strongly correlated with the mean square slope of the oceanic waves. In addition, the variation of the neutral drag coefficient, which represents the variation of the aerodynamic roughness length at a constant observation height, is well correlated with the atmospheric bulk Richardson number. With off-shore flow, the observed momentum flux significantly decreases with off-shore distance. The relationships 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 influence of the upstream land surface within the field of view of the turbulence measurement. However, these relationships agree well with those for the on-shore flow cases, as the off-shore distance increases until the field of view of the momentum transfer is entirely occupied by the ocean. The results in this study suggests that the influence of the strong turbulence from the land surface in off-shore flow may lead to ambiguous physical interpretation of the correlation between the momentum flux and the derived sea state such as the wave age due to self-correlations through the friction velocity.