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| Titel |
Evaluating the assumptions of surface reflectance and aerosol type selection within the MODIS aerosol retrieval over land: the problem of dust type selection |
| VerfasserIn |
T. Mielonen, R. C. Levy, V. Aaltonen, M. Komppula, G. Leeuw, J. Huttunen, H. Lihavainen, P. Kolmonen, K. E. J. Lehtinen, A. Arola |
| 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 ; 4, no. 2 ; Nr. 4, no. 2 (2011-02-09), S.201-214 |
| Datensatznummer |
250001671
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| Publikation (Nr.) |
 copernicus.org/amt-4-201-2011.pdf |
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| Zusammenfassung |
| Aerosol Optical Depth (AOD) and Ångström exponent (AE) values derived
with the MODIS retrieval algorithm over land (Collection 5) are compared with
ground based sun photometer measurements at eleven sites spanning the globe.
Although, in general, total AOD compares well at these sites (R2 values
generally over 0.8), there are cases (from 2 to 67% of the measurements
depending on the site) where MODIS clearly retrieves the wrong spectral
dependence, and hence, an unrealistic AE value. Some of these poor AE
retrievals are due to the aerosol signal being too small (total
AOD < 0.3) but in other cases the AOD should have been high enough to
derive accurate AE. However, in these cases, MODIS indicates AE values close
to 0.6 and zero fine model weighting (FMW), i.e. dust model provides the
best fitting to the MODIS observed reflectance. Yet, according to evidence
from the collocated sun photometer measurements and backtrajectory analyses,
there should be no dust present. This indicates that the assumptions about
aerosol model and surface properties made by the MODIS algorithm may have
been incorrect. Here we focus on problems related to parameterization of the
land-surface optical properties in the algorithm, in particular the
relationship between the surface reflectance at 660 and 2130 nm. The
retrieval assumes that there is a linear equation that relates the
reflectance in these two channels, with the value of the slope
(slope660/2130) determined, in part, by the infrared Normalized
Difference Vegetation Index, (NDVISWIR). However, the assumed
dependence of the slope on the NDVISWIR is not supported by a MODIS
based surface albedo climatology. The use of a modified relationship based on
the albedo data improves the AE retrieval at the studied sites. The increase
in the AE agreement fraction between MODIS and AERONET measurements is
between 3 and 22 percentage units depending on the site. These results
indicate that the surface reflectance assumptions, especially the
slope660/2130 in the MODIS algorithm is the major reason for the
inaccurate AE values and the flawed aerosol model combining in the retrieval.
However, at some of these sites, the new relationship slightly reduces the
correlation between the MODIS and AERONET AOD. This decrease indicates that
the combination of the assumed surface and aerosol properties still do not
match the actual properties under investigation. |
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