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| Titel |
Implications of MODIS bow-tie distortion on aerosol optical depth retrievals, and techniques for mitigation |
| VerfasserIn |
A. M. Sayer, N. C. Hsu, C. Bettenhausen |
| 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 (2015-12-17), S.5277-5288 |
| Datensatznummer |
250116727
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| Publikation (Nr.) |
 copernicus.org/amt-8-5277-2015.pdf |
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| Zusammenfassung |
| The scan geometry of the Moderate Resolution Imaging Spectroradiometer
(MODIS) sensors, combined with the Earth's curvature, results in a pixel shape distortion known as the "bow-tie
effect". Specifically, sensor pixels near the edge of the swath are
elongated along-track and across-track compared to pixels near the centre of
the swath, resulting in an increase of pixel area by up to a factor of ∼ 9 and,
additionally, the overlap of pixels acquired from consecutive scans. The Deep
Blue and Dark Target aerosol optical depth (AOD) retrieval algorithms
aggregate sensor pixels and provide level 2 (L2) AOD at a nominal horizontal
pixel size of 10 km, but the bow-tie distortion means that they also
suffer from this size increase and overlap. This means that the spatial
characteristics of the data vary as a function of satellite viewing zenith
angle (VZA) and, for VZA > 30°, corresponding to approximately
50 % of the data, are areally enlarged by a factor of 50 %
or more compared to this nominal pixel area and are not spatially
independent of each other. This has implications for retrieval uncertainty
and aggregated statistics, causing a narrowing of AOD distributions near the
edge of the swath, as well as for data comparability from the application of
similar algorithms to sensors without this level of bow-tie distortion.
Additionally, the pixel overlap is not obvious to users of the L2 aerosol
products because only pixel centres, not boundaries, are provided within the
L2 products. A two-step procedure is proposed to mitigate the effects of this
distortion on the MODIS aerosol products. The first (simple) step involves
changing the order in which pixels are aggregated in L2 processing to reflect
geographical location rather than scan order, which removes the bulk of the
overlap between L2 pixels and slows the rate of growth of
L2 pixel size vs. VZA. This can be achieved without significant
changes to existing MODIS processing algorithms. The second step involves
additionally changing the number of sensor pixels aggregated across-track as
a function of VZA, which preserves L2 pixel size at around
10 km × 10 km across the whole swath but would
require algorithmic quality assurance tests to be re-evaluated. Both of these
steps also improve the extent to which the pixel locations a user would infer
from the L2 data products represent the actual spatial extent of the
L2 pixels. |
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