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| Titel |
Characteristics of CALIOP attenuated backscatter noise: implication for cloud/aerosol detection |
| VerfasserIn |
D. L. Wu, J. H. Chae, A. Lambert, F. F. Zhang |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 6 ; Nr. 11, no. 6 (2011-03-22), S.2641-2654 |
| Datensatznummer |
250009512
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| Publikation (Nr.) |
 copernicus.org/acp-11-2641-2011.pdf |
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| Zusammenfassung |
| A research algorithm is developed for noise evaluation and feature detection
of the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Level 1
(L1) backscatter data with an emphasis on cloud/aerosol features in the
upper troposphere and lower stratosphere (UT/LS). CALIOP measurement noise
of the version v2.01 and v2.02 L1 backscatter data aggregated to (5 km)
horizontal resolution is analyzed with two approaches in this study. One is
to compare the observed and modeled molecular scatter profiles by scaling
the modeled profile (with a fitted scaling factor α) to the observed
clear-sky backscatter profiles. This scaling α value is sensitive to errors
in the calibrated backscatter and the atmospheric model used. Most of the
nighttime 532-nm α values are close to unity, as expected, but an abrupt drop
occurred in October 2008 in the daytime 532-nm α, which is likely indicative
of a problem in the v2.02 daytime calibrated data. The 1064-nm night α is
generally close to 2 while its day α is ~3. The other approach to
evaluate the lidar measurement noise is to use the calibrated lidar
backscatter data at altitudes above 19 km. With this method, the 532-nm and
1064-nm measurement noises are analyzed and characterized individually for
each profile in terms of the mean (μ) and standard deviation (σ), showing
larger σ values in general over landmasses or bright surfaces during day and
in radiation-hard regions during night. A significant increasing trend is
evident in the nighttime 1064-nm σ, which is likely responsible for the
increasing difference between the feature occurrence frequencies (532-nm vs.
1064-nm) derived from this study. For feature detection with the research
algorithm, we apply a σ–based method to the aggregated L1 data. The derived
morphology of feature occurrence frequency is in general agreement with that
obtained from the Level 2 (L2) 05 km_CLAY+05 km_ALAY products at 5 km
horizontal resolution. Finally, a normalized probability density function
(PDF) method is employed to evaluate the day-night backscatter data in which
noise levels are largely different. CALIOP observations reveal a higher
probability of daytime cloud/aerosol occurrence than nighttime in the
tropical UT/LS region for 532-nm total backscatters >0.01 km−1 sr−1. |
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