THE DEVELOPMENT OF NARROW BAROCLINIC ZONES AND OTHER SMALL-SCALE STRUCTURE IN THE STRATOSPHERE DURING SIMULATED MAJOR WARMINGS

Numerical simulations of stratospheric major warmings with a multi‐level, primitive equation model of the stratosphere and mesosphere reveal several examples of processes giving rise to small‐scale structure in the simulated flow. For a simulation of the warming of 1984/85, the following examples are cited: (1) the maintenance of the Aleutian high by injections of air with low values of potential vorticity; (2) the erosion of the main cyclonic vortex as it is stripped of air with high values of potential vorticity; (3) the merger of a pair of anticyclones at the height of the warming; (4) differential advection of potential vorticity in a vertically sheared flow; and (5) the generation in the stratosphere of narrow baroclinic zones with strong temperature gradients in the horizontal and in the vertical. For a simulation of the warming of 1979, the example given is of the in situ generation in the stratosphere of inertio‐gravity waves near a developing baroclinic zone. The dynamics involved in the formation of narrow baroclinic zones in the simulated stratosphere is studied in some detail. It is shown that the ageostrophic flow (mainly the vertical motion) provides a positive feedback on the frontogenetic effect of the geostrophic (large‐scale) flow. Similarities and differences with frontogenesis at the ground and in the upper troposphere are enumerated. Copyright © 1990 Royal Meteorological Society