A New Empirical Approach to Detect Surface Currents Using Doppler Marine Radar

During three research cruises in the Pacific in 2016 and 2017, a Helmholtz-Zentrum Geesthacht rotating vertically-polarized (VV) X-band Doppler Marine Radar (DMR) was installed onboard the R/V Roger Revelle and the R/V Oceanus. Ocean currents with marine radar are typically acquired with the use of the three-dimensional fast Fourier transform (3D FFT) on a radar image sequence in order to measure the velocity of the ocean waves in comparison to the theoretical wave velocity in the absence of a current. However, dispersion shell currents cannot resolve very fine-scale current variations, such as at the interface of a current front or an internal wave. To fill this gap, the marine radar data were used in a different way. The raw data include the phase of the backscattered signal of each radar pulse, which can be converted to radial Doppler velocities. The Doppler velocity itself includes contributions from a multitude of ocean surface processes. A linear regression was set up to determine a functional relationship between Doppler velocity and surface current. The Doppler Velocity Synthesis algorithm (DoVeS) was developed to synthesize radial Doppler velocity information from a moving ship in order to obtain a two-dimensional vector surface current field around the ship track. Finally, to showcase the fine-scale capabilities of this method, an example is shown where DoVeS is able determine the small-scale variations of the Doppler velocity associated with an internal wave.