Abstract
Abstract. In this paper, we describe in situ observations of mid-latitude cirrus cloud band carried out on 16 May 2007 during the CIRCLE-2 experiment. The Polar Nephelometer and the Cloud Particle Imager (CPI) instruments with PMS FSSP-300 and 2D-C probes were used for the description of the optical and microphysical cloud properties. Two selected cloud regions are compared and discussed in detail. Significant differences in optical properties are evidenced in terms of 22° halo occurrences even though prevalent planar-plate ice crystals are observed in both cloud regions. Featureless scattering phase functions are measured in the first cloud region located near the trailing edge of the cirrus-band at about 11 800 m/−57 °C. In contrast, well pronounced 22° halo peaks are observed with predominant similar-shaped ice crystals near the cirrus-band leading edge at 7100 m/−27 °C. CPI ice crystal images with Polar Nephelometer observations are carefully analysed and interpreted from a theoretical light scattering model in order to explain occurrence and non-occurrence of the 22° halo feature. The results highlight that the halo peaks are inherent only in perfect plate ice crystals (or pristine crystals). On the basis of previous datasets 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 that are smooth and featureless best represent cirrus scattering properties.
Highlights
Light-scattering properties of ice crystals have been the subject of considerable work over the last few years, addressing the global climate effect of cirrus clouds (Stephens et al, 1990)
A representative example of cirrus properties with nonobserved 22◦ halo feature is illustrated from the observations carried out in a cloud region located near the trailing edge of the cirrus-band at about 11 800 m/−57 ◦C
High halo ratio values (> 1.0) correspond to sharp peaks with a well-pronounced 22◦ halo, whereas smoothed peaks and/or smoothed scattering phase functions with no 22◦ halo feature are characterised by a smaller halo ratio (< 1)
Summary
Light-scattering properties of ice crystals have been the subject of considerable work over the last few years, addressing the global climate effect of cirrus clouds (Stephens et al, 1990). Optical phenomena such as the 22◦ halo (and 46◦ halo) were first explained by Mariotte (1717) as being due to the refraction of light (in the visible) by randomly oriented hexagonal ice crystals. Roughness of the ice surface or imperfect internal structure are known to hamper the formation of halos (see among others Baran and Labonnote, 2007) This may explain the rare ground-observed occurrences of these optical phenomena
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