Optical properties of pristine ice crystals in mid-latitude cirrus clouds: a case study during CIRCLE-2 experiment

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 planarplate 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.

Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements

“…It is defined by the ratio of the scattered energy values measured at the scattering angles of 22 • and 18.5 • . As such, it is sensitive to the presence in ice crystals of the 60 • prism angle and to the smoothness of the prismatic faces (Auriol et al, 2001;Gayet et al, 2011). Here it was calculated by averaging SID-3 image brightness between the scattering angles from 21.5 • to 22.5 • , and from 18 • to 19 • , and taking the ratio of the respective averages.…”

“…Roughness may have been responsible for unexpectedly low values of the asymmetry parameter measured in ice clouds by Garrett et al (2001). Gayet et al (2011) found prevalent particles with imperfect or complex shapes at the trailing edge of mid-latitude frontal cirrus, while the leading edge was dominated by more regular plates that produced a 22 • halo signature. For one full day of PARASOL data over ocean, very rough faceted particles provided an improved fit to polarized reflectances (Cole et al, 2013).…”

Bulk microphysical properties of semi-transparent cirrus from AIRS: a six year global climatology and statistical analysis in synergy with geometrical profiling data from CloudSat-CALIPSO

“…Statistics is averaged over the period 2003-2009. that suggest that irregular is the dominant habit of ice particle grown in natural clouds (e.g. Francis et al, 1999;Korolev and Isaac, 2003;Gayet et al, 2011). Considering Fig.…”

Halo ratio from ground based all-sky imaging

“…Unlike in previous studies which tended to use slightly smaller angles (Auriol et al, 2001;Gayet et al, 2011;Ulanowski et al, 2014;Forster et al, 2017) we have used a ratio at scattering angles of 23…”

“…In general roughening, internal inclusions and complex shapes are all major factors contributing to removing halo features from the SPFs (Shcherbakov, 2013). The asymmetry parameter on the other hand is expected to be positively correlated with the HR (Ulanowski et al, 2006;Gayet et al, 2011; The colour camera has a Bayer mosaic used for filtering the red, blue and green wavelengths. The Matlab built-in function "demosaic" is applied to the raw image data to reconstruct the full colour image.…”