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| Titel |
Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter |
| VerfasserIn |
B. Diedenhoven, B. Cairns, A. M. Fridlind, A. S. Ackerman, T. J. Garrett |
| 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 ; 13, no. 6 ; Nr. 13, no. 6 (2013-03-18), S.3185-3203 |
| Datensatznummer |
250018527
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| Publikation (Nr.) |
 copernicus.org/acp-13-3185-2013.pdf |
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| Zusammenfassung |
| A new method to retrieve ice cloud asymmetry parameters from
multi-directional polarized reflectance measurements is applied to
measurements of the airborne Research Scanning Polarimeter (RSP)
obtained during the CRYSTAL-FACE campaign in 2002. The method
assumes individual hexagonal ice columns and plates serve as proxies
for more complex shapes and aggregates. The closest fit is searched
in a look-up table of simulated polarized reflectances computed for
cloud layers that contain individual, randomly oriented hexagonal
columns and plates with a virtually continuous selection of aspect
ratios and distortion. The asymmetry parameter, aspect ratio and
distortion of the hexagonal particle that leads to the best fit with
the measurements are considered the retrieved values. Two cases of
thick convective clouds and two cases of thinner anvil cloud layers
are analyzed. Median asymmetry parameters retrieved by the RSP range
from 0.76 to 0.78, and are generally smaller than those currently
assumed in most climate models and satellite retrievals. In all
cases the measurements indicate roughened or distorted ice crystals, which is
consistent with previous findings. Retrieved aspect ratios in three
of the cases range from 0.9 to 1.6, indicating compact particles
dominate the cloud-top shortwave radiation. Retrievals for the
remaining case indicate plate-like ice crystals with aspect ratios
around 0.3. The RSP retrievals are qualitatively consistent with the
CPI images obtained in the same cloud layers. Retrieved asymmetry
parameters are compared to those determined in situ by the Cloud
Integrating Nephelometer (CIN). For two cases, the median values of
asymmetry parameter retrieved by CIN and RSP agree within 0.01,
while for the two other cases RSP asymmetry parameters are about
0.03–0.05 greater than those obtained by the CIN. Part of this bias
might be explained by vertical variation of the asymmetry parameter or ice shattering on the CIN probe, or both. |
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