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| Titel |
Distinguishing cirrus cloud presence in autonomous lidar measurements |
| VerfasserIn |
J. R. Campbell, M. A. Vaughan, M. Oo, R. E. Holz, J. R. Lewis, E. J. Welton |
| 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. 1 ; Nr. 8, no. 1 (2015-01-27), S.435-449 |
| Datensatznummer |
250116065
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| Publikation (Nr.) |
 copernicus.org/amt-8-435-2015.pdf |
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| Zusammenfassung |
| 2012 Level-2 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)
satellite-based cloud data sets are investigated for thresholds that
distinguish the presence of cirrus clouds in autonomous lidar measurements,
based on temperatures, heights, optical depth and phase. A thermal
threshold, proposed by Sassen and Campbell (2001) for cloud top temperature
Ttop ≤ −37 °C, is evaluated versus CALIOP algorithms
that identify ice-phase cloud layers using polarized backscatter
measurements. Derived global mean cloud top heights (11.15 vs. 10.07 km
above mean sea level; a.m.s.l.), base heights (8.76 km a.m.s.l. vs. 7.95 km a.m.s.l.),
temperatures (−58.48 °C vs. −52.18 °C and
−42.40 °C vs. −38.13 °C, respectively, for tops and
bases) and optical depths (1.18 vs. 1.23) reflect the sensitivity to this
constraint. Over 99 % of all Ttop ≤ −37 °C clouds are
classified as ice by CALIOP Level-2 algorithms. Over 81 % of all ice
clouds correspond with Ttop ≤ −37 °C. For instruments
lacking polarized measurements, and thus practical estimates of phase,
Ttop ≤ −37 °C provides sufficient justification for
distinguishing cirrus, as opposed to the risks of glaciated liquid-water
cloud contamination occurring in a given sample from clouds identified at
relatively "warm" (Ttop > −37 °C) temperatures.
Although accounting for uncertainties in temperatures collocated with lidar
data (i.e., model reanalyses/sondes) may justifiably relax the threshold to
include warmer cases, the ambiguity of "warm" ice clouds cannot be fully
reconciled with available measurements, conspicuously including phase. Cloud
top heights and optical depths are investigated, and global distributions
and frequencies derived, as functions of CALIOP-retrieved phase. These data
provide little additional information, compared with temperature alone, and
may exacerbate classification uncertainties overall. |
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