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The January 2006 low ozone event over the UK

by M. Keil, D. R. Jackson, M. C. Hort
Atmospheric Chemistry and Physics Discussions ()
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In this paper we present a case study of a record low ozone event\nobserved over the UK in January 2006. We focus on the dynamical processes\nthat cause this event. This is done by examining the observations,\nmeteorological analyses and back trajectories calculated by the NAME\nIII atmospheric dispersion model. We show that this model, hitherto\nonly used for tropospheric pollution studies, can be an important\nand effective tool for the examination of transport in the upper\ntroposphere/lower stratosphere (UTLS) and mid-stratosphere regions.\nA record low total ozone column of 177 DU was observed at Reading,\nUK, on 19 January 2006. Low ozone values were also recorded at other\nstations in Northwest Europe around this date. Ozonesonde measurements\nindicate the depletion is occurring in two distinct vertical regions,\nwith around a third of the reduction in total ozone column values\noriginating from the mid-stratosphere and the rest from the UTLS\nregion. Evidence suggests that air inside the stratospheric polar\nvortex was poor in ozone prior to 19 January and the occurrence of\na major stratospheric warming shifted this air over Northwest Europe.\nIn addition we show that moderate ozone depletion, related to the\nlifting of the tropopause and divergence in the lower stratosphere\nassociated with the presence of an anticyclone, is also a plausible\nmechanism for the record low ozone column that is observed. In order\nto confirm that both mid-stratosphere and UTLS transport processes\nare responsible for the record low ozone values, we perform turbulent\nback trajectory calculations using the Met Office NAME III model.\nThe results show that air parcels in the mid-stratosphere that arrive\nover the British Isles on 19 January originate in the polar vortex,\nand furthermore that air parcels near the tropopause arrive from\nlow latitudes and are transported anticyclonically. Therefore this\nstrongly suggests that the record low ozone values are due to a combination\nof a raised tropopause with increased divergence in the lower stratosphere\nand the presence of low ozone stratospheric air aloft.

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