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| Titel |
MISR research-aerosol-algorithm refinements for dark water retrievals |
| VerfasserIn |
J. A. Limbacher, R. A. Kahn |
| 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. 11 ; Nr. 7, no. 11 (2014-11-27), S.3989-4007 |
| Datensatznummer |
250115967
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| Publikation (Nr.) |
 copernicus.org/amt-7-3989-2014.pdf |
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| Zusammenfassung |
| We explore systematically the cumulative effect of many assumptions made in
the Multi-angle Imaging SpectroRadiometer (MISR) research aerosol retrieval
algorithm with the aim of quantifying the main sources of uncertainty over
ocean, and correcting them to the extent possible. A total of 1129
coincident, surface-based sun photometer spectral aerosol optical depth (AOD)
measurements are used for validation. Based on comparisons between these data
and our baseline case (similar to the MISR standard algorithm, but without
the "modified linear mixing" approximation), for 558 nm AOD
< 0.10, a high bias of 0.024 is reduced by about one-third when (1)
ocean surface under-light is included and the assumed whitecap reflectance at
672 nm is increased, (2) physically based adjustments in particle
microphysical properties and mixtures are made, (3) an adaptive pixel
selection method is used, (4) spectral reflectance uncertainty is estimated
from vicarious calibration, and (5) minor radiometric calibration changes are
made for the 672 and 866 nm channels. Applying (6) more stringent cloud
screening (setting the maximum fraction not-clear to 0.50) brings all median
spectral biases to about 0.01. When all adjustments except more stringent
cloud screening are applied, and a modified acceptance criterion is used, the
Root-Mean-Square-Error (RMSE) decreases for all wavelengths by 8–27%
for the research algorithm relative to the baseline, and is 12–36%
lower than the RMSE for the Version 22 MISR standard algorithm (SA, with no
adjustments applied). At 558 nm, 87% of AOD data falls within the
greater of 0.05 or 20% of validation values; 62% of the 446 nm AOD
data, and > 68% of 558, 672, and 866 nm AOD values fall
within the greater of 0.03 or 10%. For the Ångström exponent
(ANG), 67% of 1119 validation cases for AOD > 0.01 fall
within 0.275 of the sun photometer values, compared to 49% for the SA.
ANG RMSE decreases by 17% compared to the SA, and the median absolute
error drops by 36%. |
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