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| Titel |
Multistation intercomparison of column-averaged methane from NDACC and TCCON: impact of dynamical variability |
| VerfasserIn |
A. Ostler, R. Sussmann, M. Rettinger, N. M. Deutscher, S. Dohe, F. Hase, N. Jones, M. Palm, B.-M. Sinnhuber |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 7, no. 12 ; Nr. 7, no. 12 (2014-12-03), S.4081-4101 |
| Datensatznummer |
250115972
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| Publikation (Nr.) |
 copernicus.org/amt-7-4081-2014.pdf |
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| Zusammenfassung |
Dry-air column-averaged mole fractions of methane (XCH4) retrieved from
ground-based solar Fourier transform infrared (FTIR) measurements provide
valuable information for satellite validation, evaluation of
chemical-transport models, and source-sink-inversions. In this context,
Sussmann et al. (2013) have shown that midinfrared (MIR) soundings from the
Network for the Detection of Atmospheric Composition Change (NDACC) can be
combined with near-infrared (NIR) soundings from the Total Carbon Column
Observing Network (TCCON) without the need to apply an overall
intercalibration factor. However, in spite of efforts to reduce a priori
impact, some residual seasonal biases were identified, and the reasons
behind remained unclear. In extension to this previous work, which was based
on multiannual quasi-coincident MIR and NIR measurements from the stations
Garmisch (47.48° N, 11.06° E, 743 m a.s.l.) and
Wollongong (34.41° S, 150.88° E, 30 m a.s.l.), we now
investigate upgraded retrievals with longer temporal coverage and include
three additional stations (Ny-Ålesund, 78.92° N,
11.93° E, 20 m a.s.l.; Karlsruhe, 49.08° N,
8.43° E, 110 m a.s.l.; Izaña, 28.31° N,
16.45° W, 2.370 m a.s.l.). Our intercomparison results (except
for Ny-Ålesund) confirm that there is no overall bias between MIR and
NIR XCH4 retrievals, and all MIR and NIR time series reveal a
quasi-periodic seasonal bias for all stations, except for Izaña.
We find that dynamical variability causes MIR–NIR differences of up to
~ 30 ppb (parts per billion) for Ny-Ålesund, ~ 20 ppb for
Wollongong, ~ 18 ppb for Garmisch, and ~ 12 ppb
for Karlsruhe. The mechanisms behind this variability are elaborated via two
case studies, one dealing with stratospheric subsidence induced by the polar
vortex at Ny-Ålesund and the other with a deep stratospheric intrusion
event at Garmisch. Smoothing effects caused by the dynamical variability
during these events are different for MIR and NIR retrievals depending on
the altitude of the perturbation area. MIR retrievals appear to be more
realistic in the case of stratospheric subsidence, while NIR retrievals are
more accurate in the case of stratosphere–troposphere exchange (STE) in the
upper troposphere/lower stratosphere (UTLS) region. About 35% of the
FTIR measurement days at Garmisch are impacted by STE, and about 23% of
the measurement days at Ny-Ålesund are influenced by polar vortex
subsidence. The exclusion of data affected by these dynamical situations
resulted in improved agreement of MIR and NIR seasonal cycles for
Ny-Ålesund and Garmisch.
We found that dynamical variability is a key factor in constraining the
accuracy of MIR and NIR seasonal cycles. To mitigate this impact it is
necessary to use more realistic a priori profiles that take these dynamical
events into account (e.g., via improved models), and/or to improve the FTIR
retrievals to achieve a more uniform sensitivity at all altitudes (possibly
including profile retrievals for the TCCON data). |
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