A spatial filter for use in finite area calculations

A multidimensional filter that selectively removes 2 Delta x and 2 Delta y noise is introduced. The filter is designed to be effective in finite-area models with nonperiodic horizontal boundary conditions. An economical splitting technique, requiring an intermediate boundary condition, is used in the calculations. The filter is tested on a finite-difference representation of two-dimensional linear advection and compared with the Shapiro eighth-order filter. Tests on both uniform and variable grid configurations are conducted, as well as a finite-element calculation. Influence of repeated applications of the filters upon a rectangular column is analyzed, and comparisons between filter factors are made. Lastly, the proposed filter is inserted into the three-dimensional Penn State-NCAR regional model to test its performance in a state-of-the-art model and to study the importance of horizontal diffusion.