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| Titel |
The sensitivity of land emissivity estimates from AMSR-E at C and X bands to surface properties |
| VerfasserIn |
H. Norouzi, M. Temimi, W. B. Rossow, C. Pearl, M. Azarderakhsh, R. Khanbilvardi |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 15, no. 11 ; Nr. 15, no. 11 (2011-11-25), S.3577-3589 |
| Datensatznummer |
250013032
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| Publikation (Nr.) |
 copernicus.org/hess-15-3577-2011.pdf |
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| Zusammenfassung |
| Microwave observations at low frequencies exhibit more sensitivity to
surface and subsurface properties with little interference from the
atmosphere. The objective of this study is to develop a global land
emissivity product using passive microwave observations from the Advanced
Microwave Scanning Radiometer – Earth Observing System (AMSR-E) and to
investigate its sensitivity to land surface properties. The developed
product complements existing land emissivity products from SSM/I and AMSU by
adding land emissivity estimates at two lower frequencies, 6.9 and 10.65 GHz
(C- and X-band, respectively). Observations at these low frequencies
penetrate deeper into the soil layer. Ancillary data used in the analysis,
such as surface skin temperature and cloud mask, are obtained from
International Satellite Cloud Climatology Project (ISCCP). Atmospheric
properties are obtained from the TIROS Operational Vertical Sounder (TOVS)
observations to determine the small upwelling and downwelling atmospheric
emissions as well as the atmospheric transmission. A sensitivity test
confirms the small effect of the atmosphere but shows that skin temperature
accuracy can significantly affect emissivity estimates. Retrieved
emissivities at C- and X-bands and their polarization differences exhibit
similar patterns of variation with changes in land cover type, soil
moisture, and vegetation density as seen at SSM/I-like frequencies (Ka and
Ku bands). The emissivity maps from AMSR-E at these higher frequencies agree
reasonably well with the existing SSM/I-based product. The inherent
discrepancy introduced by the difference between SSM/I and AMSR-E
frequencies, incidence angles, and calibration has been assessed.
Significantly greater standard deviation of estimated emissivities compared
to SSM/I land emissivity product was found over desert regions. Large
differences between emissivity estimates from ascending and descending
overpasses were found at lower frequencies due to the inconsistency between
thermal IR skin temperatures and passive microwave brightness temperatures
which can originate from below the surface. The mismatch between day and
night AMSR-E emissivities is greater than ascending and descending
differences of SSM/I emissivity. This is because of unique orbit time of
AMSR-E (01:30 a.m./p.m. LT) while other microwave sensors have orbit time of
06:00 to 09:00 (a.m./p.m.). This highlights the importance of considering
the penetration depth of the microwave signal and diurnal variability of the
temperature in emissivity retrieval. The effect of these factors is greater
for AMSR-E observations than SSM/I observations, as AMSR-E observations
exhibit a greater difference between day and night measures. This issue must
be addressed in future studies to improve the accuracy of the emissivity
estimates especially at AMSR-E lower frequencies. |
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