A Lagrangian method for solving the spherical shallow water equations using power diagrams

Abstract

Numerical simulations of the air in the atmosphere and water in the oceans are essential for numerical weather prediction. The state-of-the-art for performing these fluid simulations relies on an Eulerian viewpoint, in which the fluid domain is discretized into a mesh, and the governing equations describe the fluid motion as it passes through each cell of the mesh. However, it is unclear whether a Lagrangian viewpoint, in which the fluid is discretized by a collection of particles, can outperform Eulerian simulations in global atmospheric simulations. To date, Lagrangian approaches have shown promise, but tend to produce smoother solutions. In this work, a new Lagrangian method is developed to simulate the atmosphere in which particles are represented with spherical power cells. We introduce an efficient algorithm for computing these cells which are then used to discretize the spherical shallow water equations. Mass conservation is enforced by solving a semi-discrete optimal transport problem and a semi-implicit time stepping procedure is used to advance the solution in time. We note that, in contrast to previous work, artificial viscosity is not needed to stabilize the simulation. The performance of the spherical Voronoi diagram calculation is first assessed, which shows that spherical Voronoi diagrams of 100 million sites can be computed in under 2 minutes on a single machine. The new simulation method is then evaluated on standard benchmark test cases, which shows that momentum and energy conservation of this new method is comparable to the latest Lagrangian approach for simulating the spherical shallow water equations.