Xiaopeng Fan

The role of wind on the detachment of low salinity water in the Changjiang Estuary in summer

in July 2006, and the role of wind on detaching the LSW in particular, are explored with a three-dimensional numerical model. The real-case simulation and the sensitivity experiments results show that wind plays a crucial role in the detachment events and is highlighted in three aspects. First, wind is the most important dynamic factor in the two detachment processes of the LSW. Wind mixing, wind-driven northward current and wind-induced upwelling are three driving forces on detaching the LSW, which increase the salinity in the upper layer in the detachment region along the 30 m isobath and separate the offshore LSW from the nearshore main body of LSW. The diagnostic analysis further indicates that the increase of salinity in the detachment region is mainly due to northward current which transports high salinity water from the south. Second, a critical wind speed, namely a southeasterly wind above 8.0 m/s, is found to be related to the timing of the detachment events. A sensitivity experiment further confirms this critical wind speed and no detachment occurs when the wind speed is below 8.0 m/s. Third, the southwesterly wind plays a key role in the magnitude of the spatial size of the detached LSW. Before the detachment occurs, a persistent southwesterly wind induces northeastward expansion of the LSW and consequently forms larger LSW offshore after detachment, which is verified by another sensitivity experiment with modified wind direction.

Tidal and residual currents in the Qiongzhou Strait estimated from shipboard ADCP data using a modified tidal harmonic analysis method

In spring 2013, 33 repeat shipboard Acoustic Doppler Current Profile (ADCP) surveys were conducted to measure the tidal current in the Qiongzhou Strait (QS). The major tidal currents and the residual current along a section across the QS were estimated using a modified tidal harmonic analysis method based on the inverse technique. A simple simulation and comparisons with previous observations demonstrated that the tidal currents estimated using the modified tidal harmonic analysis method are reasonable, and this method was able to control the magnitude and deviation of the estimation error. The direction of the major axis of tidal current ellipses is generally along the strait. Diurnal tidal constituents are dominant among the five tidal current constituents (K1, O1, M2, S2, and MSf). The ratio of the amplitudes of O1, K1, M2, S2, and MSf, averaged along the section across the QS is 1:0.79:0.42:0.27:0.29. The residual current along the entire section is all westward; the averaged velocity over the section is 6.0±2.1 cm s−1; the associated volume transport through the section is −0.065 ± 0.046 Sv (Sv = 106 × m3 s−1), in which the second value denotes the uncertainty of first value. Dynamic analysis indicates that tidal current activity is more dominant than mean current and eddy activity, and tidal rectification and sea level difference between two entrances of the QS are important in maintaining the residual current through the strait.

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.

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.

A coastal acoustic tomography experiment in the Qiongzhou Strait

An experiment for synchronous mapping of current velocity (V CAT ) and temperature (T CAT ) distributions was successfully carried out using four coastal acoustic tomography (CAT) systems, from March 17 to April 1, 2013, in the Qiongzhou Strait, China. During the CAT observation period, shipboard acoustic Doppler current profiler (ADCP) surveys were also performed to observe the current velocity (V ADCP ) and temperature (T ADCP ) for the purpose of comparison with the CAT data. Position offsets, including a long-time averaged offset and a short-period fluctuation offset, were corrected for temperature estimation. The horizontal distribution of V CAT and T CAT was reconstructed by an inverse technique using the CAT data. V CAT was nearly parallel to the strait while T CAT was higher in the southern area than in the northern area. During the whole observation period, T CAT gradually increased by approximately 2°C, indicating seasonal warming. V CAT was in good agreement with V ADCP , while the root-mean-square differences (RMSDs) between V CAT and V ADCP for eastward and northward velocities were 0.10 m/s and 0.08 m/s, respectively. T CAT was in approximate agreement with T ADCP , with an RMSD of 0.30°C.

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...