Resonating neutral modes of the Eady model

The resonance between a neutral Eady mode and a continuous mode with the same phase speed is used to interpret the growth of neutral modes discussed by Farrell. It is shown that the existence of such a resonance may lead to spurious linear growth of error in a discretized quasigeostrophic (QG) model if one of the levels lies very close to the steering level of the neutral Eady normal mode. For a primitive equation model, this spurious resonance manifests itself as an exponentially growing-decaying pair of short waves. Such waves are similar to those found by Arakawa and Moorthi for the Lorenz grid for the QG equations. However, here it is found that for the primitive equation model, these spurious short waves exist both for the Lorenz grid and the Charney-Phillips (CP) grid, although the growth rate is somewhat smaller for the CP grid. -from Author