Frontal Cyclogenesis

A dry baroclinic spectral model is used to study the initation of frontal cyclones in a baroclinic life cycle. The mature cyclone exhibits a frontal cyclone development associated with a finite amplitude interaction between an upper tropospheric potential vorticity (PV) cut off and a surface cold front. In comparison, a linear stability study of the front identifies only weakly growing modes at the 1000–2000 km wavelength, although with a latent heat release parameterization included in ascending regions, the baroclinic modes are considerably destabilized. It is found that the cold front in the growing baroclinic wave is in fact stabilized by the deformation present in the frontal region, an aspect neglected in studies of small amplitude perturbations to steady basic states. However, it is proposed that small scale waves often observed on the cold front baroclinicity near the warm front and seen in the life cycle experiments are the result of linear upstream development initiated by the wrap-up of low-level isotherms. The main source of upper tropospheric PV anomalies in the atmosphere is provided by the large scale baroclinic waves which advect stratospheric air down sloping isentropes toward the surface and equator. The meridional shear present in the basic state strongly determines the nonlinear evolution of the trough, the PV advection in it, and thus the frontal cyclone developments.