Multi-Scale FVCOM Model System For The East China Sea And Changjiang Estuary And Its Applications

Several works have been done in this thesis, with the focus on multi-scale dynamic simulations over the East China Sea shelf, Changjiang Estuary and Deep Waterway Project region:1) An unstructured-grid high-resolution multi-scale model system based on Finite-Volume Coastal Ocean Model (FVCOM) has been set up, with the geographic coverage of East China Sea, Yellow Sea, Bohai Sea, Taiwan Strait, Japan Sea and northwest Pacific, and with the inclusion of Kuroshio Current, Taiwan Warm Current, Changjiang River discharge and other physical dynamics, and this model system contains multi-scale weather model (WRF), regional East China Sea FVCOM (shelf scale ECS-FVCOM), Changjiang Coastal FVCOM (regional CJ-FVCOM) and Deep Waterway FVCOM (local DW-FVCOM). Multiple physical factors are included, such as open ocean currents, freshwater discharge, astronomical tide, monsoon, surface wave and sedimentation;2) With the several updates of open boundary condition and vertical discretization in shelf-scale ECS-FVCOM, climatological shelf circulations have been studied with an aim to reveal the climatological characteristics of Kuroshio Current, Taiwan Warm Current and other shelf currents under tide, freshwater discharge, heat radiation and monsoon. The overall circulations and distributions of salinity and temperature around the Changjiang Estuary are used to provide the lateral boundary for the interior CJ-FVCOM.3) An unstructured-grid finite-volume dike-groyne treatment algorithm is derived and implemented into FVCOM. The unstructured triangular mesh used in this model is flexible to resolve accurately any configuration of dikes and groynes, and the finite-volume flux algorithm in FVCOM ensures the conservation of volume and mass under the boundary condition of no flux into or out of the construction. With the same MPI framework, this model can efficiently use either a single computer or multi processor clusters for the fast computation. An idealized case is designed to validate the capability of the dike-groyne module in resolving the realistic water exchange across the construction and the accuracy of parallel computing under memory-distributed multiple-node architecture. The comparison between dike-groyne algorithm and bed-conforming treatment method suggests that resolving the kinematic boundary condition on dike-groyne construction is critical to capture the realistic flow and tracer fields in the system and the bed-conforming approach can cause an unrealistic overestimation of water transport across the construction and also fails to resolve the geometrically controlled formation of eddies around the construction.4) With the implementation of finite-volume dike-groin treatment algorithm, four versions of DW-FVCOM have been setup with the grid resolutions of200m,100m,50m and25m, which exactly resolve the geometries of dikes, groins and shipping channel around the Deep Waterway Project of Changjiang River mouth. DW-FVCOM has successfully been applied to simulate the flow field around the project region. The constructions of dikes and groins around the Deep Waterway Project have resulted in the significant changes on the local circulations, including the small-size eddies generated in the groin region, cross-over current along the dikes during the high tide when the water level is higher than the dike elevation, and converting the original rotational current system into alternating flow along the shipping channel. From Phase I to Phase II, the along-channel current has some changes on the velocity amplitude and current direction. Significant velocity increasing due to the constraint effect of dikes is revealed in the eastern part of DWP. Small amplitude changes at the upstream western part have been found, with decreasing amplitude about20cm/s during the flood tide. Strong bottom saltwater intrusion is also revealed in the model-data comparison and the idealized experiments. The significant stratification indicates a two-layer system in eastern part of DWP, with the low-salinity water dominated the upper column and salty ocean water controlled the lower column.5) With the combination of observation in the Changjiang Estuary and inner shelf of East China Sea, a high-resolution, full three-dimensional, triangle-grid FVCOM model is applied to study the physical mechanisms of plume distribution, variation and detachment during the monsoon summertime under typical freshwater discharge from the Changjiang River. This numerical model has got reasonable simulation for horizontal and vertical structures for currents and salinity around the Changjiang Estuary. A two-layer system is revealed from the observation and numerical simulation, with a～15m thickness buoyancy-driven upper layer and tide-dominant lower column. The two-layer system caused significant baroclinic instability and produces bulge shape around the deep region with bathymetry from20m to50m, however, it has no capability to detach the low-salinity plume into the outer shelf under the moderate tidal mixing. Monsoon wind effect during the summertime enhances the baroclinic instability around the plume front region, and causes significant local plume detachment around the localized high-speed wind stirring.