Development Of An Unstructured Quadrilateral Grid,Finite-differencing,Estuarine And Coastal Ocean Model

Multi-scale simulation is a hot spot of current ocean numerical model research.In order to balance the need to increase the grid resolution and the computational costs,local grid refinement is a common approach for some specific research subjects,such as small-scale processes in the research area,or numerical domains with complex topography.Unstructured grid models are inherently suitable for multi-scale simulation,but they are less mature than their structured grid counterparts in some aspects,like numerical schems.the structured grid model requires grid nesting to implement multiscale simulation,but still maintains its original characteristics and advantages,which means it is convenient to apply finite difference method and construct high-order accuracy numerical scheme with high efficiency.In this dissertation,an unstructured quadrilateral grid,finite-differencing,estuarine and coastal ocean model called as UFDECOM,is developed based on the two-way nesting approach.The model is developed on the ocean primitive equations where the non-orthogonal curvilinear coordinates transformation and vertical ?coordinate transformation are introduced.The model grid is capable of multi-region nesting and multi-level refinement.Only valid cells are numbered in one-dimensional order to avoid redundant computations and accelerate calculations,and independent variables are distributed on an Arakawa C grid.As for solving the momentum and tracer equations,all the terms except for the vertical diffusion terms are treated explicitly.And the model code is parallelized with the Open MP tool to optimize the calculation speed.The information exchange between coarse and fine grids relies on the original virtual grid method,and the virtual grid variables are obtained from the actual grid variables.The model uses the HSIMT-TVD(high-order spatial interpolation at the middle temporal level with a TVD limiter)advection scheme with high-order accuracy and non-oscillation to calculate the advection terms in the tracer transport equations.Two novel interpolation schemes,the HSIMT parabolic interpolation scheme and HSIMT advection-equivalent interpolation scheme,are devised base on the advection scheme for information exchange.Their advantages lie in strong compatibility with the model and high-order accuracy.UFDECOM applies flux correction algorithm to ensure the conservation of nested grid model.The conclusions are as follows:(1)The results of the ideal salinity advection experiment show that the high-order quadratic interpolation scheme and HSIMT parabolic interpolation scheme maintain the maximum value of the computational domain most smoothly during the transition of the saline water mass from the coarse grid to the fine grid.The interpolation schemes can cause oscillations in the numerical solution,and the HSIMT parabolic interpolation scheme delivers the most stable performance.The quadratic interpolation scheme and the HSIMT advection-equivalent interpolation scheme give more reasonable performaces in conserving the total mass compared with the other two schemes.(2)UFDECOM was applied to a realistic simulation in the North Passage of the Changjiang Estuary to validate the model and further evaluate different interpolation schemes.The validation statistics,as well as the flow velocity and the salinity time series showed that the new model could successfully simulate the hydrodynamic process,but when simulating the salinity transport process,it significantly deteriorated compared to the traditional structural grid model.The HSIMT parabolic interpolation scheme performed the best among all the interpolation schemes.Through a further experiment and discussions,it was confirmed that the flux correction method reduced the accuracy of the nested grid model in solving the salinity transport equation.The new model also performed well in further validation of the measured data of the six buoy stations in May 2020.(3)UFDECOM was used to simulate a extremely severe saltwater intrusion in the Changjiang Estuary in February 2014.The model grid was locally refined in the areas of the critical passages for saltwater intrusion.The simulated elevation and salinity fitted well with the measured data.The new model successfully reproduced the severe saltwater intrusion,which gives a strong proof of its ability to conduct realistic simulations.