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| Titel |
The recent increase of methane as seen by IASI onboard MetOp-A in the tropical band |
| VerfasserIn |
Cyril Crevoisier, Delphine Nobileau, Raymond Armante, Laurent Crépeau, Thibaud Thonat, Jérôme Pernin, Noëlle A. Scott, Alain Chédin |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055970
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| Zusammenfassung |
| Since July 2007, monthly averages of mid-tropospheric methane are retrieved in the tropics
over land and sea, by day and night, in clear-sky conditions from the Infrared Atmospheric
Sounding Interferometer (IASI) flying onboard MetOp-A, yielding a complete view of the
geographical distribution, seasonality and long-term tendency of methane in the
mid-troposphere.
With its high spectral resolution, IASI provides nine channels in the 7.7 μm band highly
sensitive to CH4 with reduced sensitivities to other atmospheric variables. However, infrared
radiances being sensitive to both temperature and methane, simultaneous observations from
the Advanced Microwave Sounding Unit (AMSU) also flying onboard MetOp-A and
sensitive to temperature only are used to help decorrelating both signals through a non-linear
inference scheme based on neural networks. A key point of this approach is that no use
is made of prior information in terms of gas seasonality, trend, or geographical
patterns. The precision of the IASI retrieval is estimated to be about 20 pbbv (1 month,
5°Ã5°).
Retrieved methane displays a clear seasonal cycle of  25 ppbv in the northern tropics,
with a maximum in November and a minimum in April-May, a more complex cycle of  15
ppbv in the southern tropics, and a South-to-North gradient of  30 ppbv. The retrievals are in
good agreement with simulations from atmospheric transport models, which help analysing
the signatures of surface emissions transported to the mid-troposphere, and to aircraft
measurements.
In 2007 and 2008, IASI shows an increase of mid-tropospheric methane of 9.5±2.8 and
6.3±1.7 ppbv yr-1 respectively, in good agreement with the rate of increase measured at the
surface after almost a decade of near-zero growth. IASI also indicates a slowing down of this
increase in the following years, with methane reaching a near-zero increase in 2010. This
result might indicate a decrease in methane surface emissions in the tropics (predominantly
wetlands and biomass burning).
With the launch of two other successive IASI-like instruments scheduled for 2012 and
2016, more than 20 years of observations of mid-tropospheric methane will be available for
climate studies. |
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