Optical properties of pristine ice crystals in mid-latitude cirrus clouds: a case study during CIRCLE-2 experiment
1Laboratoire de Météorologie Physique, UMR 6016 CNRS/Université Blaise Pascal, France
2LaMP – Institut Universitaire de Technologie de Montluçon, Avenue A. Briand-BP 2235, 03101 Montluçon Cedex, France
3Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
Abstract. Preferential horizontally-oriented ice crystals with a prevalent hexagonal-plate shape revealed by the Cloud Particle Imager can explain systematic larger Lidar CALIOP extinctions when compared with extinction derived from co-located in situ measurements. Surprisingly, the Polar Nephelometer does not reveal any signature of 22° (and 46°) halos, showing a rather featureless scattering phase function in this case. In contrast, well pronounced 22° halo peaks are observed with predominant similar-shaped ice crystals in other cirrus situations. This paper discusses the results of a careful examination of CPI images with Polar Nephelometer observations in order to explain occurrence and non occurrence of the 22° halo feature. Observations highlight that halo peaks are evidenced only by the presence of perfect plate ice crystals (or pristine crystals). On the basis of previous data sets in mid-latitude cirrus it is found that simple pristine crystals are uncommon whereas particles with imperfect or complex shapes are prevalent. As a result, phase functions are smooth and featureless and best represent cirrus scattering properties.