Three-dimensional views of midlatitude stratospheric intrusions are presented. The views are obtained by plotting a surface of constant potential vorticity (PV), where the PV is diagnosed from a 6-day run of the U.K. Universities Global Atmospheric Modelling Project General Circulation Model. The PV = 1 × 10−6 K kg−1 m2 s−1 (=1 PVU) isosurface is chosen as representative of the tropopause. The evolution of this surface is examined during the development of baroclinic waves in the Northern Hemisphere during October 1990. The developments show a number of features expected during the evolution of upper-level troughs, such as vortex roll-up, and the generation of tropopause folds, in which air from the stratosphere intrudes downward into the troposphere. However, it is shown that the combined effects of deformation and convergence lead to the rapid collapse of folded features to leave low-level tubes of PV, together with higher-level filaments. The result is that the air intruded in the vicinity of the upper-level fold or filament is rapidly removed to other regions (cutoff lows/highs, low-level tubes, or the stratosphere). It is also shown that high pressure regions can possess similar folded structures, which also rapidly collapse to the model grid scale. These effects are examined in more detail using a contour advection technique. There is evidence for the existence of the low-level tubes both in assimilated datasets and in other models. If they are real structures, they should be observable as temperature and humidity anomalies in the same way as folds, but ground-based observations are unlikely to be able to separate the two kinds of structure—aircraft flights would be required.
Corresponding author address: Dr. M. Bithell, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire, OXII OQX United Kingdom.