Diagnosing cyclogenesis by partitioning energy and potential enstrophy in a linear quasi‐geostrophic model

Baroclinic development is studied with 2 linear, quasi‐geostrophic models. One model is the Eady model, the other uses more realistic wind, density, Coriolis, and static stability. Initial‐value solutions are diagnosed using time series of potential enstrophy ( H ), total energy ( E ), the components of H and E , and the amplitude norm. Two vertical structures for the initial condition are used for both models. One initial condition is representative of a class of initial conditions studied previously having enhanced nonmodal growth (NG). The other initial condition approximates observed conditions prior to cyclogenesis. Results are shown for the most unstable normal mode wavelength of each model. The growth rates of the components of H and E evolve quite differently for different initial states and models tested. NG in H is shown to be sensitive to the contribution of the boundary potential vorticity (BPV) of the initial state; small adjustments in eddy structure at the boundary significantly alter BPV and H growth rates. The amount of NG is related to how far BPV present initially differs from the asymptotic normal mode. The effect upon NG of each approximation present in the Eady model (but not in the other model) are considered. Using realistic mean flow shear, static stability, or compressibility can significantly reduce NG but including linearly varying Coriolis parameter did not. Two conceptual models of NG are considered. Growth by increasingly favorable superposition remains relevant. Growth by "tilting into the vertical" is shown to be incorrect.