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| Titel |
Comparing satellite- to ground-based automated and manual cloud coverage observations – a case study |
| VerfasserIn |
A. Werkmeister, M. Lockhoff, M. Schrempf, K. Tohsing, B. Liley, G. Seckmeyer |
| 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. 5 ; Nr. 8, no. 5 (2015-05-06), S.2001-2015 |
| Datensatznummer |
250116357
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| Publikation (Nr.) |
 copernicus.org/amt-8-2001-2015.pdf |
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| Zusammenfassung |
In this case study we compare cloud fractional cover measured by radiometers
on polar satellites (AVHRR) and on one geostationary satellite (SEVIRI) to
ground-based manual (SYNOP) and automated observations by a cloud camera
(Hemispherical Sky Imager, HSI). These observations took place in Hannover,
Germany, and in Lauder, New Zealand, over time frames of 3 and 2 months,
respectively.
Daily mean comparisons between satellite derivations and the ground-based HSI
found the deviation to be 6 ± 14% for AVHRR and
8 ± 16% for SEVIRI, which can be considered satisfactory.
AVHRR's instantaneous differences are smaller (2 ± 22%) than
instantaneous SEVIRI cloud fraction estimates (8 ± 29%) when
compared to HSI due to resolution and scenery effect issues. All spaceborne
observations show a very good skill in detecting completely overcast skies
(cloud cover ≥ 6 oktas) with probabilities between 92 and 94%
and false alarm rates between 21 and 29% for AVHRR and SEVIRI in Hannover,
Germany. In the case of a clear sky (cloud cover lower than 3 oktas) we find
good skill with detection probabilities between 72 and 76%. We find poor
skill, however, whenever broken clouds occur (probability of detection is
32% for AVHRR and 12% for SEVIRI in Hannover, Germany).
In order to better understand these discrepancies we analyze the influence of
algorithm features on the satellite-based data. We find that the differences
between SEVIRI and HSI cloud fractional cover (CFC) decrease (from a bias of 8 to almost 0%) with
decreasing number of spatially averaged pixels and decreasing index which
determines the cloud coverage in each "cloud-contaminated" pixel of the
binary map. We conclude that window size and index need to be adjusted in
order to improve instantaneous SEVIRI and AVHRR estimates. Due to its
automated operation and its spatial, temporal and spectral resolution, we
recommend as well that more automated ground-based instruments in the form of
cloud cameras should be installed as they cover larger areas of the sky than
other automated ground-based instruments. These cameras could be an essential
supplement to SYNOP observation as they cover the same spectral wavelengths
as the human eye. |
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