非静力平衡模式的中尺度非均匀初始场试验和锋面模拟

用非静力平衡二维中β－γ尺度大气模式,选取１９８７年欧洲的一次冷锋过程为个例,以法国的静力平衡模式的模拟结果为初始场,选用５ｋｍ,２５ｋｍ,４５ｋｍ３种水平格距进行了水平范围为２０００ｋｍ的模拟,结果表明：（１）中尺度非静力平衡模式应用非均匀初始场,而不加人为的初始扰动是可行的;（２）扰动量的初始场中π′的精度要求很高,应严格符合静力平衡条件,否则就会产生严重误差,甚至在一个时步内改变垂直速度;（３）５ｋｍ,２５ｋｍ,４５ｋｍ３种格距的对比模拟表明,５ｋｍ格距的非静力平衡模式能模拟出锋面上的多个中β尺度对流,且垂直速度场分布与实际最接近,而后两种格距只能模拟出锋面上的大范围上升区,不能细致地描述锋面对流结构。     

Efficient computation of surf zone waves using the nonlinear shallow water equations with non-hydrostatic pressure

A numerical method for non-hydrostatic, free-surface, irrotational flow governed by the nonlinear shallow water equations including the effects of vertical acceleration is presented at the aim of studying surf zone phenomena. A vertical boundary-fitted grid is used with the water depth divided into a number of layers. A compact finite difference scheme is employed for accurate computation of frequency dispersion requiring a limited vertical resolution and hence, capable of predicting the onset of wave breaking. A novel wet–dry algorithm is applied for a proper handling of moving shoreline. Mass and momentum are strictly conserved at discrete level while the method only dissipates energy in the case of wave breaking. The numerical results are verified with a number of tests and show that the proposed model using two layers without ad-hoc assumptions enables to resolve propagating nonlinear shoaling, breaking waves and wave run-up within the surf and swash zones in an efficient manner.     

A non-hydrostatic ocean model with a scalable multigrid Poisson solver

A non-hydrostatic ocean model using an effective Poisson solver is developed. The Poisson solver is a combination of the multigrid method, the Krylov-subspace method, and the sparse approximate inverse. Its numerical cost only linearly increases with total number of computational cells, and it also has high parallel computing efficiency. The numerical cost of the non-hydrostatic model described in the present paper remains only twice of that of a hydrostatic model, even with non-smooth topography and with a huge number of computational grid cells on massively parallelized computer systems. Therefore, it has a potential to expand the applicability of non-hydrostatic ocean models. We also present the preliminary result of the high-resolution non-hydrostatic experiment on Ice Shelf Water overflow in the southern Weddell Sea, which shows good agreement with observations in terms of the pathway of dense water and velocity field.     

A non-hydrostatic numerical model for calculating free-surface stratified flows

A three-dimensional non-hydrostatic numerical model for simulation of the free-surface stratified flows is presented. The model is a non-hydrostatic extension of free-surface primitive equation model with a general vertical coordinate and horizontal orthogonal curvilinear coordinates. The model equations are integrated with mode-splitting technique and decomposition of pressure and velocity fields on hydrostatic and non-hydrostatic components. The model was tested against laboratory experiments on the steep wave transformation over the longshore bar, solitary wave impact on the vertical wall, the collapse of the mixed region in the thin pycnocline, mixing in the lock-exchange flows and water exchange through the sea strait. The agreement is generally fair.Responsible Editor: Hans Burchard     

A wave-resolving model for nearshore suspended sediment transport

This paper presents a wave-resolving sediment transport model, which is capable of simulating sediment suspension in the field-scale surf zone. The surf zone hydrodynamics is modeled by the non-hydrostatic model NHWAVE (Ma et al., 2012). The turbulent flow and suspended sediment are simulated in a coupled manner. Three effects of suspended sediment on turbulent flow field are considered: (1) baroclinic forcing effect; (2) turbulence damping effect and (3) bottom boundary layer effect. Through the validation with the laboratory measurements of suspended sediment under nonbreaking skewed waves and surfzone breaking waves, we demonstrate that the model can reasonably predict wave-averaged sediment profiles. The model is then utilized to simulate a rip current field experiment (RCEX) and nearshore suspended sediment transport. The offshore sediment transport by rip currents is captured by the model. The effects of suspended sediment on self-suspension are also investigated. The turbulence damping and bottom boundary layer effects are significant on sediment suspension. The suspended sediment creates a stably stratified water column, damping fluid turbulence and reducing turbulent diffusivity. The suspension of sediment also produces a stably stratified bottom boundary layer. Thus, the drag coefficient and bottom shear stress are reduced, causing less sediment pickup from the bottom. The cross-shore suspended sediment flux is analyzed as well. The mean Eulerian suspended sediment flux is shoreward outside the surf zone, while it is seaward in the surf zone.     

A semi-implicit three-dimensional numerical model for non-hydrostatic pressure free-surface flows on an unstructured,sigma grid

A semi-implicit 3-D numerical formulation for solving non-hydrostatic pressure free-surface flows on an unstructured,sigma grid is proposed.Pressure-splitting and 9 semi-implicit methods are inherited and reformed from Casulli’s z-coordinate formulation.The non-orthogonal sigma-coordinate transformation leads to additional terms. The resulting linear system for the non-hydrostatic correction is diagonally dominant but unsymmetric,and it is solved by the BiCGstab method.In contrast with z-coordinate non-hydrostatic models,the new model fits vertical boundaries much better,which is important for the long-time simulation of sediment transport and riverbed deformation.A lock-exchange density flow is computed to determine whether the new scheme is able to simulate non-hydrostatic free-surface flows.The new model is further verified using the field data of a natural river bend of the lower Yangtze River.Good agreement between simulations and earlier research results,field data is obtained, indicating that the new model is applicable to hydraulic projects in real rivers.     

中尺度山脉对流群的动力和微物理数值模拟

大山脉春夏季受太阳加热和地形抬升作用易于形成对流过程。为研究这类对流特点，在沙特阿拉伯ＡＳＩＲ山区组织了一个ＳＡＣＰＥＸ实验计划。本文应用中国强风暴实验室ＭＢＧ（ＭＥＳＯ—Ｂｅｔａ＆Ｇａｍｍａ）非静力模式，选取适当模拟通道模拟了该计划中１９９０年５月３日一次对流降水过程。模拟结果揭示了大山脉气流强迫与对流环流相互作用的一些基本特点，展示了大山脉对流过程多种复杂的宏微观特征及其生消演变。包括山坡激发对流，大对流对小对流的抑制，云自然引晶效应等，并指出在这类对流系统中ＣＣＮ浓度对其降水类型的重要影响。此外，本文还进行了采取细致的雹云微物理参数化和采用简单的暖云微物理参数化的对比试验，结果表明两者模拟的动力场差别甚小，而云形态和降水发展有明显差异。上述结果总体上与该个例外场观测一致。