Momentum flux in off-shore flow

Aircraft measurements of the mean flow and turbulence during the Shoaling Waves Experiment near Duck, NC, USA are analyzed to examine the spatial structure of off-shore flow. Repeated aircraft passes were flown at multiple levels on flight tracks both parallel and perpendicular to the coast. An interpolation technique is applied to develop fetch-height cross-sections that are suitable for future work on verification of numerical models.

For stable flow of warm air over cool water, the low-level momentum flux over the sea decreases rapidly with increasing fetch for the first few kilometers and gradually reaches equilibrium values by 10 km off the coast. This decrease is thought to be associated with decay of advected land-based turbulence by the offshore flow, which accelerates downstream from the coast. A few kilometers off-shore, an increase in the downward momentum flux with height is observed in the lowest 100 m. The initial formation of an elevated wind stress maxima is attributed in part to the height-dependence of advection.

A simple relaxation model is developed that relates the decrease in the sea surface stress to travel time from the coast. Variations in the time scale that describes the travel time-dependence of the stress over the sea are related to variations in boundary layer depth over land.