Transport timescales and tracer properties in the extratropical UTLS P. Hoor1, H. Wernli2, M. I. Hegglin3, and H. Bönisch4 1Institute for Atmospheric Physics, University of Mainz, 55128 Mainz, Germany 2Institute for Atmospheric and Climate Sciences, Federal Institute of Technology (ETH), Zürich 3Department of Physics, University of Toronto, Toronto, Canada 4Institute for Atmospheric and Environmental Sciences, Goethe University, Frankfurt/Main, Germany
Abstract. A comprehensive evaluation of seasonal backward trajectories initialized in the
northern hemisphere lowermost stratosphere (LMS) has been performed to
investigate the factors that determine the temporal and spatial structure of
troposphere-to-stratosphere-transport (TST) and it's impact on the LMS.
In particular we explain the fundamental role of the transit time since last
TST (tTST) for the chemical composition of the LMS. According to our
results the structure of the LMS can be characterized by a layer with
tTST<40 days forming a narrow band around the local tropopause.
This layer extends about 30 K above the local dynamical tropopause,
corresponding to the extratropical tropopause transition layer (ExTL)
as identified by CO. The LMS beyond this layer shows a relatively well
defined separation as marked by an aprupt transition to longer tTST
indicating less frequent mixing and a smaller fraction of tropospheric air.
Thus the LMS constitutes a region of two well defined regimes of tropospheric
influence. These can be characterized mainly by different transport times from
the troposphere and different fractions of tropospheric air.
Carbon monoxide (CO) mirrors this structure of tTST due to it's finite lifetime on the
order of three months. Water vapour isopleths, on the other hand, do not uniquely
indicate TST and are independent of tTST, but are determined by the
Lagrangian Cold Point (LCP) of air parcels. Most of the backward trajectories
from the LMS experienced their LCP in the tropics and sub-tropics, and TST
often occurs 20 days after trajectories have encountered their LCP. Therefore,
ExTL properties deduced from CO and H2O provide totally different
informations on transport and particular TST for the LMS.
Citation: Hoor, P., Wernli, H., Hegglin, M. I., and Bönisch, H.: Transport timescales and tracer properties in the extratropical UTLS, Atmos. Chem. Phys., 10, 7929-7944, doi:10.5194/acp-10-7929-2010, 2010.