A three-dimensional view of the evolution of midlatitude stratospheric intrusions

Three-dimensional views of midlatitude stratospheric intrusions are present ed. The views are obtained by plotting a surface of constant potential vort icity (PV), where the PV is diagnosed from a 6-day run of the U.K. Universi ties Global Atmospheric Modelling Project General Circulation Model. The PV = 1 X 10(-6) K kg(-1) m(2) s(-1) (= 1 PVU) isosurface is chosen as represe ntative of the tropopause. The evolution of this surface is examined during the development of baroclinic waves in the Northern Hemisphere during Octo ber 1990. The developments show a number of features expected during the ev olution of upper-level troughs, such as vortex roll-up, and the generation of tropopause folds, in which air from the stratosphere intrudes downward i nto the troposphere. However, it is shown that the combined effects of defo rmation and convergence lead to the rapid collapse of folded features to le ave 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 filamen t is rapidly removed to other regions (cutoff lows/highs, low-level tubes, or the stratosphere). It is also shown that high pressure regions can posse ss 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 sep arate the two kinds of structure-aircraft flights would be required.