Synchronous Assimilation of Tidal Current-Related Data Obtained Using Coastal Acoustic Tomography and High-Frequency Radar in the Xiangshan Bay, China (original) (raw)
Continental Shelf Research, 2015
Quantifying the tidal current and volume transport through the Qiongzhou Strait (QS) is vital to understanding the circulation in the northern South China Sea. To measure the tidal current in the strait, a 15-day coastal acoustic tomography (CAT) experiment was carried out at four acoustic stations in March 2013. The horizontal distributions of the tidal currents were calculated by inverse analysis of CAT data. The diurnal tidal current constituents were found to dominate: the ratio of the amplitudes O 1 , K 1 , M 2 , S 2 , and MSF was 1.00:0.60:0.47:0.21:0.11. The residual currents were found to flow westward in the northern QS and turn southward in the southern QS. The residual current velocities were larger in the northern area than in the southern area, with a maximum westward velocity of 12.4 cm s À 1 in the northern QS. Volume transport estimated using the CAT data varied between À 0.710 Sv and 0.859 Sv, with residual current transport of À 0.044 Sv, where negative values indicate westward. We conducted a dynamic analysis of the observations made during the study, which suggested that tidal rectification and sea level difference between the two entrances of the QS are important in maintaining the residual current through the strait. This is the first estimation, from synchronous measurements, of major tidal current constituents, residual currents, and volume transport in this strait.
Mapping Tidal Currents and Residual Currents by Use of Coastal Acoustic Tomography
Proceedings of International Conference "Managinag risks to coastal regions and communities in a changinag world" (EMECS'11 - SeaCoasts XXVI), 2016
A coastal acoustic tomography (CAT) experiment for mapping the tidal currents in the Zhitouyang Bay was successfully carried out with seven acoustic stations during July 12 to 13, 2009. The horizontal distributions of tidal current in the tomography domain are calculated by the inverse analysis in which the travel time differences for sound traveling reciprocally are used as data. Spatial mean amplitude ratios M2 : M4 : M6 are 1.00 : 0.15 : 0.11. The shallow-water equations are used to analyze the generation mechanisms of M4 and M6. In the deep area, velocity amplitudes of M4 measured by CAT agree well with those of M4 predicted by the advection terms in the shallow water equations, indicating that M4 in the deep area where water depths are larger than 60 m is predominantly generated by the advection terms. M6 measured by CAT and M6 predicted by the nonlinear quadratic bottom friction terms agree well in the area where water depths are less than 20 m, indicating that friction mechanisms are predominant for generating M6 in the shallow area. Dynamic analysis of the residual currents using the tidally averaged momentum equation shows that spatial mean values of the horizontal pressure gradient due to residual sea level and of the advection of residual currents together contribute about 75% of the spatial mean values of the advection by the tidal currents, indicating that residual currents in this bay are induced mainly by the nonlinear effects of tidal currents.
Coastal tomographic mapping of nonlinear tidal currents and residual currents
Continental Shelf Research, 2017
Depth-averaged current data, which were obtained by coastal acoustic tomography (CAT) July 12-13, 2009 in Zhitouyang Bay on the western side of the East China Sea, are used to estimate the semidiurnal tidal current (M 2) as well as its first two overtide currents (M 4 and M 6). Spatial mean amplitude ratios M 2 :M 4 :M 6 in the bay are 1.00:0.15:0.11. The shallow-water equations are used to analyze the generation mechanisms of M 4 and M 6. In the deep area, where water depths are larger than 60 m, M 4 velocity amplitudes measured by CAT agree well with those predicted by the advection terms in the shallow water equations, indicating that M 4 in the deep area is predominantly generated by the advection terms. M 6 measured by CAT and M 6 predicted by the nonlinear quadratic bottom friction terms agree well in the area where water depths are less than 20 m, indicating that friction mechanisms are predominant for generating M 6 in the shallow area. In addition, dynamic analysis of the residual currents using the tidally averaged momentum equation shows that spatial mean values of the horizontal pressure gradient due to residual sea level and of the advection of residual currents together contribute about 75% of the spatial mean values of the advection by the tidal currents, indicating that residual currents in this bay are induced mainly by the nonlinear effects of tidal currents. This is the first ever nonlinear tidal current study by CAT.
High-precision measurement of tidal current structures using coastal acoustic tomography
Estuarine, Coastal and Shelf Science, 2017
A high-precision coastal acoustic tomography (CAT) experiment for reconstructing the current variation in Dalian Bay (DLB) was successfully conducted by 11 coastal acoustic tomography systems during March 7e8, 2015. The horizontal distributions of tidal currents and residual currents were mapped well by the inverse method, which used reciprocal travel time data along 51 successful sound transmission rays. The semi-diurnal tide is dominant in DLB, with a maximum speed of 0.69 m s À1 at the eastern and southwestern parts near the bay mouth that gradually decreases toward the inner bay with an average velocity of 0.31 m s À1. The residual current enters the observational domain from the two flanks of the bay mouth and flows out in the inner bay. One anticyclone and one cyclone were noted inside DLB as was one cyclone at the bay mouth. The maximum residual current in the observational domain reached 0.11 m s À1 , with a mean residual current of 0.03 m s À1. The upper 15-m depth-averaged inverse velocities were in excellent agreement with the moored Acoustic Doppler Current Profiler (ADCP) at the center of the bay, with a rootemeanesquare difference (RMSD) of 0.04 m s À1 for the eastward and northward components. The precision of the present tomography measurements was the highest thus far owing to the largest number of transmission rays ever recorded. Sensitivity experiments showed that the RMSD between CAT and moored-ADCP increased from 0.04 m s À1 to 0.08 m s À1 for both the eastward and northward velocities when reducing the number of transmission rays from 51 to 11. The observational accuracy was determined by the spatial resolution of acoustic ray in the CAT measurements. The costoptimal scheme consisted of 29 transmission rays with a spatial resolution of acoustic ray of 2.03 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi km 2 =ray numbers p. Moreover, a dynamic analysis of the residual currents showed that the horizontal pressure gradient of residual sea level and Coriolis force contribute 38.3% and 36.0%, respectively. This indicates that the two terms are the dominant factors of the residual currents in DLB.
Geophysical Research Letters, 2005
1] The Kanmon Strait acoustic tomography data acquired at a 5-min interval are assimilated sequentially into a 2D ocean model on the basis of the ensemble Kalman filter scheme to image strong tidal current structures occurring in the strait. When the accurate range-averaged currents obtained for the sound transmission lines connecting eight tomography stations are used as assimilation data, the complicated vortex-imbedded currents are imaged with horizontal resolution and accuracy much better than the result of tomographic inversion. The assimilated currents are well compared to the shipboard ADCP data with a RMS difference of about 24 cm s À1 for both the horizontal velocity components. The assimilated volume transport across the strait also shows a good agreement with the transport estimated from the range-averaged current on a pair of transmission lines crossing the strait, making a RMS difference of 3700 m 3 s À1 . Citation: Lin, J., , Accurate imaging and prediction of Kanmon Strait tidal current structures by the coastal acoustic tomography data, Geophys.
2004
A dominant part of the circulation in nearly enclosed bays, estuaries, or sounds is dictated by tidal inflow at its mouth, called co-oscillatory forcing. The remaining flow component is usually due to winds. HF radar measurements across the region at the entrance can be used to extract the sinusoidal tidal velocity constituents along a line across the mouth. We use this complex (i.e., amplitude and phase) spatial profile at different times of the tidal cycle as the boundary excitation condition to solve a scalar second-order partial differential equation (PDE) for tide height. For the boundary condition over the remaining enclosing coast, the flow normal to the shore is taken to be zero. The bathymetry of the bay is included in this scalar PDE. This is then solved by a powerful finite-element code, PDE2D. From the tide height distribution, the velocity circulation is simply calculated as its gradient.
Measurements of tidal flow around a headland with a shipboard acoustic Doppler current profiler
Journal of Geophysical Research: Oceans, 1990
A shipboard acoustic Doppler current profiler (ADCP) and moored current meters were used to obtain detailed measurements of the spatial structure of the tidal flow around a headland in Vineyard Sound, Massachusetts, where tidal currents typically range from 50 to 70 cm s−1. Eight shipboard surveys were conducted, each of which followed one of five trapezoidal tracks in the vicinity of the headland, completing 11–12 circuits in the course of the tidal cycle (12.4 hours). The measurements from the ADCP compare favorably with moored velocity measurements at two locations (rms deviations of ±3 cm s−1), but the comparison showed more scatter at two other sites (rms deviations of ±7 cm s−1). The ADCP measurements from separate cruises were merged to form a composite spatial representation of the tidal and residual currents, providing resolution of the spatial structure of the flow around the headland at scales from several hundred meters to 10 km. The semidiurnal tidal flow is relatively ...