Equatorial Total Column of N2O as Measured by IASI : Implications to Transport Processes

1. Equatorial Total Column of N2O as Measured by IASI : Implications to Transport Processes P. Ricaud(1), J.-L. Attié(1), B. Barret(1), L. El Amraoui(1,2), H. Teyssèdre(2), V.-H. Peuch(2) (1) Laboratoire d’Aérologie, CNRS/Université Toulouse, Toulouse, France (2) CNRM, Météo-France, Toulouse, France

2. Contents • Scientific Issues • IASI Total column of N2O • MOCAGE CTM • Comparisons MOCAGE vs. IASI • Conclusions

3. Scientific Issues • Time evolution of long-lived and tropospheric-origin nitrous oxide (N2O) in the equatorial UTLS and stratosphere was examined by combining satellite measurements and 3D CTM results (Ricaud et al., ACP, 2007 and ACPD, to be submitted, 2008). • Particular attention was given in the Equatorial UTLS over different regions (Western Pacific vs. Africa) where Troposphere-to-Stratosphere Transport was found to be more intense over Africa during the March-May 2002-2004 season • Model results show, whatever the season considered, a maximum of N2O in the troposphere over Africa • Potentially important effect of the Walker and the Hadley cells on the tropospheric distribution of N2O producing a local maximum in N2O above Africathat behaves as a convergence zone • Check whether IASI N2O measurements do show this modelled African maximum in the troposphere

4. MAM season OPF Overshooting Probability Function (Liu and Zipfser, JGR, 2005) OLR • At 400 K, all measured gases (N2O, CH4 and CO) show significant longitudinal variations, not captured by the model (Ricaud et al., ACP, 2007). • The maximum amounts are primarily located over Africa in MAM 2002-2004. • The suggestion is of strong overshooting over land convective regions, particularly Africa, very consistent with the TRMM maximum overshooting features over the same region during the same season. ODIN N2O 400 K 400 K MOCAGE N2O

5. Tropospheric distribution of N2O from MOCAGE @ 340 K DJF MAM JJA SON Ricaud et al., ACPD, to be submitted, 2008

6. DJF Vertical distribution of N2O from MOCAGE (0-20 km) MAM • Africa appears to be a Convergence Zone all over the year JJA SON Ricaud et al., ACPD, to be submitted, 2008

7. MetOp • The MetOp platform has been launched on 19 October 2006. Expected lifetime is 5 years. • It carries • Advanced Microwave Sounding Unit (AMSU) • Microwave Humidity Sounder (MHS) • High Resolution Infrared Radiation Sounder (HIRS) • Advanced Very High Resolution Radiometer (AVHRR) • Infrared Atmospheric Sounding Interferometer (IASI) • … • It has a Sun-synchronous orbit crossing the Equator at 09:30 mean local solar time in descending node with an inclination of 98.7° to the Equator at a mean altitude of ~815 km.

8. IASI • IR remote sensing using an accurately calibrated Fourier Transform Spectrometer operating in the 3.6-15.5 µm (645-2760 cm-1) spectral range with a spectral resolution 0.25 cm-1. • The signature of several atmospheric constituents can be measured with a good signal-to-noise ratio: H2O, CO2, N2O, CH4, O3 and CO. • IR N2O spectral signatures are basically covering two bands: 1200-1350 cm-1 and 2150-2250 cm-1, although the first band is contaminated by CH4 lines • Official level 2 data (geophysical data) provided by EUMETSAT that produces near real time total amounts of N2O • From a selection of a subset of IASI channels, an iterative process minimizes a cost function (a priori information, calculated brightness temperatures and the Jacobians) with the measured radiances considering several parameters (e.g. fractional cloud cover) retrieved from the other instruments aboard the platform (AMSU, AVHRR). • Averaged data over the March-May 2008 period into 2°x2° bins only considered cloud-free data (CLOUD_FORMATION flag sets to 0).

9. N2O spectral lines Clerbaux et al., ACP, 2003.

10. N2O Jacobians • 2150-2250 cm-1 (centred at 2223.76 cm-1 (3))

11. MOCAGE CTM • Météo-France, Toulouse, France • 2000-2005 • 5.6°x5.6°; 60 layers from the surface to about 0.07 hPa • Detailed tropospheric and stratospheric chemistry • Vertical velocities calculated from the ECMWF forcing analyses • N2O surface emissions from the Global Emissions Inventory Activity and are climatologies representative of the year 1990 (Bouwman et al., 1995). • N2O emissions include anthropogenic and biogenic sources, for a total emission rate of 14.7 Tg(N) yr-1. • No monthly variability and trends in N2O emission but diurnal variations • Focus on March-May 2002-2004 period • The total column of N2O has been calculated considering a surface standard pressure of 1013.25 hPa.

12. N2O trends N2O2008=1.25% x N2O2002-2004 • Surface data • 20°S-20°N • 6 GAW/WDCGG stations • ~0.75 ppbv/yr • (0.24%/yr) • WMO: 0.74±0.02 ppbv/yr • MOCAGE: 10°S-10°N (SLIMCAT)

13. IASI N2O MAM 2008 MOCAGE N2O x 1.25% MAM 2002-2004 N2O Emissions

14. N2O South America Africa Indonesia Western Pacific OLR N2O Emissions

15. Conclusions • Recent theoretical studies (Ricaud et al., 2007 and 2008) have shown the potentially important effect of the Walker and the Hadley cells on the tropospheric distribution of N2O in the equatorial band (10°S-10°N), by producing a local maximum above the African continent. • In the present study, we are making use of the total columns of N2O as measured by the IASI instrument aboard the MetOp platform and delivered by EUMETSAT during the March-May 2008 period. • We compared the measured data set with the outputs from the chemical-transport model MOCAGE during the period MAM 2002-2004 rescaled by a factor 1.25% in order to represent the positive N2O linear trend until MAM 2008. • IASI N2O equatorial measurements do obviously show a maximum over Africa and a minimum over South America in very good agreement with the outputs from MOCAGE whilst emissions of N2O are more intense over America than over Africa. • Our study gives measurement evidence that Africa is a convergence zone (or accumulation zone) to where airmasses coming from different convective regions focus whilst Western Pacific behaves more like a divergence zone.