Vertical mode decomposition and model resolution

The sensitivity of the vertical mode expansion to the location of the upper boundary of the model atmosphere and to the vertical resolution is studied. The vertical modes are determined using a σ-coordinate linearized primitive equation model with basic state at rest. The σ-levels are defined at standard pressure levels so as to avoid interpolation influence on the results. Two basic sets of experiments are performed: (a) varying the top pressure where σ is zero while maintaining the vertical resolution in the interior unchanged: (b) fixing the top pressure and varying the vertical resolution. The results are analysed in terms of the energetics of the projection of gridded FGGE level III-b data at one particular grid point over tropical South America during the summer. It is found that the peak energy which occurs at an equivalent depth of the order of 220 m is approximately invariant with respect to experiments a and b.