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| Titel |
Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements |
| VerfasserIn |
B. Diedenhoven, B. Cairns, I. V. Geogdzhayev, A. M. Fridlind, A. S. Ackerman, P. Yang, B. A. Baum |
| 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 ; 5, no. 10 ; Nr. 5, no. 10 (2012-10-08), S.2361-2374 |
| Datensatznummer |
250003119
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| Publikation (Nr.) |
 copernicus.org/amt-5-2361-2012.pdf |
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| Zusammenfassung |
| We present a new remote sensing technique to infer the average
asymmetry parameter of ice crystals near cloud top from
multi-directional polarization measurements. The method is based on
previous findings that (a) complex aggregates of hexagonal crystals
generally have scattering phase matrices resembling those of their
components; and (b) scattering phase matrices systematically vary
with aspect ratios of crystals and their degree of microscale
surface roughness. Ice cloud asymmetry parameters are inferred from
multi-directional polarized reflectance measurements by searching
for the closest fit in a look-up table of simulated polarized
reflectances computed for cloud layers that contain individual, randomly oriented
hexagonal columns and plates with varying aspect ratios and
roughness values. The asymmetry parameter of the hexagonal particle
that leads to the best fit with the measurements is considered the
retrieved value. For clouds with optical thickness less than 5, the
cloud optical thickness must be retrieved simultaneously with the
asymmetry parameter, while for optically thicker clouds the
asymmetry parameter retrieval is independent of cloud optical
thickness. Evaluation of the technique using simulated measurements
based on the optical properties of a number of complex particles and
their mixtures shows that the ice crystal asymmetry parameters are
generally retrieved to within 5%, or about 0.04 in absolute
terms. The retrieval scheme is largely independent of calibration
errors, range and sampling density of scattering angles and random
noise in the measurements. The approach can be applied to
measurements of past, current and future airborne and satellite
instruments that measure multi-directional polarized reflectances of
ice-topped clouds. |
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