Relationship between optical properties and ice crystal shapes in mid-latitude cirrus clouds from the polar nephelometer and cloud particle imager instruments

The analysis of concomitant CALIPSO satellite and airborne observations carried out during the CIRCLE-2 experiment (May 2007) in mid-latitude cirrus clouds over Western Europe has shown systematic larger Lidar CALIOP extinctions compared to extinction derived from co-located in situ measurements. This feature is explained by preferential horizontally oriented ice crystals with prevalent hexagonal-plate shape as revealed by the Cloud Particle Imager. Surprisingly the Polar Nephelometer does not reveal any signature of 22° (and 46°) halos with rather a featureless scattering phase function in this case. In contrast, very well identified 22° halo features are observed with predominant similarly plate-shaped ice crystals in other cirrus situation. This contribution discusses the results of a careful examination of CPI images related to Polar Nephelometer observations in order to explain the occurrence and no-occurrence of the 22° halo feature. Implications for realistic modelling of scattering properties of cirrus clouds are given.