Tropopause Folds and Synoptic-Scale Baroclinic Wave Life Cycles

The linear stability of three midlatitude zonal mean states of varying baroclinicity and barotropy is examined using a primitive equation stability analysis under the anelastic approximation. Each of these basic states includes both tropospheric and stratospheric regions with realistic properties compared with those of the earth's atmosphere in midlatitudes. In all three cases, the fastest growing synoptic-scale modes delivered by the linear stability analysis have wavelengths of approximately 4000 km. The nonlinear evolution of these synoptic-scale modes is explored using a three-dimensional anelastic finite-difference model whose zonal scale is chosen to be equal to the wavelength of the fastest growing mode of linear theory. During the nonlinear evolution of the wave, deep tropopause folds are shown to form (generically) and the depth to which the fold penetrates the troposphere is seen to increase with the baroclinicity of the mean state. These folds, descending along the sloping frontal zones within the parent wave, invariably have their maximum depth of tropospheric penetration located at the southernmost edge of the upper-level wave. This paper focuses on three important issues: 1) the impact of the background baroclinicity on these generic tropopause deformations, 2) the stage in the life cycle of the parent wave at which the deformation reaches its greatest vertical extent, and 3) the physical processes that limit this vertical extent.