Two types of vertical grids that are used for atmospheric models are the Lorenz grid (L-grid) and the Charney-Philips grid (CP-grid). Arakawa (1996) pointed out that, due to the existence of an extra degree of freedom in the vertical distribution of the temperature when using the L-grid, there is a computational mode in the calculations. The existence of the computational mode may have serious consequences, especially in the models that include moist processes, since the mode can interact spuriously with the physical mode as a result of nonlinearity in the equations.

This paper compares the two grids in the minimal hurricane model of Zhu et al. (2001) We show that with the L-grid, the temperature field in the version of the model with only an explicit representation of moist processes is dominated by short waves, and the temperature anomalies propagate horizontally due to the computational mode. As a result, the vortex environment warms and the vortex expands, while with the CP-grid, there is no evidence of short-wave growth and also no expansion of the vortex core.