In the framework of the linearized inviscid adiabatic Eady model, the optimal perturbations are calculated for the L2 norm. It is shown that the superposition of the interior PV (i.e. the Orr mechanism) is the main mechanism providing most of the amplification of the optimal perturbations for small optimization times on both sides of the Eady model stability cutoff. On the longwave side of the cutoff, as optimization time increases, the SV approaches the adjoint of the unstable non-singular growing mode, which implies that the initial structure of disturbance tends to include increasingly larger components of the unstable growing mode. Initially, the large component of the growing mode is masked by the singular modes making up the continuous spectrum. As time evolves, the exponential growth associated with the unstable mode becomes the dominant mechanism for the amplification. On the shortwave side, there is no unstable mode. The results show that as optimization time increases, larger components of the two non-singular normal modes, which are the edge waves, are included in SV's. Initially, the large components of the non-singular modes are masked by the continuous modes. As the interior PV is tilted downshear, the boundary thermal perturbation associated with the continuous modes becomes in phase with that of the edge waves. Therefore, the superposition of boundary thermal perturbations is the dominant mechanism for the growth of disturbances.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics