Nonlinear Wave Packet Evolution on a Baroclinicaily Unstable Jet

Abstract

Numerical simulations of the inviscid, nonlinear life cycle of wave packets emerging from a zonally localized initial disturbance on a baroclinically unstable jet are studied. The study reveals the following. 1) Linear wave packet theory accurately bounds the upstream and downstream development of the synoptic disturbance throughout the nonlinear evolution. 2) The short, shallow, slowly propagating waves at the upstream fringe of the wave packet equilibrate at low amplitudes and never come to dominate the wave packet evolution. 3) Upslxeampropagating Rossby waves are excited in the vicinity of the synoptic wave packet, which cause the generation of strong barotropic jets mostly upstream of the dominant body of the wave packet. This prevents the barotropicgovernor mechanism from limiting the growth Of the disturbances in the main body of the packet; the main equilibration mechanism is a simple homogenization of the lower-layer potential vorticity in the vicinity of the wave packet. 4) The waves that reach the largest amplitude are the weakly growing waves at the leading edge of the wave packet. These have group speeds comparable to the upper-level jet maximum and wavelengthscomparable to that of the most unstable normal mode, but are slowly growing. The growing leading-edge disturbances leave behind a zonally spreading finite-amplitude disturbance with nearly zero phase speed; there is no evidence of a barotropic decay stage.