Shipboard Marine X-band Radar Current and Bathymetry Retrieval in the Louisiana Bight

This study evaluates shipboard marine X-band radar (MR) near-surface current and bathymetry measurements from the shallow waters of the Louisiana Bight, directly west of the Mississippi River mouth. The retrieval algorithm employed here is based on the surface wave signal within three-dimensional wavenumber-frequency MR backscatter intensity variance spectra. A numerical least-squares technique solves for the near-surface current vector and bathymetry that minimize the signal's deviation from the linear wave dispersion relationship. The MR data were collected during a research cruise that investigated submesoscale processes and their impact on oil spill transport across the continental shelf. The MR results are validated using measurements from 500 GPS-equipped surface drifters, a shipboard acoustic Doppler current profiler, and a shipboard singlebeam echo sounder. Earlier results from the same experiment but using a different set of sensors indicate strong upper ocean vertical current shear over a 3.5 h period, despite only mild wind forcing. Here, the MR currents are derived as a function of wavenumber, providing a measure of vertical shear for the full duration of the cruise. Strong vertical shear is frequently observed, with a maximum difference of 0.42 m s⁻¹ between the MR high (effective depth of ~0.9 m) and low (~2.4 m) wavenumber bins. Treating the drifter and echo sounder measurements as truth, the accuracies of the MR near-surface current and bathymetry results are 0.04-0.07 m s⁻¹ and 1.2 m (or 7% of the mean water depth). However, it is shown that ~50% of the MR–drifter current differences is due to vertical shear. The shallow water MR near-surface current accuracy thus matches findings from an earlier validation in the deep waters of the DeSoto Canyon, where vertical shear was much weaker.