Reexamining the Dynamics of Short-Scale, Diabatic Rossby Waves and Their Role in Midlatitude Moist Cyclogenesis

Abstract

As a step toward a more comprehensive study of the physical processes that underlie explosive cyclogenesis, a two-dimensional, semigeostrophic model with a now standard parameterization of latent heat release is used to diagnose the structure, energetics, and propagation characteristics of short-scale, diabatic normal modes in a moist, baroclinic atmosphere with the Eady basic state. Upon revisiting the inviscid problem, it is found that when a thermodynamically consistent vertical profile of latent heat release is used, the short-wave cutoff vanishes, and growth rates become independent of zonal wavelength for zonal wavelengths shorter than approximately 1900 km. The destabilized short-scale modes, identified previously as diabatic Rossby waves, owe their existence to the continuous generation of potential vorticity by moist processes associated with warm air advection, rising motion, and latent heat release. To determine if these short-scale, diabatic Rossby wave modes continue to grow at small but finite amplitudes in the presence of frictional damping, a standard Ekman boundary layer with quadratic surface drag is employed. It is found that exponential growth is robust over a wide range of zonal wavelengths during the incipient phase of moist cyclogenesis and that, once generated, the short-scale, diabatic modes persist even in the presence of ?realistic? surface friction. Finally, observations have shown that a number of explosive cyclones exhibit two stages of development. The first stage, prior to the period of most rapid deepening, involves the spinup of surface vorticity that can occur independent of upper-level forcing. The results of this study, in conjunction with recent observational work, provide compelling evidence that a diabatic Rossby wave may possibly serve as a precursor, low-level cyclonic disturbance. It is advocated here that an understanding of these small-scale, ?moist? coherent structures is vital to describe the life cycle of extratropical cyclones in moist, baroclinic environments.