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| Titel |
Evaluation of IASI-derived dust aerosol characteristics over the tropical belt |
| VerfasserIn |
V. Capelle, A. Chédin, M. Siméon, C. Tsamalis, C. Pierangelo, M. Pondrom, C. Crevoisier, L. Crépeau, N. A. Scott |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 17 ; Nr. 14, no. 17 (2014-09-10), S.9343-9362 |
| Datensatznummer |
250119017
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| Publikation (Nr.) |
 copernicus.org/acp-14-9343-2014.pdf |
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| Zusammenfassung |
IASI (Infrared Atmospheric Sounder Interferometer)-derived monthly mean infrared (10 μm) dust aerosol optical depth
(AOD) and altitude are evaluated against ground-based Aerosol RObotic
NETwork of sun photometers (AERONET) measurements
of the 500 nm coarse-mode AOD and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) measurements of altitude at 38
AERONET sites (sea and land) within the tropical belt (30° N–30° S).
The period covered extends from July 2007 to June 2013.
The evaluation goes through the analysis of Taylor diagrams and box-and-whiskers plots, separating situations over oceanic regions and over land.
For the AOD, such an evaluation raises the problem of the difference between
the two spectral domains used: infrared for IASI and visible for AERONET.
Consequently, the two measurements do not share the same metrics. For that
reason, AERONET coarse-mode AOD is first "translated" into IASI-equivalent
infrared AOD. This is done by the determination, site by site, of an
infrared to visible AOD ratio. Because translating visible coarse-mode AOD
into infrared AOD requires accurate knowledge of variables, such as the
infrared refractive index or the particle size distribution, quantifying
the bias between these two sources of AOD is not straightforward. This
problem is detailed in this paper, in particular in Appendix A. For the
sites over oceanic regions, the overall AOD temporal correlation comes to
0.86 for 786 items (IASI and AERONET monthly mean bins). The overall
normalized standard deviation (i.e. ratio of the standard deviation of the
test data (IASI) to that of the reference data (AERONET)) is 0.93, close
to the desired value of 1. Over land, essentially desert, correlation is 0.74 for 619 items and the normalized standard deviation is 0.86. This
slight but significant degradation over land most probably results from the
greater complexity of the surface (heterogeneity, elevation) and, to a
lesser extent, to the episodic presence of dust within the boundary layer
(particularly for sites close to active sources) to which IASI, as any
thermal infrared sounder, is poorly sensitive, unlike AERONET. Site by
site, disparities appear that are principally due to either
the insufficient number of AERONET observations throughout the period
considered, to the complexity of the location leading to the mixing of several aerosol types
(in the case of the Persian Gulf, for example), to surface heterogeneities
(elevation, emissivity, etc.), or to the use of a single aerosol model
(called "MITR"). Results using another aerosol model, with a different refractive
index, are presented and discussed. Concerning altitude over oceanic
regions, correlation is 0.70 for 853 items and the normalized standard
deviation is 0.92. A systematic bias of −0.4 km (IASI–CALIOP) is
observed, with a standard deviation of 0.48 km. This result is satisfactory,
considering the important differences between the two instruments
(space–time coverage, definition of the altitude). Altitude results over
land, essentially over deserts, are not satisfactory for a majority of
sites. The smaller sensitivity of IASI to altitude compared to its
sensitivity to AOD, added to the difficulties met for the determination of
the AOD over land (surface heterogeneities), explain this result. Work is in
progress to solve this difficulty.
We conclude that the present results demonstrate the usefulness of IASI
data, which are planned to cover a long period of time, as an additional
constraint to a better knowledge of the impact of aerosols on the climate
system. |
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