Measurements of the Upper Surface Current Profile

We present measurements of surface currents within the upper meter using coherent X-band marine radars (MR), video cameras (VC) and an uplooking Acoustic Doppler Current Profiler (ADCP). We will introduce the different measurement techniques applied to MR and VC data and inter compare the measurement results. The radar Doppler speed measurements correlate well to the surface wind speeds as well as the upper centimeter currents measured by VC and therefore provide a good estimate of the upper cm surface currents. We will also show current profiles of the upper meter using the different methods.

To measure the upper cm of the surface currents we use Doppler speed measurements of the MR, which at X-band result from the ~1.5 cm long surface waves propagating in radar look direction. Their propagation speed is the sum of the phase speed and the upper cm surface current. Utilizing a quadrocopter based VC we estimate the upper 10 cm surface currents by Particle Tracking Velocimetry (PTV). This method is typically applied to surfactants, which are often influenced by wind stress. To minimize this issue we deployed oranges as Lagrangian drifters, which are easily tracked by PTV and represent very well the currents in the upper 10 cm. In another approach, we use the VC image sequences to measure wave properties, e.g. wave direction and length as well as phase velocity. These properties allow estimating the surface current, which results from the difference of the observed phase velocity to that given by the linear dispersion relation of gravity waves. Due to the high spatial (up to 1 cm) and temporal resolution (0.04 s) of the VC data we resolve waves starting at wavelengths of a cm. As waves of different wavelengths feel the currents in different water depths, a current profile is deduced. Finally, surface currents are measured with an uplooking ADCP, which resolves currents between 0.25 m and 2 m with a 0.2 m resolution.