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| Titel |
A cloud detection algorithm using the downwelling infrared radiance measured by an infrared pyrometer of the ground-based microwave radiometer |
| VerfasserIn |
M.-H. Ahn, D. Han, H. Y. Won, V. Morris |
| 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 ; 8, no. 2 ; Nr. 8, no. 2 (2015-02-03), S.553-566 |
| Datensatznummer |
250116120
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| Publikation (Nr.) |
 copernicus.org/amt-8-553-2015.pdf |
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| Zusammenfassung |
| For better utilization of the ground-based microwave radiometer, it is
important to detect the cloud presence in the measured data. Here, we
introduce a simple and fast cloud detection algorithm by using the optical
characteristics of the clouds in the infrared atmospheric window region. The
new algorithm utilizes the brightness temperature (Tb) measured by an infrared
radiometer installed on top of a microwave radiometer. The two-step
algorithm consists of a spectral test followed by a temporal test. The
measured Tb is first compared with a predicted clear-sky Tb obtained by an
empirical formula as a function of surface air temperature and water vapor
pressure. For the temporal test, the temporal variability of the measured
Tb during one minute compares with a dynamic threshold value, representing the
variability of clear-sky conditions. It is designated as cloud-free data
only when both the spectral and temporal tests confirm cloud-free data.
Overall, most of the thick and uniform clouds are successfully detected by
the spectral test, while the broken and fast-varying clouds are detected by
the temporal test. The algorithm is validated by comparison with the
collocated ceilometer data for six months, from January to June 2013. The
overall proportion of correctness is about 88.3% and the probability of
detection is 90.8%, which are comparable with or better than those of
previous similar approaches. Two thirds of discrepancies occur when the new
algorithm detects clouds while the ceilometer does not, resulting in
different values of the probability of detection with different cloud-base
altitude, 93.8, 90.3, and 82.8% for low, mid, and high clouds,
respectively. Finally, due to the characteristics of the spectral range, the
new algorithm is found to be insensitive to the presence of inversion
layers. |
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