Optical probes are frequently used for the detection of microphysical cloudparticle properties such as liquid and ice phase, size and morphology. Theseproperties can eventually influence the angular light scattering propertiesof cirrus clouds as well as the growth and accretion mechanisms of singlecloud particles. In this study we compare four commonly used optical probesto examine their response to small cloud particles of different phase andasphericity. Cloud simulation experiments were conducted at theCosmics Leaving OUtdoor Droplets (CLOUD) chamber at European Organisation forNuclear Research (CERN). The chamber was operated in a series of multi-stepadiabatic expansions to produce growth and sublimation of ice particles atsuper- and subsaturated ice conditions and for initial temperatures of−30, −40 and −50 °C. The experiments were performed for icecloud formation via homogeneous ice nucleation. We report the opticalobservations of small ice particles in deep convection and in situ cirrussimulations. Ice crystal asphericity deduced from measurements of spatiallyresolved single particle light scattering patterns by the Particle PhaseDiscriminator mark 2 (PPD-2K, Karlsruhe edition) were compared with Cloud andAerosol Spectrometer with Polarisation (CASPOL) measurements and imageroundness captured by the 3View Cloud Particle Imager (3V-CPI). Averaged pathlight scattering properties of the simulated ice clouds were measured usingthe Scattering Intensity Measurements for the Optical detectioN of icE(SIMONE) and single particle scattering properties were measured by theCASPOL.We show the ambiguity of several optical measurements in ice fractiondetermination of homogeneously frozen ice in the case where sublimatingquasi-spherical ice particles are present. Moreover, most of the instrumentshave difficulties of producing reliable ice fraction if small aspherical iceparticles are present, and all of the instruments cannot separate perfectlyspherical ice particles from supercooled droplets. Correlation analysis ofbulk averaged path depolarisation measurements and single particlemeasurements of these clouds showed higher values at highconcentrations and small diameters, but these results require furtherconfirmation. We find that none of these instruments were able to determineunambiguously the phase of the small particles. These results haveimplications for the interpretation of atmospheric measurements andparametrisations for modelling, particularly for low particle numberconcentration clouds.
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