Instability of finite-amplitude lower-neutral Eady waves

The problem of the stability of the two-dimensional neutral Eady wave, studied numerically by Fantini and Davolio (2001) as a possible mechanism leading to strong orographic cyclogenesis, is re-examined. An approximate analytic solution is found, and a simple condition for the appearance of meridionally structured unstable secondary modes is proposed, based on the relative vorticity of the primary wave. The approximate analytic solution compares well with the numerical simulations and extends previous results.