Atmos. Chem. Phys. Discuss., 10, 12953-12991, 2010
www.atmos-chem-phys-discuss.net/10/12953/2010/
doi:10.5194/acpd-10-12953-2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Transport timescales and tracer properties in the extratropical UTLS
P. Hoor1, H. Wernli2, and M. I. Hegglin3
1Institute for Atmospheric Physics, University of Mainz, 55128 Mainz, Germany
2Institute for Atmospheric and Climate Sciences, Federal Institute of Technology (ETH), Zürich, Switzerland
3Department of Physics, University of Toronto, Toronto, Canada

Abstract. A comprehensive evaluation of seasonal backward trajectories initialized in the Northern Hemisphere lowermost stratosphere (LMS) has been performed to investigate the origin of air parcels and the main mechanisms determining characteristic structures in H2O and CO within the LMS. In particular we explain the fundamental role of the transit time since last tropopause crossing (tTST) for the chemical structure of the LMS as well as the feature of the extra-tropical tropopause transition layer (ExTL) as identified from CO profiles. The distribution of H2O in the background LMS above Θ=320 K and 340 K in northern winter and summer, respectively, is found to be governed mainly by the saturation mixing ratio, which in turn is determined by the Lagrangian Cold Point (LCP) encountered by each trajectory. Most of the backward trajectories from this region in the LMS experienced their LCP in the tropics and sub-tropics. The transit time since crossing the tropopause from the troposphere to the stratosphere (tTST) is independent of the H2O value of the air parcel. TST often occurs 20 days after trajectories have encountered their LCP. CO, on the other hand, depends strongly on tTST due to its finite lifetime. The ExTL as identified from CO measurements is then explained as a layer of air just above the tropopause, which on average encountered TST fairly recently.

Citation: Hoor, P., Wernli, H., and Hegglin, M. I.: Transport timescales and tracer properties in the extratropical UTLS, Atmos. Chem. Phys. Discuss., 10, 12953-12991, doi:10.5194/acpd-10-12953-2010, 2010.
 
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