Structure and evolution of upper-tropospheric jet streaks in a stratified quasigeostrophic model

Observational evidence indicates that jet streaks in the upper-troposphere frequently result from the superposition of coherent monopolar and dipolar vortices of mesoscale dimensions with the enhanced potential vorticity gradients that coincide with the polar-front jet stream. Previous investigation by the authors into the possibility of interpreting jet streaks in terms of coherent vortices shows that the analytical solution due to Berestov of a vortex dipole applicable to the stratified quasigeostrophic (QG) system provides a realistic dynamical representation of an isolated, straight jet streak. Jet streaks in the atmosphere are rarely isolated or straight, however, and typically are embedded in a larger-scale flow with horizontal and vertical shear. Furthermore, observations suggest that the coherent vortices associated with jet streaks are primarily monopolar in nature. The present study represents an attempt to extend the previous investigation to account for these observed attributes of jet streaks.

Numerical simulations of a continuously stratified QG model will be presented that describe the interactions between monopolar vortices and zonal jets with nonuniform horizontal and vertical shear (i.e., the Hoskins-West jet; a baroclinic Bickley jet) on an f-plane. Although these interactions display complex evolutions, it is demonstrated that the simulations provide idealized analogs to the behaviour of observed jet streaks. In particular, the jet streaks in these simulations exhibit: (i) an elongated entrance region and a compact exit region; (ii) a two-cell pattern of vertical velocity with maximum ascent beneath the cyclonic-shear side of the exit region; (iii) an ageostrophic wind directed towards lower geopotential height in the entrance region and towards higher geopotential height in the exit region that is dominated by its rotational component; and (iv) a developing surface cyclone beneath the left exit region. In addition, it is shown that the vortices are capable of maintaining vertical coherence even in the presence of significant vertical shear, a consequence of the so-called alignment process. It is of interest that the combined effects of vertical shear and alignment result in a tilted vertical structure in the simulations that is in accord with observations of jet streaks in the atmosphere. Finally, the limitations of this investigation will be discussed, along with potential extensions to include the role and importance of unbalanced flow in jet-streak dynamics.