Chuanzheng Zhang

Tomographic mapping of a coastal upwelling and the associated diurnal internal tides in Hiroshima Bay, Japan

Temperature variations caused by a typhoon were measured in the northern part of Hiroshima Bay by four coastal acoustic tomography (CAT) systems. The horizontal distributions of depth-averaged temperature from 0 to 8 m were mapped at 10 min intervals between the 11 and 25 September 2013. The horizontal distributions of a coastal upwelling and the associated diurnal internal tides were reconstructed well by regularized inversion based on the grid segmented method, using one-way travel time data along five successful sound transmission lines. Station-to-station ranges were corrected in such a way that sound speed (determined from one-way travel time data) was equal to sound speed calculated from a couple of CTD (conductivity-temperature-depth) data sets on each transmission line. In addition, all station positions were adjusted to make focal points at the geographical positions of the transducers. The corrections increased the accuracy of temperature measurements to make temperature errors as small as 0.073–0.079°C. The high accuracy made it possible to map the temperature structure with a variation range of less than 0.5°C. An upwelling grew from 16 to 17 September, due to a typhoon-derived northerly wind. The diurnal internal tide resonated with the semidiurnal external tide, which was pronounced after the upwelling decayed (18 September), around the time the spring tide occurred. The upwelling and mixing fractions were formulated. These fractions increased continuously as the upwelling grew. Complete mixing was observed during the upwellings mature phase.

Reciprocal sound transmission experiments for current measurement in a tidal river

Six repeat reciprocal sound transmission experiments for current measurement were successful carried out in the upstream region of the Qiantang River in Hangzhou city, about 90 km from the mouth of Hangzhou Bay during April to December 2009. Two Coastal Acoustic Tomography (CAT) Systems were set up at a distance of 3050 m for making a reciprocal transmission between the both banks of the Qiantang River. During the sound transmission experiments, thirty-two repeat shipboard Acoustic Doppler Profiler (ADP) surveys were also performed along the sound transmission line to get a comparison with the reciprocal sound transmission data. Range-averaged current velocities, determined from the travel time differences along the transmission line were in good agreement with those from the ADP, producing a root-mean-square difference of 0.03 m/s. The travel time differences data were well correlated with the river discharge estimated from the ADP data. The time series of river discharge during the experimental period was estimated through an empirical formula, relating to both the parameters. The variations of river discharge caused by tidal bores were well captured. The river discharge changed in the range of −7626 m3/s and 5096 m3/s, with a mean of 1246 m3/s. The above results suggest that the CAT is a powerful instrument for measuring continuously the river discharge in such tidal rivers with quite heavy shipping traffic.

The first Chinese coastal acoustic tomography experiment

The first Chinese coastal acoustic tomography (CAT) experiment was successfully carried out during July 12 to 13, 2009 with seven acoustic stations for mapping the tidal currents in the Zhitouyang Bay near the Zhoushan Island. The CAT results showed the strong eastward tidal current which entered into the tomography site from the Loutou Channel at the western part of the bay, and separated into two branches, flowing into the Laoshumen Channel, Qingzimeng Channel and Fodu Channel during the ebb tide. On the other hand, the strong westward tidal current, joined with the currents from Laoshumen Channel, Qingzimeng Channel and Fodu Channel, went back to the Loutou Channel during the flood tide. A clockwise tidal vortex of diameter about 5 km was generated in the eastern part of the bay in the transition phase from the ebb to flood. No tidal vortices were generated in the weak tidal currents in the transition phase from the flood to ebb, changing the current direction from westward to eastward. The range-averaged current obtained along the transmission lines after the inverse analysis was in agreement with the acoustic Doppler current profiler (ADCP) results, producing the root-mean-square difference (RMSD) of 1.77 cm/s. The horizontal distributions of tidal current calculated by the inverse analysis were in rough agreement with that of the shipboard ADCP velocity. The RMSD is 11.70 cm/s for the eastward current and 16.50 cm/s for the northward current. It is suggested that the CAT is a powerful instrument for continuously mapping the horizontal tidal current structures in coastal regions in China with heavy shipping traffic and active fishing.

Nontidal sea level changes in Hiroshima Bay, Japan

Nontidal sea level changes generated in Hiroshima Bay of the Seto-Inland Sea in Japan are studied over various time scales, from the sub-tidal (2 d to 1 month) to inter-annual scales (>2 years). The total sea level variation produces a standard deviation (STD) of 12.5 cm. The inter-annual component of the sea level variation in Hiroshima Bay oscillates with a STD of 3.4 cm, forming a long-term trend of 4.9 mm/a. The STD of the sea level variation is 9.8 cm for the seasonal component (8 months to 2 years) and 4.7 cm for the intra-seasonal one (1 month to 8 months). Significant sea level variations with a STD of 4.2 cm also occur in the sub-tidal range. Special attention is paid to the sub-tidal sea level changes. It is found that the upwelling and associated transient sea level changes generated along the north coast of Hiroshima Bay (opened southward) by the strong northerly wind, play a significant role in sub-tidal sea level changes. The transient sea level changes are over 10 cm in most cases when caused by typhoons that pass through the Pacific Ocean off the Kii Peninsula, located at about 400 km east of Hiroshima Bay. Reasonable sea level changes are evaluated by the balance of pressure forces at the onshore and offshore boundary of the study domain.

Mapping of a Typhoon‐Driven Coastal Upwelling by Assimilating Coastal Acoustic Tomography Data

A typhoon-driven upwelling event was observed with coastal acoustic tomography in Hiroshima Bay during September 2013. The tomography data were used to obtain state estimates from an ocean model, employing the ensemble Kalman filter (EnKF) for data assimilation. Hiroshima Bay was represented as a two-layer system with a fresh, near-surface layer overlaying a more saline, lower layer. Wind from the typhoon forced the surface layer southward, thus drawing the lower layer northward and causing upwelling. After the upwelling, these currents reversed for a period. During the upwelling, the total volume transports for the upper and lower layers were well balanced, showing continuity of the two layers. However, the total upper layer volume transported northward during the reverse-flow period was significantly smaller than the total upper layer volume transported southward during the upwelling, and significantly larger than the total lower layer volume transported southward during the reverse-flow period. The downstream reductions of transported volume indicated mixing fractions of 24%–30%, with water mixing at the interface of the upper and lower layers. Errors in state estimates were less than the changes in current and salinity associated with the upwelling.

Assimilation of coastal acoustic tomography data using an unstructured triangular grid ocean model for water with complex coastlines and islands

A coastal acoustic tomography (CAT) experiment was carried out using seven sets of CAT systems between September 6 to 9, 2009, in the Sanmen Bay region. Both assimilation and inversion of the data were used to reconstruct the current fields. The assimilated velocities agreed better with the independent acoustic Doppler current profiler (ADCP) data than the velocities obtained through inversion. Root mean square difference between current velocities obtained by data assimilation and those from ADCPs was 0.07 m s−1, which was less than the corresponding differences obtained by inversion (0.12 m s−1). Sensitivity experiments showed that missing sound transmission lines had less impact on assimilation than on inversion. These results indicate that, compared with inversion, data assimilation of the CAT data with an unstructured triangular grid is the optimal method for reconstructing the current field. This was the first challenge to assimilate coastal acoustic tomography (CAT) data into an unstructured triangular grid ocean model using the ensemble Kalman filter scheme.

Recent progress in coastal acoustic tomography in China: Experiments and data assimilation

We will review three recent coastal acoustic tomography (CAT) experiments carried out in China: (1) A 15-day tomography experiment that was carried out for the first time in the Qiongzhou Strait to measure the major tidal current constituents, residual currents, and volume transport; (2) A high precision CAT experiment with 11 CAT systems performed in the winter of 2015 in the Dalian bay, China. The number of successful reciprocal transmission lines reached 51, which may be the highest number in ocean acoustic tomography history. The CAT results showed a very high accuracy of velocity measurements with a root-mean-square difference of 4 cm/s compared with moored ADCP measurements; (3) Rapid sampling CAT measurements were used to map the structure of nonlinear tidal (M4 and M6) currents. The results indicate that M4 is predominantly generated by the advection terms, while friction mechanisms are predominant for generating M6. Finally, we will introduce the CAT data assimilated into an unstructured triangul...

A coastal acoustic tomography inverse method based on Legendre polynomials and its application in Sanmen Bay field experiment, China

Seven sets of shelf-contained type coastal acoustic tomography systems (CATs) were successfully deployed in Sammen Bay near the Taizhou city, China, during September 6 to 9, 2009 to measure tidal current structures at 3-min interval. An inverse model based on Legendre polynomials and Tikhonov regularization technology is developed to reconstruct the tidal current distributions using travel time differences obtained from the reciprocal sound transmission between the paired CAT stations. Variation of tidal current fields from food tide to ebb tide and tidal vortexes are well reconstructed. The horizontal distributions of tidal current obtained from the inverse analysis are in good agreement with that of Acoustic Doppler current Profiler (ADCP) observation and FVCOM ocean model. Comparison between CAT inversion and FVCOM model results, the spatial averaged mean square deviation (RMSD) is about 0.09 m/s for the eastward current and 0.04 m/s for the northward current, respectively. The time average RMSD between CAT inversion and ADCP measurement is 0.09 m/s for eastward current component and 0.002 m/s for northward current component, respectively. The investigation suggests that the coastal acoustic tomography inverse method based on Legendre polynomials is a promising alternative method for the ocean acoustic tomography inverse problem.

The Chinese Coastal Acoustic Tomography system and its application to the Luotou Channel, China

The Chinese Coastal Acoustic Tomography (CAT) system was developed for measurement of currents in the coastal sea. The CAT system was composed of two stations and each station was equipped with a GPS and a transducer. The operating frequency range of CAT system was 3–7.5 kHz. The reciprocal sound transmission experiment was carried out in March 25–26, 2009 in the Luotou Channel, near the Zhoushan Island, China. The sound signals using a carrier of 5 kHz modulated with M sequence of 9th order, were successful received at two stations. The mean signal to noise ratio reached about 19–21 dB. The range-averaged current velocity obtained along the transmission line measured by CAT was in good agreement with that measured by the lowed acoustic Doppler current profiler, producing the root-mean-square difference of 0.095 m/s. The above field experiment results indicated that the CAT system operated correctly.

A coastal acoustic tomography inverse method based on Chebyshev polynomials and its application in Zhoushan field experiment

Seven sets of shelf-contained type coastal acoustic tomography systems (CATs) were deployed in Zhitouyang Bay near the Zhoushan Island during July 12 to 13, 2009 to measure current structures at 3-min interval. The acoustic ray travel time tomography algorithm based on Chebyshev polynomials and truncated total least squares (TTLS) technology for reconstruction of two-dimensional flow field are constructed and tested using the CAT observation data. It can completely avoid spatial periodic of inverse results using Chebyshev polynomials. The TTLS technology, which simultaneously considers operator error and observational noise in system, is used for solutions of ill-posed problem. Variation of tidal current fields from food tide to ebb tide and tidal vortexes are well reconstructed by the inverse analysis of travel-time difference data. The investigation suggests that the CAT is a powerful tool to continuously map tidal current fields in the coastal seas. The coastal acoustic tomography inverse method based on Chebyshev polynomials and truncated total least squares technology can substantially improve reconstruction of current field and is a promising alternative method for the ocean acoustic tomography inverse problem.