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| Titel |
Remote sensing of aerosols over snow using infrared AATSR observations |
| VerfasserIn |
L. G. Istomina, W. Hoyningen-Huene, A. A. Kokhanovsky, E. Schultz, J. P. Burrows |
| 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. 6 ; Nr. 4, no. 6 (2011-06-17), S.1133-1145 |
| Datensatznummer |
250002013
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| Publikation (Nr.) |
 copernicus.org/amt-4-1133-2011.pdf |
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| Zusammenfassung |
Infrared (IR) retrievals of aerosol optical thickness (AOT) are challenging
because of the low reflectance of aerosol layer at longer wavelengths. In
this paper we present a closer analysis of this problem, performed with
radiative transfer (RT) simulations for coarse and accumulation mode of four
main aerosol components. It shows the strong angular dependence of aerosol
IR reflectance at low solar elevations resulting from the significant
asymmetry of aerosol phase function at these wavelengths. This results in
detectable values of aerosol IR reflectance at certain non-nadir observation
angles providing the advantage of multiangle remote sensing instruments for
a retrieval of AOT at longer wavelengths. Such retrievals can be of
importance e.g. in case of a very strong effect of the surface on the top of
atmosphere (TOA) reflectance in the visible spectral range. In the current
work, a new method to retrieve AOT of the coarse and accumulation mode
particles over snow has been developed using the measurements of Advanced
Along Track Scanning Radiometer (AATSR) on board the ENVISAT satellite. The
algorithm uses AATSR channel at 3.7 μm and utilizes its dual-viewing
observation technique, implying the forward view with an observation zenith
angle of around 55 degrees and the nadir view. It includes cloud/snow
discrimination, extraction of the atmospheric reflectance out of measured
brightness temperature (BT) at 3.7 μm, and interpolation of look-up
tables (LUTs) for a given aerosol reflectance. The algorithm uses LUTs,
separately simulated with RT forward calculations. The resulting AOT at
500 nm is estimated from the value at 3.7 μm using a fixed Angström
parameter.
The presented method has been validated against ground-based Aerosol Robotic
Network (AERONET) data for 4 high Arctic stations and shows good agreement.
A case study has been performed at W-Greenland on 5 July 2008. The day before
was characterized by a noticeable dust event. The retrieved AOT maps of the
region show a clear increase of AOT in the Kangerlussuaq area. The area of
increased AOT was detected on 5 July on the ice sheet east of Kangelussuaq,
opposite to the observed north easterly wind at ground level. This position
can be explained by a small scale atmospheric circulation transporting the
mobilized mineral dust upslope, after its intrusion into the upper branch of
the circulation.
The performed study of atmospheric reflectance at 3.7 μm also shows
possibilities for the detection and retrievals of cloud properties over snow
surfaces. |
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