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 Africa that behaves as a convergence zone

Check whether IASI N2O measurements do show this modelled African maximum in the troposphere

4

MAM season

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.

OLR

ODIN N2O

MOCAGE N2O

Overshooting Probability Function (Liu and Zipfser, JGR, 2005)

OPF

400 K

400 K

5

Tropospheric distribution of N2O from MOCAGE @ 340 K

DJF

MAM

JJA

SON

Ricaud et al., ACPD, to be submitted, 2008

6

Vertical distribution of N2O from MOCAGE (0-20 km)

DJF

MAM

JJA

SON

Africa appears to be a Convergence Zone all over the year

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

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)

N2O2008=1.25% x N2O2002-2004

13

MOCAGE N2O x 1.25%MAM 2002-2004

IASI N2OMAM 2008

N2OEmissions

14

OLR

N2O

N2OEmissions

South America

Africa Indonesia WesternPacific

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.