Significant wave height is one of the most important parameters for characterizing ocean waves and essential for coastal protection, shipping, as well as off shore industry operations. Within this paper, a robust method is introduced for retrieving significant wave heights from Doppler speed measurements acquired with a coherent-on-receive marine radar. The Doppler velocity is caused by the surface scattering in the line of site of the radar. To a huge extent its periodic component is induced by the orbital motions associated with surface waves. The proposed methodology is based on linear wave theory, accounts for projection effects caused by the fixed antenna look direction, and was applied to a coherent-on-receive radar operating at X-band with vertical polarization in transmit and receive. To show the overall performance of the method, a data set consisting of approximately 100 days of radar measurements was analyzed and used to retrieve significant wave heights. Comparisons to wave measurements collected by a wave rider buoy resulted in a root-mean-square (rms) error of 0.21 m and a bias of 0 m without any calibration parameters needed. To further improve the accuracy of significant wave height, a calibration factor needs to be accounted for, which improves the rms error to 0.15 m with a negligible bias of -0.01 m.

Video imagery of surface waves recorded from a small off-the-shelf quadcopter with a self-stabilizing camera gimbal is analyzed to estimate the surface current field. The nadir looking camera acquires a short image sequence, which is geocoded to Universal Transverse Mercator coordinates. The resulting image sequence is used to quantify characteristic parameters (wavelength, period, and direction) of short (0.1-1 m) surface waves in space and time. This opens the opportunity to fit the linear dispersion relation to the data and thus monitor the frequency shift induced by an ambient current. The fitting is performed by applying a spectral energy-based maximization technique in the wavenumber-frequency domain. The current field is compared with measurements acquired by an acoustic Doppler current profiler mounted on a small boat, showing an overall good agreement. The root-mean-square error in current velocity is 0.09 m/s with no bias.

The Lagrangian Submesoscale Experiment (LASER) involved the deployment of ~1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V F. G. Walton Smith was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at ~500-m resolution up to a maximum range of ~3 km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR–drifter pairs, the root-mean-square error for the current speed is 4 cm and that for the current direction is 12°. The MR samples currents at a greater effective depth than the CARTHE drifters (1–5 m vs ~0.4 m). The mean MR–drifter differences are consistent with a wave- and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 15°). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP.

In this paper, the retrieval of sea surface current velocity from vertically polarized (V-pol) X-band marine radar data is presented. Three different methods, including the iterative least square approach, the normalized scalar product method, and the polar current shell algorithm, that have been used for horizontally polarized data are employed here. A comprehensive comparison of the performance of the three methods is conducted using the radar images collected by the V-pol radar deployed on the Forschungsplattformen in Nord- und Ostsee No. 3 (FINO3) offshore research platform and the acoustic Doppler current profiler (ADCP) data in the North Sea. The results indicate that all three methods are capable of providing reliable current speed and direction measurements from V-pol data, with similar performance. Based on the experimental data for which the current magnitude is less than 0.5 m/s, the radar-derived results agree best with the ADCP data at a depth of 6-8 m, with the root mean square difference for current velocity x- and y-components being 7.2-8.9 cm/s. The correlation coefficients between the radar-derived and ADCP-measured current velocity components are as high as 0.87-0.93.

In February 2020, a 120‐km‐wide freshwater plume was documented by satellite and in situ observations near the Demerara Rise (7°N/54°W‐56°W). It was initially stratified in the upper 10 m with a freshwater content of 2–3 m of Amazon water distributed down to 40 m. On February 2nd, ship transects indicate an inhomogeneous shelf structure with a propagating front in its midst, whereas minimum salinity close to 30 pss was observed close to the shelf break on February 5th. The salinity minimum eroded in time but was still observed 13–16 days later with 33.3 pss minimum value up to 400 km from the shelf break. At this time, the mixed layer depth was close to 20 m. The off‐shelf flow lasted 10 days, contributing to a plume area extending over 100,000 km² and associated with a 0.15 Sv (10⁶ m³ s⁻¹) freshwater transport. The off‐shelf plume was steered northward by a North Brazil Current ring up to 12°N and then extended westward toward the Caribbean Sea. Its occurrence followed 3 days of favorable wind direction closer to the Amazon estuary, which contributed to north‐westward freshwater transport on the shelf. Other such events of freshwater transport in January–March are documented since 2010 in salinity satellite products in 7 out of 10 years, and in 6 of those years, they were preceded by a change in wind direction between the Amazon estuary and the Guianas favoring the north‐westward freshwater transport toward the shelf break.

In this paper, a novel current inversion algorithm from X-band marine radar images is proposed. The routine, for which deep water is assumed, begins with 3-D FFT of the radar image sequence, followed by the extraction of the dispersion shell from the 3-D image spectrum. Next, the dispersion shell is converted to a polar current shell (PCS) using a polar coordinate transformation. After removing outliers along each radial direction of the PCS, a robust sinusoidal curve fitting is applied to the data points along each circumferential direction of the PCS. The angle corresponding to the maximum of the estimated sinusoid function is determined to be the current direction, and the amplitude of this sinusoidal function is the current speed. For validation, the algorithm is tested against both simulated radar images and field data collected by a vertically-polarized X-band system and ground-truthed with measurements from an acoustic Doppler current profiler (ADCP). From the field data, it is observed that when the current speed is less than 0.5 m/s, the root mean square differences between the radar-derived and the ADCP-measured current speed and direction are 7.3 cm/s and 32.7°, respectively. The results indicate that the proposed procedure, unlike most existing current inversion schemes, is not susceptible to high current speeds and circumvents the need to consider aliasing. Meanwhile, the relatively low computational cost makes it an excellent choice in practical marine applications.