Estimation of 3-D current fields near the Rhine outflow from HF radar surface current data

HF radar systems are designed to measure spatially variable sea surface currents. A methodology was developed to complement these data with information about the current variability over the water column in a stratified shallow sea. Current profiles were estimated using a diagnostic model driven by surface current measurements from an HF radar system and by sea surface slopes derived from tide gauge data. The structure of the model has a physical basis but its parameters were derived from an analysis of in-situ current profile measurements. Application of the model to HF radar data from the SCAWVEX Rhine outflow experiment showed fair agreement with in-situ current data. As applications, estimation and tidal analysis of current fields are demonstrated.     

Very high-frequency radar mapping of surface currents

An ocean surface current radar (OSCR) in the very high frequency (VHF) mode was deployed in South Florida Ocean Measurement Center (SFOMC) during the summer of 1999. During this period, a 29-d continuous time series of vector surface currents was acquired starting on 9 July 1999 and ending 7 August 1999. Over a 20-min sample interval, the VHF radar mapped coastal ocean currents over a 7.5 km × 8 km domain with a horizontal resolution of 250 m at 700 grid points. A total of 2078 snapshots of the two-dimensional current vectors were acquired during this time series and of these samples, only 69 samples (3.3%) were missing from the time series. During this period, complex surface circulation patterns were observed that included coherent, submesoscale vortices with diameters of 2 to 3 km inshore of the Florida Current. Comparisons to subsurface measurements from moored and ship-board acoustic Doppler current profiles revealed regression slopes of close to unity with biases ranging from 4 to 8 cm s^-1 between surface and subsurface measurements at 3 to 4 m beneath the surface. Correlation coefficients were 0.8 or above with phases of - 10 to - 20° suggestive of an anticyclonic veering of current with depth relative to the surface current. The radar-derived surface current field provided spatial context for an observational network using mooring-, ship- and autonomous underwater vehicle-sensor packages that were deployed at the SFOMC     

象山港牛鼻山水道潮流场的高频地波雷达观测与分析

作为LORCE计划中构建高频地波雷达观测网的试点,面向象山港牛鼻山水道,在六横岛郭巨山和白马礁各设置了1台OSMAR S50高频地波雷达。在2台雷达合成表面流场有效区域的中间地带,利用Valeport旋桨式海流仪和ADCP定点开展了周日连续观测,以验证高频地波雷达合成表面流场的精度。对比定点流场和高频地波雷达对应数据发现,两者变化相关性较好,高频地波雷达在该点获得的流场有较高精度。借助SCHISM建立的区域模型结果,检验了高频地波雷达数据大面上的可用性。比较发现,观测和模拟值在大的趋势上是一致的：牛鼻山水道为规则半日潮流海域,M2是主要分潮,分潮M2和K1以往复流为主;涨潮时外海海水先通过牛鼻山水道流向象山港内,一段时间后再流向佛渡水道;落潮时象山港内海水率先经牛鼻山水道流出至外海,随后佛渡水道海水再逐渐流入牛鼻山水道。     

Experimental method to measure surface signature generation by seabottom undulations

An experimental method to investigate submarine bedform signatures at the sea surface is described. The study area was the Lister Tief in the German Bight of the southeastern North Sea, a semienclosed tidal basin with asymmetric and very large sand waves. In-situ and remote measurements of surface roughness were obtained simultaneously. An X-band wave monitoring radar, an oceanographic multisensor sea surface buoy, an acoustic Doppler current profiler, and a standard echo sounder were operated on and from board a research vessel while drifting along the tidal channel     

A comparison of multifrequency HF radar and ADCP measurements ofnear-surface currents during COPE-3

A high-frequency multifrequency coastal radar operating at four frequencies between 4.8 and 21.8 MHz was used as part of the third Chesapeake Bay Outflow Plume Experiment (COPE-3) during October and November, 1997. The radar system surveyed the open ocean east of the coast and just south of the mouth of Chesapeake Bay from two sites separated by about 20 km. Measurements were taken once an hour, and the eastward and northward components of ocean currents were estimated at four depths ranging from about 0.5 m to 2.5 m below the surface for each location on a 2 by 2 km grid. Direction of arrival of the signals was estimated using the MUSIC algorithm. The radar measurements were compared to currents measured by several moored acoustic Doppler current profilers (ADCPs) with range bins 2-14 m below the water surface. The vertical structure of the current was examined by utilizing four different radar wavelengths, which respond to ocean currents at different depths, and by using several ADCP range bins separated by 1-m intervals. The radar and ADCP current estimates were highly correlated and showed similar depth behavior, and there was significant correlation between radar current estimates at different wavelengths and wind speed     

The role of the gravity-wave dispersion relation in HF radar measurements of the sea surface

Recent experimental and theoretical findings raise interesting questions about the applicability of the normal gravity-wave dispersion relation at wave frequencies that exceed the spectral peak frequency. The use of the dispersion relation in analysis of HF radar Doppler sea echo is examined in this paper. Drawing on the results of perturbation theory for wave-wave nonlinear interactions, we show that this relation, so essential to echo interpretation in terms of current and wave information, can be employed with no degradation in accuracy for current measurement when the dominant wave frequency is considerably less (by as much as 10) than the radar Bragg resonance frequency. This finding is supported by comparisons of currents measured by HF radar with "surface truth;" the first-order echo must only be identifiable in order to be used accurately. Wave-height directional spectral information can be extracted from the second-order echo at a given radar frequency up to the point (in wave height) where the perturbation solution employed in the inversion process fails; then a lower radar frequency must be used. On the other hand, most conventional wave measuring instruments should not use the dispersion relation for interpretation of data well beyond the spectral peak, because they do not observe wave height as a function of both space and time independently, as does HF radar.     

Residual and tidal circulation revealed by VHF radar surface current measurements in the southern Channel Isles region (English Channel)

Two very high-frequency radars (VHFRs), operating in the southern Channel Isles region (English Channel) in February–March 2003, provided a continuous 27-day long dataset of surface currents at 2 km resolution over an area extending approximately 20 km offshore. The tidal range in the region of study is one of the highest in the world and the coastal circulation is completely dominated by tides. The radar data resolve two modes which account for 97% of the variability of the surface current velocities, with the major contribution of the first mode. This mode accounts for oscillating tidal currents whereas the second mode represents motions emerging from the interaction of tidal currents with capes and islands (eddy in the vicinity of the Point of Grouin and jet south of Chausey). A fortnightly modulation of the modal amplitudes causes the exceptional (more than 600%) variability of currents which is well captured by the VHFR observations. The radar data revealed that tidal circulation in the region is flood-dominated with a strong asymmetry of current velocity curve. Wind events and fortnightly variability affect the course of tidal cycle by modifying the magnitude and duration of ebb and flood. In addition to expected features of coastal circulation (tidally dominated flow, eddies) and high wind-current coupling, the residual currents revealed a strong cross-shore structure in the mean and a significant variability which has the same order of magnitude.     

高频地波雷达单站测量表层海流的原理

运用高频地波雷达测量表层海流矢量,一般均采用双站测量方案,因而,需要大量的人力和昂贵的设备投入。该项研究在分析双站测量原理的基础上,结合海洋学原理和合理的假设,推导并给出了利用单站地波雷达测量表层海流的原理和公式,从而使单站测量表层海流成为可能。可预期该方案虽然在一定程度上,适当降低了空间分辨率,但却可以大幅度降低观测成本,减少人力物力的投入,并显著提高现场观测效率。     

Doppler radar radial winds in HIRLAM. Part I: observation modelling and validation

An observation operator for Doppler radar radial wind measurements is developed further in this article, based on the earlier work and considerations of the measurement characteristic. The elementary observation operator treats radar observations as point measurements at pre-processed observation heights. Here, modelling of the radar pulse volume broadening in vertical and the radar pulse path bending due to refraction is included to improve the realism of the observation modelling.
The operator is implemented into the High Resolution Limited Area Model (HIRLAM) limited area numerical weather prediction (NWP) system. A data set of circa 133 000 radial wind measurements is passively monitored against the HIRLAM six-hourly background values in a 1-month experiment. No data assimilation experiments are performed at this stage. A new finding is that the improved modelling reduces the mean observation minus background (OmB) vector wind difference at ranges below 55 km, and the standard deviation of the radial wind OmB difference at ranges over 25 km.
In conclusion, a more accurate and still computationally feasible observation operator is developed. The companion paper (Part II) considers optimal super-observation processing of Doppler radar radial winds for HIRLAM, with general applicability in NWP.     

黄海潮波系统下的GOCI反演及OSU模式海表流场数据适用性研究

黄海呈现独有的地形条件,且该海域的潮波运动独具特征。本文利用静止海洋水色成像仪（Geostationary Ocean Color Imager,GOCI）遥感反演和俄勒冈州立大学（Oregon State University,OSU）潮流模式分别获取了黄海海域的海表流场,基于该海域独特的潮波系统提出并识别潮波干涉区,进而对GOCI反演的流场做潮流提取,并对两种潮流数据作分区可用性评价,通过实测的漂流浮标数据验证评估。结果表明:利用GOCI反演和OSU潮流模式获取的海表流场具有一定程度的可靠性,GOCI反演的海表流场的流速平均相对大小误差值为0.77,OSU潮流模式获取的海表流场流速平均相对大小误差值为0.49;在靠近潮波干涉区的黄海中部海域,GOCI潮流数据与实测数据在方向上的一致性要优于OSU潮流数据,两者平均角度误差值分别为48.45°和63.10°;在远离潮波干涉区的黄海近岸海域,OSU潮流数据与实测数据在速度大小和方向上的一致性要优于GOCI潮流数据。     

On the performance of a shipborne current mapping HF radar

High-frequency (HF) radars have been developed to map surface currents offshore by means of land-based stations. Presently available radar systems use frequencies between 25 and 30 MHz and allow a spatial resolution of 1 km and ranges of up to 50 km. This paper reports on the experience with a shipborne radar and discusses problems which arise for the azimuthal resolution on a metal ship, the correction for the ship's speed, and limitations due to pitch-and-roll motions. Current measurements during cruises to the North Atlantic are presented. It has been found that, with the support of the satellite-supported Global Positioning System, the shipborne HF radar can measure surface current velocities with an accuracy of some 5 cm·s^-1     

Radial velocity of ocean surface current estimated from SAR Doppler frequency measurements—a case study of Kuroshio in the East China Sea

Ocean currents are a key element in ocean processes and in meteorology, affecting material transport and modulating climate change patterns. The Doppler frequency shift information of the synthetic aperture radar (SAR) echo signal can reflect the dynamic characteristics of the sea surface, and has become an essential sea surface dynamic remote sensing parameter. Studies have verified that the instantaneous Doppler frequency shift can realize the SAR detection of the sea surface current. However, the validation of SAR-derived ocean current data and a thorough analysis of the errors associated with them remain lacking. In this study, we derive high spatial resolution flow measurements for the Kuroshio in the East China Sea from SAR data using a theoretical model of shifts in Doppler frequency driven by ocean surface current. Global ocean multi observation (MOB) products and global surface Lagrangian drifter (GLD) data are used to validate the Kuroshio flow retrieved from the SAR data. Results show that the central flow velocity for the Kuroshio derived from the SAR is 0.4–1.5 m/s. The error distribution between SAR ocean currents and MOB products is an approximate standard normal distribution, with the 90％ confidence interval concentrated between –0.1 m/s and 0.1 m/s. Comparative analysis of SAR ocean current and GLD products, the correlation coefficient is 0.803, which shows to be significant at a confidence level of 99％. The cross-validation of different ocean current dataset illustrate that the SAR radial current captures the positions and dynamics of the Kuroshio central flow and the Kuroshio Counter Current, and has the capability to monitor current velocity over a wide range of values.     

Residual Currents Through the Dover Strait Measured by H.F. Radar

Residual (i.e. non-tidal) components of flow in the Dover Straits are determined from measurements recorded by the OSCR H.F. Radar system. The data are divided into 10 monthly sets, obtained from 5 months of dual radar deployments on the English side of the Straits and a corresponding 5 months from the French side. For each of these sets the tidal component was removed by subtracting constituents derived from separate (monthly) harmonic analyses.In each deployment, surface currents were measured at 700 locations at 20-min intervals, providing spatial resolution on a grid as fine as 660 m. This fine spatial resolution reveals distinct patterns of monthly-mean residual circulations never previously recognized. In particular, a residual gyre is shown to be a dominating feature along the French side, with currents exceeding 20 cm s^-1 and a diameter of 20 km. The previous obscurity of this feature may be attributable to the large (> 1·5 m s^-1) and strongly spatially variable tidal currents. In these conditions, tidal advection will rapidly smear any surface signature obtained from satellite observations. Likewise, fine resolution (grid spacings of an order of 1 km) is required to reproduce these gyre dynamics in numerical models.The time-varying residual currents were correlated with wind recordings. Significant correlations were obtained for an 'open-sea' response (as represented by Ekman theory), with the wind-driven surface current veering at angles of up to 45° to the right of the wind direction. This 'open-sea' response is modified close to the coast where the generation of surface gradients force the currents to align with the topography.Larger-scale residual motions were identified from modal analyses. The primary modes, on both sides, involved large-scale flows through the Straits partially correlated with local winds. However for winds aligned approximately north-south, the components of these flows on the English and French sides are in opposition. These modes also revealed large oscillatory motions with periods of between 20 and 40 h.Estimates of net residual flows through the Straits from the radar measurements confirm earlier calculations of the range of variability in this parameter. However the increasing complexity of the flow patterns revealed in this study emphasizes the difficulty in quantifying the long-term net flow.     

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.     

Ocean surface current retrieval and imaging with a new shore-based X-band radar based on time-shifted up-and-down linear frequency modulated signal

This paper proposes a multifunction radar that can not only measure sea currents but also perform sea-surface imaging. The fundamental aspect of the proposed radar comprises transmitting time-shifted up-and-down continuous wave linear frequency modulated signals that allow for the offset of two one-dimensional range images of the sea surface that respectively correspond to the upward linear frequency modulated(LFM) signal and the downward LFM signal. Owing to the Doppler frequency shift from the sea surface, a range offset, which is proportional to the radial velocity of the sea surface, occurs between the upward and downward LFM signals. By using the least-squares linear fitting method in the transformed domain, the range offset can be measured and the current velocity can be retrieved. Finally, we verify the accuracy of current measurement with simulation results.     

Short-wave measurements by a fixed tower-based and a drifting buoysystem

The small-scale roughness of the sea surface acts as an important link in air-sea interaction processes. Radar and sonar waves are scattered by short surface waves providing the basis for remote sensing methods of the sea surface. At high wind speeds, breaking waves occur. Bubbles penetrate into the water and drastically increase acoustical reverberation, transmission loss and ambient noise. Thus, the development of short waves and wave breaking have to be known to apply radar remote sensing to the surface and to deduce from radar backscatter which sonar conditions prevail. To measure the wind dependence of short waves an experimental device was constructed for use from stationary platforms. It is nearly all-weather capable and can easily be handled by a crane. On the other hand, frequencies of short waves measured in a fixed position are extremely frequency shifted by currents. This limits the usefulness of tower-based measurements, e.g., the short wave modulation by wind and waves or currents can only be estimated in a rough approximation. Consequently, a buoy was developed to reduce the frequency shifts. The principle of the buoy is to drift in the local surface current and to follow the amplitudes of long waves. Therefore, short waves are measured in facets of long waves and the Doppler shifts are minimized. The wind is measured at a constant height above the long wave profile and relative to the moving facets. The paper describes the conventional measuring device and points out the necessity of the drifting buoy system. Examples of wind and wave spectra are presented and short wave modulations by long waves are depicted, too. From these measurements, new insights in short wave behaviour have to be expected     

Possibilities of X-band nautical radars for monitoring of wind waves near the coast

We present the results of development and testing of a coastal X-band radar system for monitoring wind waves and currents at the Black Sea (near Gelendzhik) created on the basis of nautical radars. Radar measurements of wave heights were validated by data from a wave buoy and a moored acoustic Doppler current profiler (ADCP). The conditions for successful radar measurements of waves in the coastal environment have been determined. It was shown that a radar with an aperture 1° could successfully measure wave heights at a distance of 1.2 km from the radar, when waves arrive at an angle of ±31° to the main sensing direction. In this case, for wave height measurements, the correlation coefficient between the radar and independent data is 0.82 and the standard deviation is 0.26 m.     

三门湾外海的潮汐和潮流特征

针对2009年5月-7月三门湾外海D8和D9两个站位布放的防渔网底拖锚系ADCP连续观测获取的流速资料和水位资料,采用调和分析和功率谱分析等研究方法,分析了该海域的潮汐和潮流特征,结果显示:该海域潮汐类型为正规半日潮,近岸的D9站浅水分潮比离岸的D8站显著,潮汐呈现出回归潮特征。三门湾外海潮流半日分潮能量最大, 各层潮流呈现出旋转流性质;椭圆率随水深增加;M₂、K₁分潮流最大流速均在次表层最大;最大分潮流速发生时刻底层比中、上层提前约半小时;该海域潮流的半日分潮流以正压为主,全日分潮流则表现出较为明显的斜压性。     

A new view of near-shore dynamics based on observations from HF Radar

Since 1984 the OSCR HF Radar system has been used in over 50 deployments to measure near-shore surface currents for both scientific and engineering applications. The enhanced scope, resolution and accuracy of these measurements have yielded new insights into the tidal, wind and density driven dynamics of the near-shore zone.Tidal current ellipses obtained from these radar measurements have been shown to be in good aggrement with values calculated by numerical models both for the predominant constituents and also for higher harmonics. Coherent patterns of wind-forced currents ahve been determined with strong evidence of a “slab-like” surface response. In one deployment, with offshore winds blowing over relatively deep water, this “slab” rotated clockwise at near-inertial frequency. Strong (up to 20cm s⁻¹), persistent surface residual currents are commonly observed, these are almost certainly generated by (small) horizontal density gradients. These observed surface residuals provide ideal data for rigorous testing of 3-D numerical models.With a threatened rise in sea level, HF Radar is well-suited for observing the expected changes in the dynamics of near-shore regions. Continuing development of these radar systems offers exciting prospects of remote sensing of both surface waves and currents. Future applications may extend beyond the near-shore region to measurements along the shelf-edge, in oceanic gyres and for “beach-processes”.     

基于相参X-波段海洋雷达的海表轮廓测量研究

X-波段海洋雷达测量所得海面散射单元的多普勒信息与散射单元的雷达视向速度密切相关。首先,基于符号多普勒估计方法,对X-波段雷达海面回波的多普勒频移信息进行了估计;在此基础上,应用各分辨单元回波的多普勒频移信息,建立了海浪表面轮廓的反演算法。该算法中,同时考虑了雷达入射角、方位角和雷达空间分辨率等因素对反演结果的影响。通过将反演结果与浮标测量数据相比较,发现雷达空间分辨率起到了类似低通滤波的作用,该作用对短重力波谱影响显著。同时,还应用加拿大麦克马斯特大学的IPIX雷达数据对海表轮廓进行了反演,并将反演所得有效波高、海浪周期与现场测量数据进行了比较,反演结果与现场测量结果吻合较好。