Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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9
1
2009
10.5194/acpd-9-4899-2009
http://www.atmos-chem-phys-discuss.net/9/4899/2009/
http://www.atmos-chem-phys-discuss.net/9/4899/2009/acpd-9-4899-2009.html
http://www.atmos-chem-phys-discuss.net/9/4899/2009/acpd-9-4899-2009.pdf
4899
4930
2009-02-24
Equatorial transport as diagnosed from nitrous oxide variability
P. Ricaud
philippe.ricaud@aero.obs-mip.fr
J.-P. Pommereau
J.-L. Attié
E. Le Flochmoën
L. El Amraoui
H. Teyssèdre
V.-H. Peuch
W. Feng
M. P. Chipperfield
Université de Toulouse, Laboratoire d'Aérologie, CNRS UMR 5560, Toulouse, France
Service d'Aéronomie, CNRS, Verrières-Le-Buisson, France
CNRM, Météo-France, Toulouse, France
School of Earth and Environment, University of Leeds, Leeds, UK
The mechanisms of transport on annual, semi-annual and
quasi-biennial time scales in the equatorial
(10° S–10° N) stratosphere are
investigated using the nitrous oxide (N<sub>2</sub>O) measurements
of the space-borne ODIN Sub-Millimetre Radiometer instrument
from November 2001 to June 2005, and the simulations of the
three-dimensional Chemistry Transport Models MOCAGE and
SLIMCAT. Both models are forced with analyses from the
European Centre for Medium-range Weather Forecasts, but the
vertical transport is derived either from the forcing analyses
by solving the continuity equation (MOCAGE), or from diabatic
heating rates using a radiation scheme (SLIMCAT). The N<sub>2</sub>O
variations in the mid-to-upper stratosphere at levels above
32 hPa are shown to be generally captured by the
models though significant differences appear with the
observations as well as between the models, attributed to the
difficulty of capturing correctly the slow vertical velocities
of the Brewer-Dobson circulation. In the lower stratosphere
(LS), below 32 hPa, the variations are shown to be
principally seasonal with peak amplitude at 400 K
(~19 km), and are totally missed by the
models. The decrease in diabatic radiative heating in the LS
during the Northern Hemisphere summer is found to be out of
phase by one month and far too small to explain the observed
N<sub>2</sub>O seasonal cycle. The proposed explanation for this
annual variation is a combination of i) the annual cycle of
tropopause height of 1 km amplitude, ii) the
convective overshooting above 400 K peaking in May and
absent in the models, and iii) an annual cycle of
15 ppbv amplitude of the N<sub>2</sub>O concentration at the
tropopause, but for which no confirmation exists in the upper
troposphere in the absence of global-scale measurements. The
present study indicates i) a significant contribution of deep
convective overshooting on the chemical composition of the LS
at global scale up to 500 K, ii) a preferred region
for that over the African continent, and iii) a maximum impact
in May when the overshoot intensity is the largest and
horizontal winds are the slowest.
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