Comments on noninterpolating semi-Lagrangian advection scheme with minimized dissipation and dispersion errors

The semi-Lagrangian scheme is now firmly established as a very useful way of improving the efficiency of NWP models. The method has been successfully tested over a wide range of applications, and thus attention is now being focused on possible improvements. Rancic and Sindjic (1989), hereafter referred to as RS, note that one of the disadvantages of current semi-Lagrangian methods is the need for interpolation, which introduces error and increases the computational cost of the method. Accordingly, they propose two ``noninterpolating'' schemes by combining concepts from standard semi-Lagrangian and Eulerian finite-difference methods. Unfortunately, while their objectives are most worthwhile and their approach ingenious, it is straightforward to show that their schemes are simply interpolating semi-Lagrangian methods derived in an unusual fashion.