This article reviews the discovery, development, and use of high-frequency (HF) radio wave backscatter in oceanography. HF radars, as the instruments are commonly called, remotely measure ocean surface currents by exploiting a Bragg resonant backscatter phenomenon. Electromagnetic waves in the HF band (3–30 MHz) have wavelengths that are commensurate with wind-driven gravity waves on the ocean surface; the ocean waves whose wavelengths are exactly half as long as those of the broadcast radio waves are responsible for the resonant backscatter. Networks of HF radar systems are capable of mapping surface currents hourly out to ranges approaching 200 km with a horizontal resolution of a few kilometers. Such information has many uses, including search and rescue support and oil-spill mitigation in real time and larval population connectivity assessment when viewed over many years. Today, HF radar networks form the backbone of many ocean observing systems, and the data are assimilated into ocean circulation models.

High-Frequency Radar Observations of Ocean Surface Currents

This paper investigates the transport structure of surface currents around the Monterey Bay, CA region. Currents measured by radar stations around Monterey Bay indicate the presence of strong, spatial and temporal, nonlinear patterns. To understand the geometry of the flow in the bay, Lagrangian coherent structures (LCS) are computed. These structures are mobile separatrices that divide the flow into regions of qualitatively different dynamics. They provide direct information about the flow structure but are geometrically simpler than particle trajectories themselves. The LCS patterns were used to reveal the mesoscale flow conditions observed during the 2003 Autonomous Ocean Sampling Network (AOSN-II) experiment.

Drifter paths from the AOSN experiment were compared to the patterns induced by the LCS computed from high-frequency radar data. We verify that the fate of the drifters can be better characterized based on the LCS than direct interpretation of the current data. This property can be exploited to optimize drifter deployment.

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The correlation between surface drifters and coherent structures based on high-frequency radar data in Monterey Bay

[1] Two high-frequency radar stations (CODAR) were installed along the northern California coast in May 2001. Comparisons of radar data with acoustic Doppler current profiler (ADCP) current data and near-surface drifter tracks indicate considerable agreement, with minimum RMS differences of order 0.05–0.15 m/s and average drifter-HF-radar track separation rates of 5 ± 3 km/d. Radar data resolve the three main sources of intraseasonal current variability in the area: (1) upwelling/relaxation dynamics, (2) tidal and diurnal forcing, and (3) inertial currents. Subtidal fluctuations are the largest component of variability, accounting for 45–75% of the variance. Wind-driven dynamics are the dominant source of this subtidal variability (67% of subtidal variability). Both upwelling and relaxation periods exhibit consistent patterns of surface velocity, with nearshore currents being slower and more poleward than offshore currents, which are strongly equatorward. Analysis of tidal and inertial variability indicate that current fluctuations are polarized toward clockwise rotation and are generally weaker and more linearly polarized near the coast. M2 tidal current ellipses switch direction of rotation at the shelf break, suggesting the presence of internal tidal waves. Currents in all frequency bands are deflected and accelerated around Pt. Reyes, and there are indications of increased current variability and changes in flow direction near Cordell Bank. The presence of considerable cross-shore and alongshore gradients in the strength and direction of surface flow patterns, and in particular weak poleward currents over the inner shelf, could have important consequences for plankton retention in the area.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JC002959

HF radar observations of surface circulation off Bodega Bay (northern California, USA)

Dense arrays of surface drifters are used to quantify the flow field on time and space scales over which high-frequency (HF) radar observations are measured. Up to 13 drifters were repetitively deployed off the Santa Barbara and San Diego coasts on 7 days during 18 months. Each day a regularly spaced grid overlaid on a 1-km2 (San Diego) or 4-km2 (Santa Barbara) square, located where HF radar radial data are nearly orthogonal, was seeded with drifters. As drifters moved from the square, they were retrieved and replaced to maintain a spatially uniform distribution of observations within the sampling area during the day. This sampling scheme resulted in up to 56 velocity observations distributed over the time (1 h) and space (1 and 4 km2) scales implicit in typical surface current maps from HF radar. Root-mean-square (RMS) differences between HF radar radial velocities obtained using measured antenna patterns, and average drifter velocities, are mostly 3–5 cm s−1. Smaller RMS differences compared wi...

https://www.cordc.ucsd.edu/projects/mapping/documents/2006_Ohlmann-etal_JTECH.pdf

Interpretation of Coastal HF Radar–Derived Surface Currents with High-Resolution Drifter Data

Using multidisciplinary observations from regional- to small-scale, we examined the development of thin phytoplankton layers in water mass frontal zones of a coastal upwelling system. Two fronts developed successively in the same region of Monterey Bay, California, USA, during August and September 2003: (1) when warm, fresh offshore waters flowed into the bay following relaxation and reversal of upwelling favorable winds, and (2) when a cold upwelling filament flowed into the bay after upwelling favorable winds resumed. Thin phytoplankton layers were observed dur- ing the presence of both fronts. The layers exhibited peaks in chlorophyll fluorescence and optical backscattering, indicating biomass maxima in the layers. Maximum chlorophyll concentrations in the layers ranged from 11 to 37 µg l -1 and were 4 to 55 times greater than background levels. Layer ver- tical thickness ranged from 1 to 5 m, averaging 2.3 m. All thin layers were in the thermocline, near the 12.5°C isotherm, and the depth of the layers varied between 12 and 33 m, in parallel with varia- tions in thermocline depth. Synoptic mapping of the first frontal zone shortly before the thin layers developed showed strong phytoplankton patchiness. The role of vertical shear in thin layer formation from phytoplankton patches is supported by multiple results: (1) most thin layers (92%) were associ- ated with sharp changes in the direction of horizontal currents; (2) layer thickness was significantly (p < 0.03) negatively correlated with shear; (3) the median shear profile, computed from all thin layer velocity profiles, peaked sharply at the center of thin layers.

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Phytoplankton thin layers caused by shear in frontal zones of a coastal upwelling system

This paper focuses on the validation of remotely sensed ocean surface currents from SeaSonde-type high-frequency (HF) radar systems. Hourly observations during the period July 22, 2003 through September 9, 2003 are used from four separate radar sites deployed around the shores of Monterey Bay, CA. Calibration of direction-finding techniques is addressed through the comparisons of results obtained using measured and ideal (i.e., perfect) antenna patterns. Radial currents are compared with observations from a moored current meter and from 16 surface drifter trajectories. In addition, four overwater baselines are used for radar-to-radar comparisons. Use of measured antenna patterns improves system performance in almost all cases. Antenna-pattern measurements repeated one year later at three of the four radar locations exhibit only minor changes indicating that pattern distortions are stable. Calibrated results show root-mean-square (rms) radial velocity differences in the range of 9.8-13.0 cm/s, which suggest radar observation error levels in the range of 6.9-9.2 cm/s. In most cases, clear evidence of bearing errors can be seen, which range up to 30deg for uncalibrated radar-derived radial currents and up to 15deg for currents obtained using measured antenna patterns. Bearing errors are not, however, constant with angle. The results recommend use of measured antenna patterns in all SeaSonde-type applications. They also recommend an expanded simulation effort to better describe the effects of antenna-pattern distortions on bearing determination under a variety of ocean conditions

Kyung Cheol Kim

Papers

Calibration and Validation of Direction-Finding High-Frequency Radar Ocean Surface Current Observations