Numerical Study On Lagrangian Inter-tidal Transports Of Salinity In The Bohai Sea

Long-term variation of salinity might cause succession of marine ecological community, and also influence the circulation type. It plays an important part in the coastal circulation and long-term mass-transport. The velocity field for describing coastal circulation, and the temperature, salinity and mass concentration fields for describing long-term transport processes, are not in-situ observation data, but post-process results, from which the tidal variations of instantaneous velocity, temperature, salinity and concentration have been removed by a certain time-mean method, such as Eulerian or Lagrangian tidally -averaged method in common use.Based on a primary Lagrangian tidally-averaged theory on circulation with inter-tidal transport processes in a general nonlinear coastal/estuarine system, single-frequency tide induced Lagrangian residual circulation with inter-tidal transports of conservative substance concentration in three model seas are simulated by a 3-D baroclinic hydrodynamic model HAMSOM with trajectory tracking sub-model. Three model seas are constructed for different topography or coastal line, termed as isobathic model, stair model, and cape model. It is found that the nonlinearities of stair and cape model are stronger than that of isobathic model, shown by the observable dependence of Lagrangian residual velocity/inter-tidal concentration on tidal phases, for a set of infinite temporal-spatial fields of velocity/concentration is exhibited as the tidal phase varies continuously over one tidal cycle, each of which corresponds to a specific value of tidal phases. The convectively weakly nonlinear condition is still satisfied in the isobathic model, so its set of infinite temporal-spatial fields of velocity/concentration could be reduced to a single value as shown traditionally, i.e. mass-transport velocity /Eulerian tidally-mean concentration. ST *,the Lagrangian time-mean value of the vertical diffusion of instantaneous concentration, shows clear positive or negative at northern part of each model sea, and also at the positions of stairs or capes, which are happened to be the regions with enhanced local nonlinearity in circulation analysis. Additionally, influences of the riverine runoff and the number of tidal cycles for time-averaging, n, on Lagrangian tidally-averaged circulation with inter-tidal transports are also studied in the model seas. Conclusions of these experiments might be helpful to the numerical simulation in the Bohai Sea.When the Lagrangian tidally-averaged theory is applied to the Bohai Sea, Lagrangian residual velocity with inter-tidal transports of salinity induced by barotropic or baroclinic single-frequency tide are simulated separately. The results show that there are four strong current regions in the Bohai Sea, characterized by complicated topography or coastal lines. The residual velocity at the central basin is so weak that this region belongs to a convectively weakly nonlinear coastal system. Barotropic single-frequency tide induced Lagrangian residual velocity is quasi-stationary in the inter-tidal time scale, while baroclinic tide induced residual velocity is variable in the same scale, for the variation of its baroclinic component is mainly generated in the inter-tidal time scale. The residual velocities in both conditions could vary as the tidal phase varies, which is in the intra-tidal time scale. Inter-tidal salinity, ST * of salinity, and the difference between inter-tidal and intra-tidal (instantaneous) salinity all take on variations in both the intra- and inter-tidal time scales. Generally speaking, the simulated salinity in barotropic condition is lower than that in baroclinic condition. Beside stronger Lagrangian residual velocity, riverine runoff could also result in the variation of inter-tidal salinity with the tidal phases. In both conditions, the prominent positive or negative areas of ST * could also be marked as obvious variation of inter-tidal salinity with the tidal phases whether the runoff be large or not.It could be found in this thesis that Lagrangian tidally-averaged method is more meaningful than Eulerian time-mean method in describing the inter-tidal transport processes, for its substantial property determines the inter-tidal concentration is adaptive to the Lagrangian residual velocity. It should be pointed out that the tracking time could not be too long, or else the substantial property might be lost. The simulation of inter-tidal concentration by instantaneous concentration field and the calculation of ST * by integrating vertical diffusion of instantaneous concentration along trajectory when tracking water parcels, both of which are proposed in this thesis, successfully solve the key problems in the application of the Lagrangian theory, and also promote the combination of theory and reality in the numerical simulation.