Revisiting Horizontal Diffusion of Perturbations over Terrain for NCEP RSM

tion (NCEP) Regional Spectral Model (RSM) is defined by the difference between the RSM and its outer model, such as the NCEP global model, on the same given coordinate surfaces. This perturbation is dominated by the difference between terrains of the RSM and the global model if model ter-rain exists over the domain. After being transformed into spectral space, this difference in terrains is characterized by two distinct groups of coeffi-cients i.e., long-wave and short-wave groups. The long-wave group is con-siderably larger in amplitude than the short-wave group, and considered to be the cause of error in horizontal diffusion of perturbations on sigma surfaces. A higher order of diffusion and diffusion without long waves on sigma surfaces with perturbations are formulated to investigate the possi-bility of using diffusion on sigma surfaces. In a typical case, erroneous circulation occurred when fourth-order diffusion on a sigma surface was used; however, reasonable circulation was simulated with second-order diffusion on the pressure surface. Nevertheless