Orographic Modulation of Baroclinic Instability

A single baroclinic normal mode is perturbed by small orography. The method of analysis considered here allows one to conceptually, understand how the orography modifies the characteristics of the normal mode, and how the modulations depend upon the structural and diagnostic properties of the normal mode (such as the phase speed and the growth rate). It is found that slow phase speeds and growth rates enhance the orographically induced zonal flows, which accounts for the long-wave sensitivity to orography. In addition, the structural modulations are shown to be proportional to the square of the induced zonal wind speeds, which allows the orography to dramatically influence the flow without entering strongly into the energetics. The modulations of the phase speed can easily mitigate that due to the baroclinic wave itself at the long waves, leading to orographic instability.