Observations of the Near Surface Vertical Current Shear

We will present measurements of the vertical current profile in the upper 1-5 meters below the surface utilizing the dispersion relation (DR) method with marine X-band radar (MR) backscatter intensity image sequences. The DR method is based on the linear dispersion relation of gravity waves, which produce the dominant signal within the MR data. By observing waves of different wavelengths we have extended the DR-method to retrieve the near-surface current profile. Utilizing quadcopter based video camera (VC) data we furthermore estimate currents in the upper 0.5 m of the water column by Particle Tracking Velocimetry (PTV). This method yields the velocity of Lagrangian drifters (bamboo plates, oranges and CARTHE drifters), representing currents in 3 different layers. The drifters have a very small area above the water surface exposed to the wind, which helps minimize potential windage issues. Last but not least we applied the DR-method to the VC image sequences, allowing us to measure very high resolution current fields in space (5 m) and time (30 s) as well as the vertical current profile in the upper half meter.

We will present results from measurements acquired at German research platform FINO-3, in the German Bight of the North Sea and during the Bear Cut Inlet experiment, Miami, USA. We will show that the MR and VC DR-based current measurements correlate well with the PTV and other reference data as well as simple theoretical wind- and wave-driven profiles. The combination of MR and VC data thus yields a good estimate of the current profile from the surface to a depth of several meters.