A Fully Implicit Domain Decomposition Algorithm for Shallow Water Equations on the Cubed-Sphere

Abstract

Popular approaches for solving shallow water equations (SWEs) for climate modeling are explicit and semiimplicit methods, and both have certain constraints on the time step size. In this paper, we propose and study a fully implicit method which imposes no limit on the time step size but requires the solution of a large sparse nonlinear system of equations at every time step. The focus of the paper is a parallel, fully coupled, Newton-Krylov-RAS algorithm with a Jacobian matrix explicitly calculated on a weakly nonmatching cubed-sphere mesh. Here, RAS is a restricted additive Schwarz method. We show numerically that with such a preconditioned, nonlinearly implicit method the time step size is no longer constrained by the CFL condition, and we report superlinear speedup of the algorithm on machines with thousands of processors for problems with smooth and nonsmooth solutions.