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M. Bithell, L. J. Gray, and B. D. Cox

Abstract

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.

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M. Bithell, L. J. Gray, J. E. Harries, J. M. Russell III, and A. F. Tuck

Abstract

The degree to which the Southern Hemisphere polar vortex is isolated against horizontal (isentropic) mixing is investigated using data from the Halogen Occulation Experiment (HALOE), U.K. Meteorological Office (UKMO) potential vorticity (PV), and contour advection diagnostics. Measurements of methane and water vapor taken by HALOE during a disturbed period in the Southern Hemisphere springtime (21 September–15 October 1992) are interpreted in light of the prevailing synoptic meteorology. Daily fields of winds and PV are shown to be essential in the interpretation of the data. A climatological high pressure region is responsible for a distorted vortex, and a substantial “vortex stripping” event is present, associated with the early stages of vortex breakdown. This leads to significant temporal, zonal, and altitudinal variations in the distribution of tracers. The authors point out the difficulties this presents for the interpretation of solar occultation data, especially with regard to the use of zonal average time series. Longitude-height methane distributions from two days during the period are examined. Both days show substantial variations in abundance around a latitude circle. In particular, the authors investigate HALOE measurements at 77°S on 15 October 1992, which indicate an abundance of methane in the height region 600–2000 K (∼30-l mb) that is more typical of midlatitude air. Similar distributions, observed in the 1991 HALOE data, have previously been interpreted as evidence for the penetration of midlatitude air into the vortex. Gradients of potential vorticity and contour advection diagnostics are employed to examine whether the UKMO winds are consistent with this hypothesis in 1992. Although midlatitude air is able to penetrate poleward of the main jet core by advection processes alone, an essentially intact inner core of vortex air remains, which does not mix to any great extent with air from lower latitudes. The authors show that the high-latitude HALOE abundances that are typical of midlatitude air were observed in a region of extensive filamentation and mixing, rather than within the inner, more isolated, core.

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