A Study On Assimilation And Application Of Coastal Acoustic Tomography

Compared with traditional techniques of measuring the current field,Coastal Acoustic Tomography(CAT)can achieve long-term and synchronous measurement over a large area,obtaining the rapidly varying current field in coastal seas.This can be achieved by only deploying a few instruments on the periphery of the observational area.CAT is suitable in enclosed bodies of water,such as bays,straits,and harbors,where shipping traffic and fishing activity are heavy.This paper reconstructs the current field based on CAT data by using the inverse method and data assimilation.Then,we select three typical coastal regions(Sanmen Bay,Zhitouyang Bay,and Qiongzhou Strait)to discuss the characteristics of tidal currents,such as the structures of tidal currents and the dynamics of residual currents.For the first time,CAT data were assimilated into an ocean model with an unstructured triangular grid by using the ensemble Kalman filter scheme.First,a triangular grid with high spatial resolution was established to fit complex coastlines in coastal regions.Next,the measurement operator between the travel time differences and the state vectors of the ocean model was determined by calculating the horizontal relationship between sound transmission lines and the irregular triangular gird.Finally,the depth averaged travel time differeces datawere assimilated to reconstruct the three-dimensional current field.The three-dimensional assimilation of CAT data with an unstructured triangular grid was presented to reconstruct the current fields in Sanmen Bay.The fine horizontal and vertical current field structures around the island inside the observation region were both reproduced well.The assimilated depth-averaged velocities had better agreement with the independent acoustic Doppler current profiler(ADCP)data than the velocities obtained by inversion and simulation.The root-mean-square difference(RMSD)between depth-averaged current velocities obtained by data assimilation and those obtained by ADCPs was 0.07 m·s-1,which was less than the corresponding difference obtained by inversion and simulation(0.12 m·s-1 and 0.17 m·s-1,respectively).The assimilated vertical layer velocities also exhibited better agreement with ADCP than the velocities obtained by simulation.RMSDs between assimilated and ADCP data in vertical layers ranged from 0.02 to 0.14 m·s-1,while RMSDs between simulation and ADCP data ranged from 0.08 to 0.27 m·s-1.These results indicate that assimilation had the highest precision.Sensitivity experiments involving the elimination of sound transmission lines showed that missing data had less impact on assimilation than on inversion.Sensitivity experiments involving the elimination of CAT stations showed that assimilation with four CAT stations was the most economical and reasonable procedure in this experiment.These results indicate that,compared with inversion and simulation,data assimilation of CAT data with an unstructured triangular grid is the most optimal method for reconstructing the current field.The 15-min mean depth-averaged current data obtained by CAT,which covers about 27 hours in Zhitouyang Bay on the western side of the East China Sea,are used to estimate the semidiurnal tidal current(M2)as well as its first two overtide currents(M4 and M6).Spatial mean amplitude ratios M2:M4:M6 in the bay 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,where water depths are larger than 60 m,M4 velocity amplitudes measured by CAT agree well with those predicted by the advection terms in the shallow water equations,indicating that M4 in the deep area is predominantly generated by the advection terms.M6 measured by CAT and predicted by the nonlinear quadratic bottom friction terms agrees well in areas where water depths are less than 20 m,indicating that friction mechanisms are predominant for generating M6 in the shallow area In addition,dynamic analysis of the residual currents using the tidally averaged momentum equation shows that the spatial mean values of the horizontal pressure gradient due to the residual sea level and advection of residual currents together contribute about 75%of the total spatial mean values of the advection by tidal currents,indicating that residual currents in this bay are induced primarily by the nonlinear effects of tidal currents.A 15-day CAT experiment was carried out at four acoustic stations in March 2013 to measure the tidal current,residual current,and volume transport in the Qiongzhou Strait(QS).The diurnal tidal current constituents were predominant.The ratios of the amplitudes O1:K1:M2:S2:MSF were 1.00:0.60:0.47:0.21:0.11.The residual currents were found to flow westward in the northern QS and 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-1 in the northern QS.Volume transport estimated using CAT data varied between-0.71 Sv and 0.86 Sv,with a residual current transport of-0.04 Sv,where negative values indicate westward flow.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.The spatial structures of the diurnal tidal constituents(O1 and K1),semidiurnal tidal constituents(M2 and S2),nonlinear tidal constituents(M4 and M6),and the residual current were obtained based on CAT data.Furthermore,the dynamics of residual currents and nonlinear tidal currents were discussed.These results indicate that CAT has unique advantages in measuring tidal currents in coastal seas and plays an important role in the study of the dynamics of tidal currents.