A wave propagation method for hyperbolic systems on the sphere

Presented in this work is an explicit finite volume method for solving general hyperbolic systems on the surface of a sphere. Applications where such systems arise include passive tracer advection in the atmosphere, shallow water models of the ocean and atmosphere, and shallow water magnetohydrodynamic models of the solar tachocline. The method is based on the curved manifold wave propagation algorithm of Rossmanith, Bale, and LeVeque [A wave propagation algorithm for hyperbolic systems on curved manifolds, J. Comput. Phys. 199 (2004) 631–662], which makes use of parallel transport to approximate geometric source terms and orthonormal Riemann solvers to carry out characteristic decompositions. This approach employs TVD wave limiters, which allows the method to be accurate for both smooth solutions and solutions in which large gradients or discontinuities can occur in the form of material interfaces or shock waves. The numerical grid used in this work is the cubed sphere grid of Ronchi, Iacono, and Paolucci [The ‘cubed sphere’: a new method for the solution of partial differential equations in spherical geometry, J. Comput. Phys. 124 (1996) 93–114], which covers the sphere with nearly uniform resolution using six identical grid patches with grid lines lying on great circles. Boundary conditions across grid patches are applied either through direct copying from neighboring grid cells in the case of scalar equations or 1D interpolation along great circles in the case of more complicated systems. The resulting numerical method is applied to several test problems for the advection equation, the shallow water equations, and the shallow water magnetohydrodynamic (SMHD) equations. For the SMHD equations, we make use of an unstaggered constrained transport method to maintain a discrete divergence-free magnetic field.