Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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2009
10.5194/acpd-9-3243-2009
http://www.atmos-chem-phys-discuss.net/9/3243/2009/
http://www.atmos-chem-phys-discuss.net/9/3243/2009/acpd-9-3243-2009.html
http://www.atmos-chem-phys-discuss.net/9/3243/2009/acpd-9-3243-2009.pdf
3243
3264
2009-01-30
Equatorial total column of nitrous oxide as measured by IASI on MetOp-A: implications for transport processes
P. Ricaud
philippe.ricaud@aero.obs-mip.fr
J.-L. Attié
H. Teyssèdre
L. El Amraoui
V.-H. Peuch
M. Matricardi
P. Schlüssel
Université de Toulouse, Laboratoire d'Aérologie, CNRS UMR 5560, Toulouse, France
CNRM-GAME, Météo-France and CNRS URA 1357, Toulouse, France
ECMWF, Shinfield Park, Reading, Berkshire, RG2 9AX, UK
Department of Programme Development, EUMETSAT, Am Kavalleriesand 31, 64295 Darmstadt, Germany
In this paper we use the total columns of nitrous oxide (N<sub>2</sub>O) as
retrieved from the radiance spectra as measured by the Infrared Atmospheric
Sounding Interferometer (IASI) instrument aboard the MetOp-A platform and
distributed by the European Organisation for the Exploitation of
Meteorological Satellites (EUMETSAT) during the March–May (MAM) 2008
period. Since the total column of N<sub>2</sub>O reflects concentrations in the
middle troposphere, cloud-free columnar N<sub>2</sub>O measurements are used to
assess transport processes in the equatorial band
(10° S–10° N). We compare the measured data set
with the outputs produced by the 3-D chemical-transport model MOCAGE during
the period MAM 2002–2004. To reflect MAM 2008 concentrations, MOCAGE results
have been scaled by a factor 1.25% in order to represent the change in
concentration of N<sub>2</sub>O since 2004. IASI N<sub>2</sub>O equatorial
measurements show a maximum over Africa (4.96×10<sup>−3</sup> kg m<sup>−2</sup>)
and a minimum over South America (4.86×10<sup>−3</sup> kg m<sup>−2</sup>) in
very good agreement with the outputs from MOCAGE despite the fact that
emissions of N<sub>2</sub>O are more intense over America than over Africa. The
amplitude of the longitudinal variation of total column N<sub>2</sub>O along the
equatorial band is twice as intense in the measurements (~1.6%) than
as in the model calculations (~0.8%), and much greater than the IASI
mean random error (0.16–0.33%). A difference between the two data sets is
observed above the Western Pacific
(110° E–150° E) with a marked minimum in IASI
compared to MOCAGE. Recent theoretical studies (Ricaud et al., 2007, 2008)
have shown the potentially important effect of the Walker and the Hadley cells
on the tropospheric distribution of N<sub>2</sub>O in producing a local maximum
in N<sub>2</sub>O above Africa. Based on equatorial total columns of N<sub>2</sub>O
measured by IASI, we provide measurement evidence that Africa is a zone of
convergence of airmasses coming from different convective regions whilst
Western Pacific behaves more like a divergence zone.
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