Mapping Current Fields in a Bay Using a Coast-Fitting Tomographic Inversion.

Minmo Chen,Xiaopeng Fan,Min Wang,Chuanzheng Zhang,Ruixiang Zhao,Ju Lin,Xiao-Hua Zhu,Arata Kaneko,Ze-Nan Zhu

doi:10.3390/s20020558

Abstract

Coast-fitting tomographic inversion that is based on function expansion using three types of normal modes (the Dirichlet, Neumann, and open boundary modes) is proposed to reconstruct current fields from the coastal acoustic tomography (CAT) data. The superiority of the method was validated while using CAT data that were obtained in 2015 in the Dalian Bay. The semidiurnal tidal and residual current fields were accurately reconstructed over the entire model domain surrounded by coasts and open boundaries. The proposed method was effective, particularly around the peripheral regions of the tomography domain and the near-coast regions outside the domain, where accurate results are not expected from the conventional inverse method based on function expansion by Fourier function series with no coast fittings. The error velocity for the semidiurnal tidal currents was 2.2 cm s−1, which was calculated from the root-mean-square-difference between the CAT-observed and inverted range-averaged currents that were obtained along the nine peripheral transmission paths. The error velocity for the residual currents estimated from the 12-h mean net residual transport at the bay mouth was 0.9 cm s−1. The errors were significantly smaller than the amplitude of the tidal and residual currents.

Highlights

Coastal acoustic tomography (CAT), which was developed as a coastal-sea application for ocean acoustic tomography [1,2], still serves as innovative technology for monitoring and predicting variations in the coastal sea environment by data assimilation [3–6]
Tomographic inversion that was based on the function expansion using three types of coast-fitting normal modes three types of coast-fitting normal modes was was used to reconstruct the data set that was obtained in the 2015 Dalian Bay experiment with used to reconstruct the data set that was obtained in the 2015 Dalian Bay experiment with 51
The optimal mode number of 17 was determined at a position on the mode number-RMSD diagram, where the RMSD between the observed and inverted range-averaged currents (RACs) for the nine number-RMSD diagram, where the RMSD between the observed and inverted RACs for the nine peripheral transmission paths was as small as 2.4 cm s−1

Summary

Introduction

Coastal acoustic tomography (CAT), which was developed as a coastal-sea application for ocean acoustic tomography [1,2], still serves as innovative technology for monitoring and predicting variations in the coastal sea environment by data assimilation [3–6]. Mapping rapidly-varying current fields in coastal seas is a remarkable capability of CAT [7–13]. A CAT experiment with a horizontal resolution of 1.53 km was conducted in 2015 in the Dalian Bay, China, in which 51 sound transmission paths were constructed for 11 CAT stations [14]. The current fields were reconstructed while using conventional tomographic inversion based on Fourier function expansion with no coast constraints. Tomographic inversion that is based on function expansion using three types of coast-fitting normal modes, namely the Dirichlet, Neumann, and open boundary modes, is proposed for improving the current fields reconstructed while using conventional tomographic inversion of the. Special attention is paid to the peripheral regions of the tomography domain and the near-coast regions outside the domain

Methods

51. Theinexpansion number of

Error Estimate

Conclusions