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

Abstract

Baroclinic development is studied with 2 linear, quasi-geostrophic models. One model is theEady model, the other uses more realistic wind, density, Coriolis, and static stability. Initialvaluesolutions are diagnosed using time series of potential enstrophy (H), total energy (E), thecomponents of H and E, and the amplitude norm. Two vertical structures for the initial conditionare used for both models. One initial condition is representative of a class of initial conditionsstudied previously having enhanced nonmodal growth (NG). The other initial conditionapproximates observed conditions prior to cyclogenesis. Results are shown for the most unstablenormal mode wavelength of each model. The growth rates of the components of H and E evolvequite differently for different initial states and models tested. NG in H is shown to be sensitiveto the contribution of the boundary potential vorticity (BPV) of the initial state; small adjustmentsin eddy structure at the boundary significantly alter BPV and H growth rates. Theamount of NG is related to how far BPV present initially differs from the asymptotic normalmode. The effect upon NG of each approximation present in the Eady model (but not in theother model) are considered. Using realistic mean flow shear, static stability, or compressibilitycan significantly reduce NG but including linearly varying Coriolis parameter did not. Twoconceptual models of NG are considered. Growth by increasingly favorable superpositionremains relevant. Growth by “tilting into the vertical” is shown to be incorrect.