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| Titel |
Impact of cloud horizontal inhomogeneity and directional sampling on the retrieval of cloud droplet size by the POLDER instrument |
| VerfasserIn |
H. Shang, L. Chen, F. M. Bréon, H. Letu, S. Li, Z. Wang, L. Su |
| 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. 11 ; Nr. 8, no. 11 (2015-11-24), S.4931-4945 |
| Datensatznummer |
250116693
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| Publikation (Nr.) |
 copernicus.org/amt-8-4931-2015.pdf |
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| Zusammenfassung |
The principles of cloud droplet size retrieval via Polarization and
Directionality of the Earth's Reflectance (POLDER) requires that clouds be
horizontally homogeneous. The retrieval is performed by combining all
measurements from an area of 150 km × 150 km to compensate for
POLDER's insufficient directional sampling. Using POLDER-like data simulated
with the RT3 model, we investigate the impact of cloud horizontal
inhomogeneity and directional sampling on the retrieval and analyze which
spatial resolution is potentially accessible from the measurements. Case
studies show that the sub-grid-scale variability in droplet effective radius
(CDR) can significantly reduce valid retrievals and introduce small biases
to the CDR (~ 1.5 μm) and effective variance (EV)
estimates. Nevertheless, the sub-grid-scale variations in EV and cloud
optical thickness (COT) only influence the EV retrievals and not the CDR
estimate. In the directional sampling cases studied, the retrieval using
limited observations is accurate and is largely free of random noise.
Several improvements have been made to the original POLDER droplet size
retrieval. For example, measurements in the primary rainbow region
(137–145°) are used to ensure retrievals of large droplet
(> 15 μm) and to reduce the uncertainties caused by cloud
heterogeneity. We apply the improved method using the POLDER global L1B data
from June 2008, and the new CDR results are compared with the operational
CDRs. The comparison shows that the operational CDRs tend to be
underestimated for large droplets because the cloudbow oscillations in the
scattering angle region of 145–165° are weak for cloud fields with
CDR > 15 μm. Finally, a sub-grid-scale retrieval case
demonstrates that a higher resolution, e.g., 42 km × 42 km, can be
used when inverting cloud droplet size distribution parameters from POLDER
measurements. |
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