Cascade interpolation for semi-Lagrangian advection over the sphere

A cascade interpolator, previously proposed by the authors for semi-Lagrangian advection in Cartesian geometry, is extended to spherical geometry. It is evaluated using two test problems-solid-body rotation and strong deformational flow-and found to be both accurate and efficient. The second problem is particularly challenging. It is a generalization to spherical geometry of Doswell's idealized cyclogenesis problem with an exact solution.