High-precision measurement of tidal current structures using coastal acoustic tomography

- The horizontal distribution of tidal currents was measured by coastal acoustic tomography (CAT).
- The velocity measured by CAT had the highest precision thus far owing to the largest number of transmission rays.
- The sensitivity experiment suggests a cost-optimal CAT measurement scheme.
- The horizontal pressure gradient of residual sea level and Coriolis force controlled the residual currents in Dalian Bay.

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 7-8, 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−1 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−1. 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−1, with a mean residual current of 0.03 m s−1. 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 root-mean-square difference (RMSD) of 0.04 m s−1 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−1 to 0.08 m s−1 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 cost-optimal scheme consisted of 29 transmission rays with a spatial resolution of acoustic ray of 2.03 km2/raynumbers. 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.