Global Ocean Modelling with Adaptive Unstructured Grid Methods

在气候变化的阵痛中，海洋环流显然对一个以水为主的星球很重要。因此，海洋模拟技术对我们预测变化的能力至关重要。然而，尽管在过去十年中取得了重大进展，海洋环流模式基本上还是基于与20世纪60年代开发的最早模式相同的有限差分方法和固定结构网格。尽管非结构化网格建模一直是许多海洋学家的目标，但由于高纵横比域的挑战以及在非结构化网格上准确稳定地处理科里奥利和浮力项，将这种方法应用于全球环流的尝试失败了。在过去几年中，针对这些问题的重要解决方案已经开发出来，并被纳入了帝国理工学院海洋模型（ICOM）。该模型具有最佳的并行网格自适应方法，一套空间导数选项（如密度/示踪平流的高分辨率方法），地转和静水平衡的新颖稳健处理，优化的测深和海岸线几何形状，以及大涡自适应湍流模型。在这个项目中，我们将利用十多年来在我们的新海洋模型ICOM上的开发工作，并在管理阶段生产一个完全结合3D自适应和非结构化网格技术的全球环流模型。因此，我们将能够以最小的参数化和最大的物理捕获同时解析从10s公里（边界流）到100米（下行流）的尺度上的流动。此外，通过利用我们最先进的网格工具，初始表面分辨率将集中在水深变化区域。这将提高计算效率，因为它将减少捕获几何复杂性所需的节点和元素的数量。我们的方法将允许网格以类似于混合坐标方法的方式响应漩涡或密度层而移动。网格的拓扑结构和节点密度也将被优化，以从我们的新颖建模方法中获得最大的灵活性、功率和鲁棒性。通过与最近的原位观测数据集和可用遥感数据集的详细比较，以及与南安普敦国家海洋学中心（NOC）目前的海洋模型（如NEMO和OCCAM）在各种网格分辨率下进行的模拟进行比较，将研究如何在多年代际整合中再现海洋的基本动力学，从而评估自适应模式对全球海洋的模拟质量。模拟将在帝国理工学院，NOC和英国HECToR超级计算设施的并行计算集群上进行，其中将确定大量处理器的缩放特性。除了在球体上开发新的自适应网格技术外，我们还将展示各种形式的网格垂直和水平运动，以及网格结构变化如何提高解决方案质量。我们将研究一系列基于流动动力学表示的误差测量的性能。我们的网格生成包Terreno将被推广，以便静态网格能够在隔离网格适应性的情况下提供高质量的模拟。我们还将证明，模型自旋可以迅速实现与逐步增加的分辨率。由此产生的海洋建模能力的进步将巩固英国在非结构化和自适应网格海洋建模的前沿地位，并有望成为英国海洋建模的一个重要里程碑。总的来说，这个项目将为英国和全世界的海洋建模社区提供一个开源的海洋模型。它还将通过提供工作包9.8，为NERC的海洋2025方案作出重要贡献。

Ocean circulation is clearly important on a water-dominated planet in the throes of climate change. Ocean modelling technologies are therefore crucially important to our abilities to forecast change. However, despite significant advances over the past decade, ocean general circulation models are based on essentially the same finite difference methods and fixed structured grids employed in the earliest models developed in the 1960s. Although unstructured mesh modelling has long been a goal of many oceanographers, attempts to apply such methods the global circulation have failed due to challenges with high aspect ratio domains and in treating the Coriolis and buoyancy terms accurately and stably on unstructured meshes. Over the past few years important solutions to these problems have been developed and incorporated into the Imperial College Ocean Model (ICOM). This model has the best available parallel mesh adaptivity methods, a suite of options for spatial derivatives (such as high-resolution methods for density/tracer advection), novel robust treatments of geostrophic and hydrostatic balance, optimised bathymetry and coastline geometries, and large eddy adaptive turbulence models. In this project we will take advantage of more than a decade of development work on our new ocean model ICOM and produce, in managed stages, a global circulation model that fully incorporates 3D adaptive and unstructured mesh technology. As a result we will be able to simultaneously resolve flow at scales ranging from 10s km (boundary currents) to 100s m (downwelling currents) with minimal parametisation and maximal capture of the physics. In addition, by taking advantage of our state-of-the-art meshing tools, initial surface resolution will be focussed on areas of bathymetric change. This will impart computational efficiency since it will reduce the number of nodes and elements required to capture geometric complexity. Our approach will allow the mesh to move in response to eddies or density layers in a manner akin to hybrid coordinate approaches. The topology of the meshes and node density will also be optimised to gain maximum flexibility, power and robustness from our novel modelling approach. The quality of the adaptive model's simulation of the global ocean will be assessed by examining how, in multi-decadal integrations, it reproduces the essential dynamics of the oceans through detailed comparisons with recent in-situ observational datasets and available remote sensing datasets and also against simulations performed at the National Oceanography Centre Southampton (NOC) by current ocean models, such as NEMO and OCCAM, at a variety of grid resolutions. Simulations will be conducted on parallel computing clusters at Imperial College, NOC, and the UK's HECToR supercomputing facility where scaling properties on large numbers of processors will be determined. In addition to developing new adaptive mesh techniques on the sphere we will show how various forms of mesh movement vertically and horizontally, and mesh structural changes can each enhance solution quality. We will study the performance of a range of error measures which are based on representations of the flow dynamics. Our mesh generation package Terreno will be generalised so that static meshes are able to provide high quality simulations in isolation from mesh adaptivity. We will also demonstrate that model spin-up can be rapidly achieved with progressively increased resolution. The resulting advances in ocean modelling capabilities will cement the UK's position firmly at the forefront of unstructured and adaptive mesh ocean modelling, and is expected to be a major ocean modelling milestone for the UK. Overall this project will contribute considerably towards the production of an open source ocean model for the UK and worldwide ocean modelling communities. It will also provide an important contribution to NERC's Oceans2025 programme by delivering Work Package 9.8.