Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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8
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2008
10.5194/acpd-8-10627-2008
http://www.atmos-chem-phys-discuss.net/8/10627/2008/
http://www.atmos-chem-phys-discuss.net/8/10627/2008/acpd-8-10627-2008.html
http://www.atmos-chem-phys-discuss.net/8/10627/2008/acpd-8-10627-2008.pdf
10627
10664
2008-06-04
Mixing processes and exchanges in the tropical and the subtropical UT/LS
R. James
james@lmd.ens.fr
B. Legras
Laboratoire de Météorologie Dynamique UMR 8539, Paris, France
Both in situ measurements and satellite observations
indicate evidence of mixing in the upper troposphere (UT) and the
lower-stratosphere (LS). In this study, the measurements performed
during the Pre-AVE and Costa-Rica AVE campaigns are analysed with
diffusive back-trajectories to assess mixing properties in the
tropical and the subtropical UT/LS. A description of
cross-tropopause pathways and mixing time scales is provided.
<br><br>
In the subtropics, Troposphere-Stratosphere mixing processes are found to
differ in the vicinity of the tropopause and at higher altitudes. Below 350 K,
the mixing line observed during Pre-AVE is shown to result from fast and local
cross-tropopause irreversible exchange, involving two initially distant air masses
with distinct chemical compositions. For measurements located above 350 K, mixing
of the tropospheric air in the subtropical stratosphere occurs over a period of a
month, the origins of the tropospheric source being localised in the tropical UT
and the tropical boundary layer.
<br><br>
In the tropics, quantitative reconstructions of CO and O<sub>3</sub> profiles
above 360 K are obtained for one month back-trajectories calculations, pointing out
that long term mixing is essential to determine the chemical composition in the
tropical ascent. In particular, the existence of two-way meridional irreversible
exchanges between 360 and 450 K is found to export tropical air in the subtropical
stratosphere and to entrain old stratospheric air in the tropical ascent. Calculations
of the Lagrangian mean age of air is shown to be in qualitative agreement with the
CO<sub>2</sub> observations and diabatic calculations.
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