The January 2006 low ozone event over the UK

In this paper we present a case study of a record low ozone event observed over the UK in January 2006. We focus on the dynamical processes that cause this event. This is done by examining the observations, meteorological analyses and back trajectories calculated by the NAME III atmospheric dispersion model. We show that this model, hitherto only used for tropospheric pollution studies, can be an important and effective tool for the examination of transport in the upper troposphere/lower stratosphere (UTLS) and mid-stratosphere regions. A record low total ozone column of 177 DU was observed at Reading, UK, on 19 January 2006. Low ozone values were also recorded at other stations in Northwest Europe around this date. Ozonesonde measurements indicate the depletion is occurring in two distinct vertical regions, with around a third of the reduction in total ozone column values originating from the mid-stratosphere and the rest from the UTLS region. Evidence suggests that air inside the stratospheric polar vortex was poor in ozone prior to 19 January and the occurrence of a major stratospheric warming shifted this air over Northwest Europe. In addition we show that moderate ozone depletion, related to the lifting of the tropopause and divergence in the lower stratosphere associated with the presence of an anticyclone, is also a plausible mechanism for the record low ozone column that is observed. In order to confirm that both mid-stratosphere and UTLS transport processes are responsible for the record low ozone values, we perform turbulent back trajectory calculations using the Met Office NAME III model. The results show that air parcels in the mid-stratosphere that arrive over the British Isles on 19 January originate in the polar vortex, and furthermore that air parcels near the tropopause arrive from low latitudes and are transported anticyclonically. Therefore this strongly suggests that the record low ozone values are due to a combination of a raised tropopause with increased divergence in the lower stratosphere and the presence of low ozone stratospheric air aloft.