A number of studies over the last decade have changed the standard view of the circulation of the extratropical winter stratosphere. McIntyre and Palmer (1984) examined the potential vorticity (PV) computed from analyzed fields based on satellite radiometer temperature measurements. They noted that the pole-to-equator gradient in PV tended to be concentrated in a narrow region around the edge of the polar vortex, which separated the high PV values inside the vortex from lower values in the subtropical "surf zone". In addition they found evidence for tongues of high PV air extending from the vortex into the surf zone. Leovy et al. (1985) found similar behaviour in satellite measurements of a quasi- conserved chemical tracer. Acknowledging the limited spatial resolution of the satellite data, McIntyre and Palmer speculated that in the real world (or in a high resolution numerical model simulation) the gradients at the boundary of the vortex and along the edge of material entrained into the surf zone may be very sharp. These speculations were strikingly confirmed by very high resolution barotropic model calculations which showed the formation of strong gradients in PV, and the presence of thin filaments of high PV air within the low PV background of the surf zone (Juckes and McIntyre, 1987). An important development was the demonstration that high-resolution 3D general circulation models (GCMs) can produce features in the stratospheric winter vortex similar to those seen in the idealized barotropic simulations.
One issue that has not been thoroughly investigated is the vertical structure of the filaments of vortex air that are entrained into the surf zone. This is an important aspect since much of the recent modelling work on the polar vortex dynamics has been conducted within the context of purely barotropic models. The recent study of Ward and Haynes (1993) emphasizes the importance of understanding the 3D structure of air mass motions in the stratosphere. They showed that the rate of dissipation of PV anomalies by the radiative diabatic heating depends on the vertical scale of the anomaly (in particular, shallow elements of PV can be expected to be dissipated rapidly). As a first step in an examination of this issue a technique for 3D visualization of polar vortex air in a model simulation is presented here.