A study of ice crystal backscatter features from 5 years of collocated POLDER, MODIS and CALIOP observations
Abstract
Significant efforts have been made in recent years to establish a realistic representation of ice crystal optical properties that can be used to characterize cirrus cloud microphysics from spaceborne observations. In particular, attention has been focused on providing models that can consistently and simultaneously explain both passive and active observations in the solar and thermal infrared spectral domains. If spectral dependencies of integral quantities such as Single Scattering Albedo or the Asymmetry Factor are key in reconciling multiple sensors observations and retrievals, particular features of the effective phase function remain of high interest. Indeed these details are very informative fingerprints of ice crystal shapes and can also be critical parameters for retrievals performed under specific viewing geometries. Among those features, the relative magnitude of the backscatter peak intensity that appears in phase functions of ice crystal potentially carries information on the crystal habits. For instance it has been demonstrated that roughened hexagonal particles do not produce as intense a backscatter peak relative to pristine crystals, and that it can even completely disappear for severely roughened particles. At the same time lidar observations and retrievals, are significantly influenced by the existence or absence of this backscatter peak. We analyzed 5 years of coincident POLDER, MODIS and CALIOP observations whereby we were able to study the angular variability of ice clouds reflectance in and around the exact backscatter direction. Statistics of the observed peak intensities from POLDER have been established in relation to coincident MODIS cirrus optical thickness and effective radius retrievals as well as CALIOP layer integrated depolarization ratio and attenuated backscatter. In the vast majority of situations analyzed, observed peak intensities were below what would be expected from pristine ice crystals. Statistics obtained through this analysis will be presented and implications for ice cloud microphysical properties and remote sensing applications will be discussed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A31J..08R
- Keywords:
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- 0319 ATMOSPHERIC COMPOSITION AND STRUCTURE / Cloud optics;
- 0321 ATMOSPHERIC COMPOSITION AND STRUCTURE / Cloud/radiation interaction