Accurate imaging and prediction of Kanmon Strait tidal current structures by the coastal acoustic tomography data (original) (raw)
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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.
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.
Measuring the Kuroshio Current with ocean acoustic tomography
The Journal of the Acoustical Society of America, 2013
Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Current southeast of Taiwan from August 20 to September 15, 2009. Sound pulses were transmitted reciprocally between two acoustic stations placed near the underwater sound channel axis and separated by 48 km. Based on the result of ray simulation, the received signals are divided into multiple ray groups because it is difficult to resolve the ray arrivals for individual rays. The average differential travel times from these ray groups are used to reconstruct the vertical profiles of currents. The currents are estimated with respect to the deepest water layer via two methods: An explicit solution and an inversion with regularization. The strong currents were confined to the upper 200 m and rapidly weakened toward 500 m in depth. Both methods give similar results and are consistent with shipboard acoustic Doppler current profiler results in the upper 150 m. The observed temporal variation demonstrates a similar trend to the prediction from the Hybrid Coordinate Ocean Model.
Remote Sensing, 2022
The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisited to elucidate the propagation and generation characteristics of the M2 and M4 tidal currents with a second CAT experiment, which was conducted from 3–6 April 2018 (local time). Two-dimensional flow fields of the M2 and M4 tidal currents and the residual current were reconstructed using a coast-fitting inversion model with the reciprocal travel-time data of five acoustic stations. The M2 tidal current is a progressive-type wave that propagates eastward at a speed of 0.7 ms−1, much slower than expected for free progressive tides in this region (19 ms−1). The M4 nonlinear current constructed an out-of-phase relationship between the western and eastern halves of the tomography domain, implying the generation of standing-type waves. Such nonlinear processes led to flood- and ebb-dominant tidal current asymmetries for the western and eastern halves of the model domain, respectively. The two-d...
Tomographic measurement of tidal current and associated 3-h oscillation in Bali Strait
Estuarine, Coastal and Shelf Science, 2020
A coastal acoustic tomography experiment involving four acoustic stations was conducted from June 1-3, 2016 in the northern part of the Bali Strait. Range-average currents obtained along four transmission paths for a 30-h observation period were investigated in two-period bands: (a) > 6 h and (b) 10 min-6 h. The dominant phenomena for period bands (a) and (b) were semidiurnal tides with periods of approximately 12 h and internal modes with periods of approximately 3 h, respectively. The phase of the semidiurnal tidal currents was significantly deviated from a sea surface height-current relation expected for the progressive waves in the observation site. The 3-h oscillation had an amplitude comparable to that of the semidiurnal tidal current (with amplitudes of 1.5 ms 1) and was embedded in a semidiurnal envelope oscillation. In relation to the semidiurnal sea surface variations, the 3-h oscillation reached a peak at low water and diminished at high water. The velocity error of 0.07 ms 1 was estimated from the differential travel-time variations in the high-frequency band (10 min-1 h). The error was significantly smaller than the amplitudes of the semidiurnal tidal currents and the 3-h oscillation. Finally, it was suggested that the 3-h oscillation is an internal seiche synchronized with the semidiurnal tides.
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.
Journal of Geophysical Research, 1996
We present a method for extracting the barotropic tide directly from the time-space series of horizontal velocity obtained by a ship-borne •coustic Doppler current profiler (ADCP). The method is conceptually straightforward, easy to implement, and suitable for operational use. It involves fitting • limited area tidal model, based on the line•rized depth-•veraged shallow water equations, to the ADCP record. The flows across the open boundaries of the model domain are assumed periodic in time with known frequencies corresponding to the tidal constituents of interest. The unknown tidal amplitudes and phases at the boundary a, re estimated from interior ADCP velocities using an inverse method; the solution of the shallow water equations is posed as a boundary value problem in the frequency domain, and the estimation procedure is based on generalized least squares regression. Results obtained include tidal maps, a tidal residual series, and associated error estimates. An application of the method to ship ADCP data collected on a cruise to the Western Bank region of the ScotJan Shelf off the east coast of Canada is described. The tidal estimates and the residual field obtMned are verified by comparison to other data collected during the cruise. The residual circulation shows an anticyclonic gyre centered on the crest of Western Bank and a northward current to the west of this region. DOWD