Numerical accuracy of the regional spectral method used in NCEP RSM

The numerical method used in NCEP RSM model is addressed. To illustrate the boundary effect and scale dependent accuracy of this method, a simple linear advection equation is used as an example. The numerical results show that there are no significant lateral boundary problems in both inflow and outflow boundaries; and no unusual scale dependent inaccuracy presents in the numerical solution, the numerical accuracy associated with large spatial scale phenomena keeps well as the numerical accuracy associated with small spatial scale phenomena. The numerical results discussing the nesting strategy between base field and regional domain show that there exits an optimal grid number in the base field through which enough information is provided to get the best solution in the regional domain; beyond that other error sources dominate the total error in the numerical solution. The effects of different time schemes and spatial differential methods are also investigated here.