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| Titel |
The effects of aerosol and cirrus clouds on the retrieval of atmospheric methane (CH4) from SCIAMACHY |
| VerfasserIn |
Diane Knappett, Hartmut Boesch, Austin Cogan, Paul Monks |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039369
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| Zusammenfassung |
| Atmospheric methane (CH4) plays a significant role in global warming despite being present
in the atmosphere in smaller quantities than carbon dioxide (CO2) as it has a radiative forcing
efficiency or ‘global warming potential’ of 21 times greater than that of CO2. The annual
global source strength of CH4 is fairly well constrained to 550 (±50) Tg (Frankenberg et al.,
2006) from the study of tropospheric OH, the dominant sink for atmospheric CH4, however
the temporal and spatial variability of individual sources and sinks is currently less well
quantified. Satellites can provide comprehensive, global datasets of CH4 which can lead to
large improvements in our understanding of the global atmospheric CH4 distribution and how
it is likely to evolve in the future. However, satellite retrievals of CH4 are often hindered
by the presence of atmospheric aerosols and/or cirrus clouds which can lead to
biases in the resulting trace gas total column if not properly accounted for. This
necessitates a new method for the inclusion of a priori aerosol and cirrus cloud data in
retrievals.
For retrievals of CH4 we utilise satellite data from the SCanning Imaging Absorption
spectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument which is a
nadir/limb viewing spectrometer of moderate resolution that observes in the UV, visible and
NIR. SCIAMACHY detects sunlight reflected from the Earth’s surface and therefore has
high sensitivity to the lowest atmospheric layers where anthropogenic trace gas
emissions peak. The FSI WFM-DOAS retrieval algorithm, previously applied to
CO2 data from SCIAMACHY (Barkley et al., 2006) has been adapted to perform
retrievals of CH4. Retrieved CH4 vertical column densities (VCD) are converted to
volume mixing ratio (VMR) by taking the ratio of the retrieved CH4 VCD with
the corresponding CO2 VCD (also retrieved using FSI WFM-DOAS) and then
multiplying by a mean model value of the CO2 VMR; using this method the effects of
spectral features due to aerosol or cloud contamination of the observed scene mostly
cancel out. However, since atmospheric extinction is wavelength dependent and we
retrieve CO2 and CH4 from separate spectral windows (centred on 1575nm and
1650nm respectively) some discrepancies remain, particularly at high optical depths
(Ï > 0.6).
Here we present the results of a series of sensitivity tests that have been carried out to
characterize the FSI algorithm for various aerosol, albedo and solar zenith angle scenarios.
Furthermore, we have investigated the spatial distribution of biases introduced when either an
inadequate representation of aerosol is used within retrievals or when aerosol is neglected
from retrievals altogether. We therefore propose a modified version of FSI WFM-DOAS
which takes aerosol extinction into consideration by incorporating the Global and
regional Earth-system Monitoring using Satellite and in-situ data (GEMS) aerosol
product from the European Centre for Medium-Range Weather Forecasts (ECMWF)
into the retrieval initialisation. The presence of cirrus clouds will also result in
detrimental spectral effects and requires further investigation to correct for within
the retrieval, however this will be taken into account in a future version of FSI
WFM-DOAS. |
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