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| Titel |
Metrology of ground-based satellite validation: co-location mismatch and smoothing issues of total ozone comparisons |
| VerfasserIn |
T. Verhoelst, J. Granville, F. Hendrick, U. Köhler, C. Lerot, J.-P. Pommereau, A. Redondas, M. Van Roozendael, J.-C. Lambert |
| 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 ; 8, no. 12 ; Nr. 8, no. 12 (2015-12-02), S.5039-5062 |
| Datensatznummer |
250116713
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| Publikation (Nr.) |
 copernicus.org/amt-8-5039-2015.pdf |
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| Zusammenfassung |
| Comparisons with ground-based correlative measurements constitute
a key component in the validation of satellite data on atmospheric
composition. The error budget of these comparisons contains not
only the measurement errors but also several terms related to
differences in sampling and smoothing of the inhomogeneous and
variable atmospheric field. A versatile system for Observing System
Simulation Experiments (OSSEs), named OSSSMOSE, is used here to
quantify these terms. Based on the application of pragmatic
observation operators onto high-resolution atmospheric fields, it
allows a simulation of each individual measurement, and consequently,
also of the differences to be expected from spatial and temporal
field variations between both measurements making up a comparison
pair. As a topical case study, the system is used to evaluate the
error budget of total ozone column (TOC) comparisons between
GOME-type direct fitting (GODFITv3) satellite retrievals from
GOME/ERS2, SCIAMACHY/Envisat, and GOME-2/MetOp-A, and ground-based
direct-sun and zenith–sky reference measurements such as those from
Dobsons, Brewers, and zenith-scattered light (ZSL-)DOAS instruments,
respectively. In particular, the focus is placed on the GODFITv3
reprocessed GOME-2A data record vs. the ground-based instruments
contributing to the Network for the Detection of Atmospheric
Composition Change (NDACC). The simulations are found to reproduce
the actual measurements almost to within the measurement
uncertainties, confirming that the OSSE approach and its technical
implementation are appropriate. This work reveals that many
features of the comparison spread and median difference can be
understood as due to metrological differences, even when using
strict co-location criteria. In particular, sampling difference
errors exceed measurement uncertainties regularly at most mid- and
high-latitude stations, with values up to 10 % and more in
extreme cases. Smoothing difference errors only play a role in the
comparisons with ZSL-DOAS instruments at high latitudes, especially
in the presence of a polar vortex due to the strong TOC gradient it
induces. At tropical latitudes, where TOC variability is lower, both
types of errors remain below about 1 % and consequently do not
contribute significantly to the comparison error budget. The
detailed analysis of the comparison results, including the
metrological errors, suggests that the published random measurement
uncertainties for GODFITv3 reprocessed satellite data are
potentially overestimated, and adjustments are proposed here. This
successful application of the OSSSMOSE system to close for the first
time the error budget of TOC comparisons, bodes well for potential
future applications, which are briefly touched upon. |
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