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1.
Measurements are reported from two side-looking Doppler systems, which were used to study the discharge front located off the mouth of Chesapeake Bay. One system was a commercial 300-kHz narrow-band acoustic Doppler current profiler (ADCP), which was mounted at a depth of 0.6 m on the port side of a research ship. The other was a prototype X-band, vertically polarized, Doppler radar mounted at a height of about 4 m on the starboard side. Both velocity and backscatter intensity were measured along two beams to ranges of 120 m (ADCP) and 200 m (radar), so that by sailing alternately on each side of the front it was possible to make nearly simultaneous across-front measurements with both systems. Despite the differences in acoustic and radar scattering mechanisms, a combined backscatter intensity surface map could be made showing a continuous frontal signature about 10-m wide and 20 dB above background levels. Each system was also able to measure the same large-scale velocity change across the front, which was dominated by the discharging buoyant bay water flowing at about 50 cm/s relative to the ambient continental shelf water. However, within a 60-m wide zone, the radar system measured velocities up to 75 cm/s larger than the ADCP. Such large velocity differences arose from the radar's sensitivity to motions associated with waves reflecting from the region of strongest across-front current convergence. This frontal convergence was resolved only by the ADCP, which showed a horizontal current change of about 25 cm/s over 10 m and appeared to extend over the upper 2 m or so of the water column. These results show that the combined information from the acoustic and radar systems provide a more complete picture of the frontal currents and wave-current interactions than either system could provide alone  相似文献   

2.
This paper explores the application of phased-array high-frequency (HF) radars to identify locations of enhanced local waveheights. Measurements of the near-surface current velocities and waveheights were obtained from HF radars deployed near the mouth of the Chesapeake Bay in the fall of 1997. The radar-derived near-surface velocities were compared with the upper bin (2-m depth) of four upward-looking acoustic Doppler current profilers (ADCPs). The slopes of the linear correlations were close to one and the root-mean-square (rms) differences were similar to previous studies. Significant waveheight (Hs) estimates from both radars were compared with a laser height gauge. The largest differences were observed during low winds due to overestimates at one of the radar stations and during storms when the laser measurement failed. Further analysis focused on the HF radar results from the more reliable of the two sites. The rms difference between this radar and the in situ sensor was 0.29 m. Synoptic observations of Hs over the Chesapeake Bay revealed regions of current-induced wave shoaling and refraction. Hs over the estuarine outflow increased between 19-50% relative to the incident Hs in light onshore winds (~5 m/s). In stronger winds (>10 m/s), Hs also increased by up to 25% when there was a tidal outflow in the surface layer, although the near-surface currents were responding to both the wind and the ebbing tide. Hs was not enhanced when the outflow was below a thicker layer (>5 m) of wind-forced onshore flow  相似文献   

3.
HF radar has become an increasingly important tool for mapping surface currents in the coastal ocean. However, the limited range, due to much higher propagation loss and smaller wave heights (relative to the saltwater ocean), has discouraged HF radar use over fresh water, Nevertheless, the potential usefulness of HF radar in measuring circulation patterns in freshwater lakes has stimulated pilot experiments to explore HF radar capabilities over fresh water. The Episodic Events Great Lakes Experiment (EEGLE), which studied the impact of intermittent strong wind events on the resuspension of pollutants from lake-bottom sediments, provided an excellent venue for a pilot experiment. A Multifrequency Coastal HF Radar (MCR) was deployed for 10 days at two sites on the shore of Lake Michigan near St. Joseph, MI. Similarly, a single-frequency CODAR SeaSonde instrument was deployed on the California shore of Lake Tahoe. These two experiments showed that when sufficiently strong surface winds (2 about 7 m/s) exist for an hour or more, a single HE radar can be effective in measuring the radial component of surface currents out to ranges of 10-15 km. We also show the effectiveness of using HF radar in concert with acoustic Doppler current profilers (ADCPs) for measuring a radial component of the current profile to depths as shallow as 50 cm and thus potentially extending the vertical coverage of an ADCP array  相似文献   

4.
利用同安湾口门断面走航ADCP观测数据,采用基于高斯基函数的Candela空间插值方法,对海流进行空间三维的调和分离,并对分离的各分潮流与余流进行流场的回归检验分析,F(α=0.01)检验显示流场的回归效果显著.观测期间同安湾口门断面平均潮差为5.15 m,最小潮差4.38m,最大潮差5.98 m,断面上涨落潮最大流速分别为92.3、80.3 cm/s,潮流特征分析表明,同安湾口门断面的潮流运动形式为往复流,以半日潮流为主,约占70%潮流信息,潮流流速从海表面向海底递减.余流最大值为12.5 cm/s,空间分布上将同安湾口门断面一分为二形成北进南出的余流进出通道,这与以往余流格局的认识相符.  相似文献   

5.
吕海滨  申辉  何宜军 《海洋科学》2013,37(11):54-58
通过对东沙群岛附近2009年夏季测到的三个孤立波事件的分析, 结合现场的X 波段雷达、ADCP和CTD 观测, 发现内孤立波经过时, 伴有突发性强流, 最大水平东西分量U大于0.5 m/s, 周期大约15 min, 各深度层海流均为西向。第三个孤立波事件中, 雷达后向散射影像中包含至少4个亮条带, 其中最后的条带雷达表面信号最强。而内波表面信号强弱不仅与内波的振幅有关, 还可能与潮流、混合层深度等环境条件有关, 该研究可加深对内波遥感观测的理解。  相似文献   

6.
Unusual temperature variations at subsurface depths were noticed while making continuous observations (for 13 h) on temperature, salinity and currents at a point in the vicinity of river Krishna in the western Bay of Bengal. Sharp temperature oscillations recorded at four different levels between 14 and 20·1 m had an average periodicity of about 1·6 h. Similar periodicities were also observed in the case of flow components. Computed Brunt-Vaisälä frequency, vertical shear and Richardson number showed remote possibilities of vertical mixing occurring in this area. These oscillations showed features of internal waves propagating with a speed of about 13·3 cm s−1 and wavelengths between 0·8 and 1·3 km and apparently generated as a result of flood current passing over a submarine obstacle in the presence of stratification.  相似文献   

7.
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.  相似文献   

8.
Ocean surface currents can be estimated, over a large coastal area, by utilizing the backscatter of high frequency (HF) radar waves from ocean gravity waves. Although the overall backscatter mechanism is complicated, the surface current information is contained within the spectral characteristics of two dominant Bragg components. The accuracy of the current estimate, following the usual FFT-based spectral estimate, is limited by the frequency resolution of the FFT and the time-varying characteristics of the Bragg components. This paper describes a high resolution parametric estimation of the ocean currents based on a recently proposed technique for analyzing time-varying signals. This technique, together with a time-domain ocean clutter model, allows all the Bragg signal information to be extracted from the two dominant eigenvalues and eigenvectors of a matrix constructed from the radar data. Using signals from an operational coastal surveillance radar, current estimates made using this technique are compared with those estimated by the conventional FFT-based method  相似文献   

9.
In this paper, we discuss the validation of water level and current predictions from three coastal hydrodynamic models and document the resource and operational requirements for each modeling system. The ADvanced CIRCulation Model (ADCIRC), the Navy Coastal Ocean Model (NCOM), and Delft3D have been configured and validated for the Chesapeake Bay region during a Navy exercise. Water level predictions are compared with a NOAA/NOS water level gauge at the Chesapeake Bay Bridge Tunnel location while current predictions are validated with Acoustic Doppler Profiler (ADP) measurement records at three locations in the lower Chesapeake Bay. Statistical metrics such as correlation coefficient and root mean square error (RMSE) are computed. Both the vertically-integrated currents and currents at varying water depths are compared as well. The model-data comparisons for surface elevation indicate all three models agreed well with water level gauge data. The two-dimensional version of ADCIRC, ADCIRC2D, and NCOM yield better statistics, in terms of correlation and RMSE, than Delft3D. For vertically-integrated currents, ADCIRC2D has the smallest RMSE at Thimble Shoal and Naval Station locations while NCOM has the smallest RMSE at Cape Henry. For the horizontal currents over the water column, the fully three-dimensional, baroclinic ADCIRC model, ADCIRC3D, and NCOM both showed better agreement with the ADP measurements.  相似文献   

10.
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  相似文献   

11.
Performance and operational feasibility of very high-frequency (VHF) Doppler radar have been demonstrated in a region dominated by strong tidal currents. An analysis of remote measurements of sea surface currents acquired by Courants de Surface MEsureacutes par Radar (COSMER)-pulsed Doppler radar during Evaluation et Preacutevision de l'Environnement Littoral (EPEL) experiment (supported by the French Navy) is presented in this paper. The VHF COSMER radar was deployed to provide continuous sea surface current measurements within an area of about 25 km times 25 km in the Normand Breton Gulf, France. This paper presents VHF measurement comparisons with observations such as acoustic Doppler current profiler (ADCP), as well as comparisons with numerical model TELEMAC 2-D. Results of tidal waves extraction, using harmonic analysis and residual currents, are shown in this paper. We also present a case where radar method is limited, due to the presence of additional peaks in the Doppler spectrum  相似文献   

12.
Three High Frequency (HF) ocean radar stations were installed around the Soya/La Perouse Strait in the Sea of Okhotsk in order to monitor the Soya Warm Current (SWC). The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are 3 km and 5 deg., respectively. The radar covers a range of approximately 70 km from the coast. The surface current velocity observed by the HF radars was compared with data from drifting buoys and shipboard Acoustic Doppler Current Profilers (ADCPs). The current velocity derived from the HF radars shows good agreement with that observed using the drifting buoys. The root-mean-square (rms) differences were found to be less than 20 cm s−1 for the zonal and meridional components in the buoy comparison. The observed current velocity was also found to exhibit reasonable agreement with the shipboard ADCP data. It was shown that the HF radars clearly capture seasonal and short-term variations of the SWC. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s−1, in summer and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately 40 km. The surface transport by the SWC shows a significant correlation with the sea level difference along the strait, as derived from coastal tide gauge records at Wakkanai and Abashiri. Deceased.  相似文献   

13.
A European Space Agency＇ s ENVISAT advanced synthetic aperture radar （ASAR） image covering Zhejiang coastal water in the East China Sea （ECS） was acquired on 1 August 2007. This image shows that there are about 20 coherent internal solitary wave （ISW） packets propagating southwestward toward Zhejiang coast. These ISW packets are separated by about 10 kin, suggesting that these ISWs are tide-generated waves. Each ISW packet contains 5-15 wave crests. The wavelengths of the wave crests within the ISW packets are about 300 m. The lengths of the leading wave crests are about 50 km. The ISW amplitude is estimated from solving KdV equation in an ideal two-layer ocean model. It is found that the ISW amplitudes is about 8 m. Further analysis of the ASAR image and ocean stratification profiles show that the observed ISWs are depression waves. Analyzing the tidal current finds that these waves are locally generated. The wavelength and amplitude of the ECS ISW are much smaller than their counter- parts in the South China Sea （SCS）. The propagation speed of the ECS ISW is also an order of magnitude smaller than that of the SCS ISW. The observed ISWs in the ECS happened during a spring tide period.  相似文献   

14.
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OSM AR-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...  相似文献   

15.
The timing and intensity of the effects of the 1997–98 El Niño on sea-surface temperature (SST) and coastal sea level along the US west coast are examined using in situ time-series measurements, and the effects on upper ocean currents on the continental shelf and slope off Oregon and northern California are examined using repeated shipborne ADCP transects, a mid-shelf mooring off Newport Oregon and an HF surface current radar. An initial transient positive anomaly was observed in both adjusted sea level and SST during May–June 1997, followed by anomalously high coastal sea levels, generally strongest during September 1997 through February 1998 and abruptly returned to normal in late February 1998, and by positive temperatures anomalies over the mid-shelf that persisted longer, into April 1998. Low-frequency coastal sea-level anomalies propagated poleward at 2.1 m/s. Poleward flow over the shelf and slope was enhanced at most depths during the El Niño, compared with following years. Northward currents in the upper 12 m over the continental shelf off Newport, Oregon averaged 13.7 cm/s stronger during August 1997 through February 1998 than during the same period the following year. Enhanced poleward flow was present at all latitudes sampled during November 1997 and February 1998, particularly over the continental slope. These transects also provided clear views of a fall/winter equatorward undercurrent, which was both strongest and had the most alongshore similarity of form, during the ENSO. Finally, subsurface-intensified anticyclonic eddies originating in the poleward undercurrent appear to be a recurrent feature of the circulation off Newport late in the upwelling season.  相似文献   

16.
In the upper Chesapeake Bay (Maryland, U.S.A.) field surveys were conducted at 18 multiple longshore sand bar sites. The multiple bar systems were found in water depths less than approximately 2 m (mean sea level), and exhibited mild bottom slopes of 0·0052 or less. The number of bars composing each system ranged from four to 17 and the spacing between the crests typically increased in the offshore direction, ranging from 12 to 70 m. Bar height also typically increased with distance offshore and ranged from 0·03 to 0·61 m. A grain size analysis of crest and trough sediment did not reveal any significant differences and the sediment was categorized as ‘fine sand’. A review of the literature data indicated that the Chesapeake Bay multiple bars possessed similar characteristics to those found in Gelding Bay (Baltic Sea); similarities in fetch, wave height and tidal range between the two bays may account for this finding. The surf-scaling parameter indicated that the multiple bar systems were extremely dissipative with regard to wave energy, and wave height appeared to be an important factor in controlling bar spacing and bar height. A multiple wave break point hypothesis was discussed as a possible mechanism for the formation of Chesapeake Bay multiple longshore bars, and limited observational evidence appeared to support such a mechanism.  相似文献   

17.
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  相似文献   

18.
Abstract

Studies of coastal bathymetry are important where littoral drift has implications on the planning of fishing and dredging operations. Also, there is a possibility of finding hitherto unknown bottom features in relatively less explored regions of the shallow seas around the globe. High resolution satellite imagery over oceans provides us with quantitative methods for estimating depth in shallow parts of the seas. One of the methods is the analysis of the refraction of coastal gravity waves observed on satellite imagery. A panchromatic image acquired by SPOT with 10 m resolution on March 22, 1986, over Bay of Bengal near Madras Coast, was used for this analysis. The image was enhanced to clearly bring out the wave structure seen on the sea surface. The image was then superimposed with a 1 km × 1 km grid. For each grid cell, 64 × 64 pixels at the center were considered for getting a Fast Fourier Transform to determine the wave spectrum and the dominant wavelength present there. The classical theory of gravity waves was used to relate the shallow water wavelengths obtained as above with the corresponding wavelengths in the deep water. The deep‐water wavelength was estimated to be 110 m using the known chart depths at a set of control points. The resulting depth estimates, when compared with standard bathymetric charts, were found, in general, to be well in agreement up to a depth of 30 m in the sea, with an r.m.s. error of 2.6 meters. The method seems to be very useful for remotely sensed bathymetric work. However, further research is required to reduce the error margin and operationalize the method.  相似文献   

19.
This study offers a new method for estimating High-Frequency (HF) radar surface current velocity error in data comparisons with other types of instrumentation. A new method is needed in order to remove the zero-mean random spatial and temporal fluctuations present in surface-current measurements from all sensors. Conventional methods for calculating radar error when comparing with another instrument have included their root mean square differences and scatter plots that provide correlation coefficient and slope/intercept of the regression line. It seems that a meaningful estimate of radar error should attempt to remove both sensors’ zero mean random fluctuations, inasmuch as possible. We offer and compare a method that does this. The method was tested on data collected in the Central San Francisco Bay, where GPS surface-drifter deployments were conducted within the coverage of four 42 MHz radars over six days in October of 2008. Drifters were continuously deployed in these areas over the sampling days, providing 525 usable drifter measurements. Drifter and radar measurements were averaged into thirty-minute time bins. The three-day long-term averages from the sampling areas were then subtracted from the thirtyminute averages to remove biases associated with comparisons done with short, disjoint time-sample periods. These were then used to develop methods that give radar error or bias after the random fluctuations have been removed. Results for error estimates in this study are commensurate with others where random fluctuations have been filtered, suggesting they are valid. The estimated error for the radars in the SF Bay is low, ranging from ?7.57 cm/s to 0.59 cm/s.  相似文献   

20.
The circulation, water masses and sea-ice of Baffin Bay   总被引：2，自引：0，他引：2  
The oceanographic, meteorological and sea-ice conditions in Baffin Bay are studied using historical hydrographic, satellite and meteorological data, and a set of current meter data from a mooring program of the Bedford Institute of Oceanography. Baffin Bay is partially covered by sea-ice all year except August and September. The interannual variation of the ice extent is shown to be correlated with winter air temperature. Available hydrographic data were used to study the water masses and the horizontal and vertical distribution of temperature/salinity. Three water masses can be identified – Arctic Water in the upper 100–300 m of all regions except the southeast, West Greenland Intermediate Water at 300–800 m in most of the interior of Baffin Bay, and Deep Baffin Bay Water in all regions below 1200 m. The temperature and salinity in Baffin Bay have limited seasonal variability except in the upper 300 m of eastern Davis Strait, northern Baffin Bay and the mouth of Lancaster Sound. Summer data have a temperature minimum at 100 m, which suggests winter convection does not penetrate deeper than this depth. Current meter data and results of a circulation model indicate that the mean circulation is cyclonic. The seasonal variation of the currents is complex. Overall, summer and fall tend to have stronger currents than winter and spring at all depths. Among the different regions, the largest seasonal variation occurs at the mouth of Lancaster Sound and the Baffin Island slope. Model generated velocity fields show a basic agreement with the observed currents, and indicate strong topographic control in the vicinity of Davis Strait and on the Greenland shelves. The model also produces a southward counter current on the Greenland slope, which may explain the observed high horizontal shears over the Greenland slope. Estimates of the volume and fresh water transports through Lancaster, Jones and Smith Sounds are reviewed. Transports through Davis Strait are computed from the current meter data. The balance of freshwater budget and sensitivity of the thermohaline circulation to freshwater transport are discussed.  相似文献   

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