Computational Framework for Multi-Scale Environmental Modelling

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.