The Choice of a Vertical Grid for a 2.5D Numerical Model of the Middle Atmosphere

Abstract

The consequences of choosing an unstaggered grid (UG), Charney-Phillips-type grid (CPG), or Lorenz-type grid (LG) in the vertical for a 2.5D model are examined. Analytical solutions for the linearized and scaled eddy equations in spherical coordinates are compared to solutions obtained by finite-difference methods. Waves with vertical structure greater than six gridpoint intervals are reliably reproduced by the model using any of these three grids. Waves with vertical structure between four and six gridpoint intervals appear in the model with shorter vertical wavelengths using the UG and CPG, and with longer vertical wavelengths using the LG. Waves with vertical structure less than four gridpoint intervals will be evanescent using the UG and CPG. Waves with vertical structure less than four gridpoint intervals will be propagated vertically using the LG; however, waves unresolved by the LG will spuriously appear and will be vertically propagated with an apparent vertical wavelength of three gridpoint intervals.