Near-Surface Current Mapping by Shipboard Marine X-Band Radar: A Validation

Abstract

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.

Airborne Remote Sensing of the Upper Ocean Turbulence during CASPER-East

“…The compared data sample is very limited, but valuable, as only few attempts of this kind were described in previous literature [28], leaving much room for a more comprehensive validation study in the future. Other most closely related active microwave current retrieval and validation studies employed methodologies based on offshore platform or ship-based marine radars and reported successful comparisons to bottom-mounted ADCP and surface drifters in situ velocity measurements [24,44]. Unlike other velocity estimates shown in this paper, MSAR obtains an instantaneous snapshot of true surface velocities (i.e., the velocity with which the surface carries ≈3 cm long capillary waves).…”

“…The ability to track the propagation of wave phases across the field of view allows for the reconstruction of the dispersion relationship curve. The Doppler shift of the curve is then used to infer the value of the underlying current [24]. Similarly, on a larger scale, shore-based HF radars broadcast low power radio frequencies seaward, and use Doppler shift distortions of the returned Remote Sens.…”

Arctic Sea Ice Drift Measured by Shipboard Marine Radar

“…They can also be used to improve our understanding of air‐sea‐ice interaction processes and sea ice modeling. The authors are presently working on combining the MR sea ice velocity data with MR‐derived near‐surface current fields, which require the presence of a surface wave signal (Lund et al, ), and shipboard wind stress measurements to estimate the ocean drag as a momentum budget residual. If carried out in near‐real time, which could be accomplished easily with a state‐of‐the‐art personal computer, MR sea ice velocity fields could also be used to aid ship routing decisions or risk assessment during on‐ice operations.…”

Measurement Characteristics of Near-Surface Currents from Ultra-Thin Drifters, Drogued Drifters, and HF Radar

“…A recent study by Lund et al [27] compared velocity measurements from shallow (0.4 m) drogued drifters used by Novelli et al [12] with shipboard x-band radar (measuring an effective depth of 1-5 m) and the results yielded an rms difference of current speed of only 4 cm s −1 . This is much less than the difference between the ultra-thin drifter speeds and the CODE-style drifter and HF radar speeds analyzed here.…”