Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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2007
10.5194/acpd-7-4673-2007
http://www.atmos-chem-phys-discuss.net/7/4673/2007/
http://www.atmos-chem-phys-discuss.net/7/4673/2007/acpd-7-4673-2007.html
http://www.atmos-chem-phys-discuss.net/7/4673/2007/acpd-7-4673-2007.pdf
4673
4703
2007-04-05
Medium-range mid-tropospheric transport of ozone and precursors over Africa: two numerical case-studies in dry and wet seasons
B. Sauvage
F. Gheusi
ghef@aero.obs-mip.fr
V. Thouret
J.-P. Cammas
J. Duron
J. Escobar
C. Mari
P. Mascart
V. Pont
Laboratoire d'Aérologie, Toulouse, France
A meso-scale model was used to understand and describe the processes
driving high ozone concentrations observed during both dry and monsoon seasons in monthly
climatologies profiles over Lagos (Nigeria, 6.6° N, 3.3° E), obtained with
the MOZAIC airborne measurements (ozone and carbon monoxide). This study focuses
on ozone enhancements observed in the upper-part of the lower
troposphere, around 3000 m.
Two individual cases have been selected in the MOZAIC dataset
as being representative of the climatological ozone enhancements, to be simulated
and analyzed with on-line Lagrangian backtracking of airmasses.
<br><br>
This study points out the role of baroclinic low-level circulations present
in the Inter Tropical Front (ITF) area. Two low-level thermal cells around a zonal axis and below 2000 m, in mirror symmetry to eachother with respect to equator,
form near 20° E and around 5° N and 5° S
during the (northern hemisphere) dry and wet seasons respectively. They are caused by surface gradients – the warm dry surface being located poleward
of the ITF and the cooler wet surface equatorward of the ITF.
<br><br>
A convergence line exists between the poleward low-level branch of each thermal cell and the equatorward low-level branch of the Hadley cell. Our main conclusion is to point out this line as a preferred location for fire products – among them ozone precursors – to be uplifted and injected into the lower free troposphere.
<br><br>
The free tropospheric transport that occurs then depends on the hemisphere and season. In the NH dry season,
the African Easterly Jet allows transport of ozone and precursors westward to Lagos. In the NH monsoon (wet) season, fire products
are transported from the southern hemisphere to Lagos by the southeasterly trade that surmounts the monsoon layer.
Additionally ozone precursors uplifted by wet convection in the Inter Tropical Convergence Zone can also mix to the ones uplifted by the baroclinic
cell and be advected up to Lagos by the trade flow.
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