Computational Framework for Multi-Scale Environmental Modelling

对流是当今大气科学中最具挑战性的问题之一。它涵盖了高能量的过程，如火山喷发或生物质燃烧羽流，以及积云或单一深度雷雨云的领域。标准大气模式，如用于天气预报或气候预报的模式，通常不能解析涉及对流活动的尺度。虽然云或烟羽的水平尺度很可能达到1公里，垂直尺度可能达到10公里，但导致极其重要的夹带(将环境特性混合到对流柱中)的关键过程，如湍流混合，尺度在几米到几十米之间。这种对流过程往往导致污染物从边界层向更高的大气层(在火山喷发或极深对流云中，直至平流层)的快速垂直输送，因此它们的正确模拟非常关键。同样，复杂地形(城市或山区环境)中不太强烈的对流但高度湍动的流动对于危险化学物质的扩散或野火的发展也很重要。这种情况对当前一代的环境数值模式是具有挑战性的。这个项目的总体目的是将现有的两个计算模型(Imperial-Flowity和Cambridge-Atham)结合起来并进行优化。阿萨姆是一种高分辨率大气模式，具有物理参数，适用于广泛的羽流和云相关应用。对于地形和与计算区域之外的流动的相互作用次要的问题，Atham成功地在有限区域内以高空间分辨率模拟了大气过程。流体包含最先进的并行自适应网格方法，能够以最佳方式分解流动，同时能够准确地表示对精度和稳定性至关重要的关键力平衡(地转和流体静力)，并以其海洋学的国际博协名义开发。流体缺乏阿萨姆将提供的大气问题的物理参数；然而，它为广泛的计算域和分辨率提供了CFD问题的一般框架。阿萨姆-流动性将结合两种模式的最佳元素。也就是说，灵活的自适应网格和保持平衡的流动有限元方法和Atham的先进物理模型，使得与全球大气模型相关的一系列新问题得以研究。一个重要的例子是对流，它通常在锋面系统内发展，这些锋面系统是大尺度流动的一部分，具有地形和差异加热，因为表面不均匀往往提供可以触发对流的扰动。组合模型将能够捕捉感兴趣区域的大规模流动以及细微尺度特征，从而在一个单一模型中更有效地进行尺度交互。在过去的十年中，编程范例已经从结构化变为模块化和面向对象的编程，在这种编程中，任何一组现代语言都可以被广泛使用。因此，将许多开源代码和库与FLOCITY和ATAM中包含的物理和高级数值技术相结合，将为开发组合的ATAM-FLOCITY模型作为下一代环境流动模型提供了极好的机会。由此产生的开放源码模型的主要优点将是：(A)问题表述的灵活性；(B)使用并行网格自适应以最佳方式表示物理的多物理建模；以及(C)先进组件技术(例如CAD几何和网格生成、线性和非线性求解器等)的模块化设计。阿萨姆-流动性将产生比现有软件代码更真实和准确的计算结果。

Convection is one of the most challenging problems in atmospheric science today. It covers highly energetic processes, like volcanic eruptions or biomass burning plumes, as well as fields of cumulus clouds or single deep thunderclouds. Standard atmospheric models, such as those used in weather forecasting or climate prediction, are generally not able to resolve the scales involved in convective activity. While cloud or plume sizes may well reach the 1km scale in the horizontal and 10km scale in the vertical, key processes, such as turbulent mixing that lead to extremely important entrainment (mixing environmental properties into the convective column) are of the scale of a few to tens of metres. Such convective processes are often responsible for fast vertical transport of pollutants from the boundary layer to higher atmospheric layers (in volcanic eruptions, or very deep convective clouds, up to the stratosphere), and therefore their correct simulation is highly crucial. Similarly, less vigorously convective but highly turbulent flows in complex topographies (urban or mountainous environments) are important for the dispersion of hazardous chemical species or for the development of wild fires. Such situations are challenging for the current generation of environmental numerical models. The overall purpose of this project is to couple and optimise two existing computational models (Imperial-FLUIDITY and Cambridge-ATHAM). ATHAM is a high-resolution atmospheric model with physical parameterisations for a wide range of plume and cloud relevant applications. ATHAM has successfully simulated atmospheric processes with high spatial resolution within a limited area for problems where topography and the interaction with the flow outside the computational domain are of secondary importance. FLUIDITY contains state-of-the-art parallel adaptive mesh methods that are able to optimally resolve flows, whilst being able to represent key force balances (geostrophic and hydrostatic) exactly which is important for accuracy and stability, and has been developed in its oceanographic guise of ICOM. FLUIDITY lacks the physical parameterisations for atmospheric problems that ATHAM will supply; however, it provides a general framework for CFD problems for a wide range of computational domains and resolutions. ATHAM-FLUIDITY will combine the best elements from both models. That is, the flexible adaptive mesh and balance maintaining finite element methods of FLUIDITY and the advanced physical models of ATHAM, allowing a new range of problems associated with global atmospheric models to be investigated. An important example is convection, which often develops within frontal systems that are part of the large-scale flow with topography and differential heating due to surface inhomogeneities often providing the perturbation that can trigger convection. The combined model will be able to capture large-scale flows as well as fine-scale features in areas of interest allowing a more efficient interaction of scales in one single model. Over the last decade, the programming paradigm has changed from structured to modular and object-oriented programming, in which any set of modern languages may be widely used. Therefore, combining a number of open-source codes and libraries with the physics and advanced numerical technologies contained within FLUIDITY and ATHAM offers an excellent opportunity to develop the combined ATHAM-FLUIDITY model as a next-generation environmental flow model. The main advantages of the resulting open-source model will be: (a) flexibility of the problem formulation; (b) multi-physics modelling to optimally represent the physics using parallel mesh adaptivity and; (c) modular design of the advanced component technologies (e.g. CAD-geometry and mesh generation, linear and non-linear solvers etc). ATHAM-FLUIDITY will be linked produce computational results that are more realistic and accurate than the existing software codes.