Surface Current Maps Research Articles

Experimental study of the Rhone plume. Part I: physics and dynamics

The complicated dynamic processes occurring when fluvial waters mix with marine waters control the nature and the fluxes of materials exported by rivers to the sea. Understanding these processes is of primary importance in evaluating budgets. In wide-open estuarine situations these processes take place under the influence of an intense turbulence induced by tides. Conversely, the Rhone waters spread into the Mediterranean Sea in the form of an easily distinguishable buoyant plume often extending far offshore from the mouth of the river. The aim of this study is to describe the dynamic and hydrological fields on the basis of eulerian VHF radar mapping of surface currents coupled with lagrangian in situ physical or geochemical measurements. This paper focuses mainly on physical processes. Data analysis provides an insight into the typical scales of variability of the phenomena, either vertically or horizontally. It is shown that morphological fluctuations can occur (mainly in orientation and offshore extent) according to wind and outflow forcing conditions, and that the vertical structure variations can range from an almost unaltered two-layer distribution to an evolving and deepening mixed layer situation, or even to a more complex superimposed multi-layered structure. The simultaneous examination of radar maps and lagrangian drifter tracking allows the main dynamic tendencies of the Rhone plume to be sketched out.

Observations of the Florida Current with two over‐the‐horizon radars

We show the space and time variability of the Florida Current in the Southern Straits of Florida using imagery derived from measurements made with two high‐frequency over‐the‐horizon (ionospheric) radars. Between March and August 1997, ten surface current maps show how the Florida Current meanders in response to mesoscale eddies propagating eastward through the Straits. Satellite altimetry aids in the interpretation of the radar imagery by showing upstream eddy development in the Gulf of Mexico.

Corrections To "A Test Of Ocean Surface-current Mapping With Over-the-horizon Radar"

Corrections To "A Test Of Ocean Surface-current Mapping With Over-the-horizon Radar"

A test of ocean surface-current mapping with over-the-horizon radar

A two-day test with a decametric over-the-horizon (OTH or ionospheric) radar in Virginia attempted to map the radial component of ocean surface currents over a 210 000 km/sup 2/ area that includes the Florida Straits and parts of the Gulf of Mexico as distant as 1500 km. Ionospheric motions distort and bias individual measurements, but their effects are mitigated by a combination of strategies that take advantage of the different space and time scales of oceanic and ionospheric motions. In addition, nearby land echoes are used as zero Doppler references to correct for ionospheric shifts. The result is a composite picture of the Florida Current and ancillary surface flows with 10-15-km resolution. The picture agrees quantitatively with known currents in the region, but reveals dynamical features with new detail. Concurrent sea-surface topography in the Gulf of Mexico, derived using tandem altimetric observations from the TOPEX/Poseidon and ERS-1 satellites, confirms that a region where the OTH radar measures a southwestward current greater than 1 m/s/sup -1/ coincides with the confluence of the Tortugas Gyre and the Gulf of Mexico Loop Current. These results suggest that consistent surface current maps can be constructed by using OTH radar to repeatedly interrogate a region of interest, perhaps over several days.

Mapping Surface Currents in Monterey Bay With CODAR-Type HF Radar

HIGH-FREQUENCY (HF) radar measurements have been employed around Monterey Bay, CA, to measure ocean surface currents since February 1992. The first array consisted of two older-generation CODAR instruments located at sites near Monterey in the south and Moss Landing, halfway around the bay to the north (Front Cover: Fig. 1). In 1994, the southern site was replaced with a newer-generation SeaSonde system at Pt. Pinos and a similar unit was installed near Santa Cruz on the northern shore of Monterey Bay. Finally, in 1996, the CODAR system in Moss Landing was replaced by a modern SeaSonde unit. Several months time series of two-dimensional surface currents have been collected for Monterey Bay since the first CODAR units were installed. These data represent the most extensive measurements collected to date from compact, directionfinding HF radar systems (Barrick and Lipa, 1997: Paduan and Graber, 1997). Indeed, Monterey Bay is the only location where continuous HF radar measurements are underway. The geometry of the bay (a curving coastline with a radius of -20 km) is ideal for a multisite HF radar network. The overwater distance is close to the typical radar range and, when three or more shore locations are utilized, the entire region can be observed without lost coverage along the baseline between radar sites. This geometry also makes Monterey Bay well suited for validation and development of the algorithms for currents, waves, and wind direction because a large ocean region is oversampled. Not only can in situ measurements within this region be compared with the remotely sensed estimates, but self-consistency (or lack thereof) in the HF radar measurements can be used to characterize errors in the radar data (Melton, 1995).

Ocean surface features and currents measured with synthetic aperture radar interferometry and HF radar

This paper describes the results of the first quantitative comparison between high‐resolution ocean surface current fields extracted from interferometric synthetic aperture radar (INSAR) measurements and those from a high‐frequency (HF) ocean surface current radar (OSCR) system. Data from each of these radar systems along with supporting measurements from shipboard and buoy‐mounted sensors were collected during the High‐Resolution Remote Sensing Experiment (High‐Res) on June 20, 1993, on the continental shelf off the coast of Cape Hatteras, North Carolina. Both components of the surface current were obtained from the INSAR system at roughly 10‐m resolution from two orthogonal flight legs over the region separated in time by about 30 min. The OSCR system measured two‐dimensional surface current vectors at about 1‐km resolution over this same region, while the USNS Bartlett was collecting hydrographic samples and near‐surface current measurements. Two‐dimensional wave spectra as well as meteorological and additional current measurements were collected at two buoys within the experimental area. We discuss in the paper two techniques for eliminating the effect of surface wave motion on the INSAR current estimates. One method relies on some knowledge of the local wind and wave field and the use of a microwave scattering model. The other method makes use of a few in situ current measurements spaced at different range locations across the INSAR image. Using either of these techniques, we find that the agreement between the INSAR and OSCR current estimates is generally very good. Furthermore, the INSAR current and magnitude imagery show the presence of undulating surface features where abrupt changes in the current speed and direction occurred. The ship surveys indicate that these features were caused by the collision of water masses of different density. We show for the first time in this paper a high‐resolution, area‐extensive vector surface current map derived from the INSAR of the two‐dimensional flow in the vicinity of these features. Our results demonstrate convincingly that high‐resolution oceanic surface current vectors can be derived from INSAR current measurements and that these measurements may be very beneficial for detailed studies of the dynamics of small‐scale surface features in regions of strong current divergences or shears.

Current meter observations from the Labrador and Newfoundland shelves and comparisons with barotropic model predictions and IIP surface currents

Abstract Recent current measurements from the southern Labrador and northeastern Newfoundland shelves confirm the presence of inshore and offshore branches of the Labrador Current with high mean currents and low standard deviations. At mid‐shelf weaker and more variable currents occur over the banks, and cross‐shelf flows are found to be associated with the shelf topography. An annual cycle of the inshore branch, in phase with wind forcing, is significant on the NE Newfoundland Shelf but not detectable on Hamilton Bank. The phase of the annual cycle in the offshore branch is consistent with buoyancy, not wind forcing. The observations compare reasonably well with results from a barotropic model for the region and the International Ice Patrol (IIP) surface current map. Differences occur particularly in regions of high bathymetrie curvature or an ill‐defined shelf break. The model location of the Labrador Current lies inshore of that indicated by the data, suggesting the need for better definition of the no...

A Case Study of Wave–Current Interaction in a Strong Tidal Current

Abstract During August 1991, a field program was carried out in the vicinity of Cape St. James, off the British Columbia coast, where a strong tidally driven flow interacts with an active wave climate. Surface current maps were obtained from a CODAR-type HF radar (Seasonde) over an area of about 350 km2 around the cape. A series of Loran-C drifters were also deployed during the experiment and used as ground truth for the radar. A comparison between the drifter and the radar surface currents indicates reasonable agreement. Wave information was acquired with three Waverider buoys deployed around the cape. A significant modulation of the wave properties at the tidal period was observed for the buoy located in the area where the currents are maximum. The tidally induced changes in the wave field are modeled with a local wave–current interaction model based on wave action conservation and on a high-frequency limiting spectral shape. The model is applied on a period of 11 days for which the wind was relatively ...

Large-scale mapping of ocean surface currents with dual over-the-horizon radars

DETAILED information about near-surface ocean currents is needed for effective fisheries management, pollution mitigation, search and rescue, and climate studies, but the present generation of measurement techniques provides only limited spatial and temporal resolution or coverage1,2. In near-coastal environments, pairs of shore-based high-frequency radars have been used to map surface currents over an area of a few hundred square kilometres3,4. The potential for mapping open-ocean current fields has been demonstrated using military high-frequency radars that can be used to 'see' over the horizon for thousands of kilometres by reflecting signals off the ionosphere. But using one radar, only one current component can be mapped by this method5. Here we report the mapping of surface-current vectors obtained from simultaneously employing two such radar systems with overlapping coverage. We obtain a current map in the Florida Straits, about 1,500km from the radars, covering two 70,000km2 areas at a resolution of 10km and O.lm s–1. As it employs only about 2% of the radars' potential coverage, the test shows the potential of this technique for mapping the more energetic features of ocean circulation—such as boundary currents and mesoscale eddy systems—over vast ocean areas.

On the statistics of the phase of microwave backscatter from the ocean surface

In this paper we describe what we believe is the first study of the distribution of the phase of microwave fields backscattered from the ocean surface. Scatterometry data from the Synthetic Aperture Radar and X band Ocean Nonlinearities experiment conducted on the German North Sea Research Platform, Forschungsplatform Nordsee, in November 1990 have been analyzed to reveal the distribution of phase differences as a function of time lag. A theoretical model for these statistics is presented based on modulated Gaussian fields. This theoretical model is shown to be in good agreement with the measured statistics. From this agreement we conclude that the backscattered fields have a Gaussian distribution on short time scales but are modulated in amplitude and frequency by the long surface waves. These results are of more than purely academic interest, with direct applications to the design and analysis of interferometric synthetic aperture radars, a relatively new class of instruments that may be capable of providing high‐resolution maps of ocean surface currents from aircraft or satellites.

Radar observation of an along-front jet and transverse flow convergence associated with a North Sea front

This paper describes the first synoptic mapping of surface currents across a strong and stable tidal mixing front by HF radar. The radar deployment took place along the coast of northeast England during August and early September 1988 in parallel with extensive ship based CTD density and ADCP (Acoustic Doppler Current Profiler) measurements which provided data in the vertical plane to complement those of the HF radar. We describe two main results. Firstly, during a spring-tide period of strengthening inshore density gradients, an along-front jet with speeds of up to 14 cm s −1 was detected in the long term IIF radar residual field. The location and spatial form of this jet correspond with estimates of geostrophic currents derived from the measured density field. Secondly, a transverse “double-sided” surface flow convergence centred close to the frontal boundary and of net magnitude 4 cm s −1 accompanied the large along-front jet. Such a weaker cross-frontal component has been anticipated on theoretical grounds but never previously observed in this detailed fashion. The experiment underlines the power of a synergistic approach, based on HF remote sensing radar and ADCP, for the study of frontal circulation in coastal zones.

Surface current mapping performance of bistatic and monostatic Delta k-radars

A theoretical analysis of the surface ocean current mapping performance of land-based bistatic and monostatic Delta k-radars is presented. Contour maps of the calculated surface current resolution achievable with both configurations are presented and compared. For the assumed surface conditions, both configurations are seen to be capable of resolving 5 cm/s currents with 100 m spatial resolution out to ranges of several kilometers using reasonable dwell times.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A review of methods of remote sensing of sea-surface conditions by HF radar and design considerations for narrow-beam systems

Present achievements in HF ground-wave radar remote mapping of surface currents and wave characteristics are reviewed, and the features of the different systems that are in use are described. Particular emphasis is given to the problems remaining to ensure effective mapping of nondirectional wave parameters and wave directional spectra over a sector of sea extending up to a range of 150-200 km from a coastal or island site. As an example of current progress, measurements in the United Kingdom of first-moment wave period by single-site radar have achieved measurements to within 15 percent of buoy data over a range from 4 to 9 s. Measurements of significant wave height gave standard deviations of 15 and 20 percent for wave heights less than and greater than 2 m, respectively. In the latter case, a bias of 40 percent was found, which later work suggests is reducible to about 14 percent, using a model-fitting technique. To achieve directional wave spectrum measurement with a single radar requires some 40-dB ratio between the Bragg line return and the noise floor of the Doppler spectra. A two-site radar has been shown to yield advantages for directional spectrum measurement. The design of steerable narrow-beam radars in the 6-25-MHz band to achieve such performance is discussed. Suitable transmitted waveforms to achieve good rejection of interference and to be acceptable in the HF band are put forward. The operational experience in the United Kingdom, obtained with a particular FM interrupted continuous-wave radar system associated with a steerable array, is reported. Results of simulating the effect of the antenna radiation pattern on the observed Doppler spectra are illustrated. The principal effects are on the weaker Bragg line and the second-order continuum around it. This can result in errors in deduced surface wind direction and directional wave spectrum. Finally, some useful results on propagation monitoring of a skywave radar with the aid of a range-Doppler graphical output are reported.

Minimizing Errors of Ocean Currents Measured by Electromagnetic Backscatter

The use of a single HF or microwave radar permits the radial component of the ocean surface current to be mapped for the radar look direction [1, 2]. The azimuthal component of the surface current can also be calculated by invoking the continuity equation for the surface current [3]. A problem with this approach is that the error in the calculated azimuthal currents becomes excessive when the number of pixels in the current map is large. In this paper we present a statistical analysis of the relevant errors. We present an algorithm which reduces the errors to alevel comparable to the optimum level that can be achieved. We compare the theoretical results with those obtained from a computer simulation of the process. The results indicate that surface current maps for large areas of the ocean are feasible using a single radar system.

The analysis and digital signal processing of NOAA's surface current mapping system

We describe newly developed numerical procedures used to analyze and process sea-echo data acquired with National Oceanic and Atmospheric Administration's (NOAA) dual-site HF-radar system called Coastal Ocean Dynamics Applications Radar (CODAR). CODAR is a transportable shore-based system that can map surface currents out to a nominal range of 50 km. Since its introduction in 1976, it has performed well in a dozen major experiments. Until recently, however, the data processing was labor intensive, difficult to understand, and slow. The processing presented here largely corrects these difficulties, giving CODAR reliable real-time mapping capabilities.