Articles | Volume 10, issue 9
Atmos. Meas. Tech., 10, 3231–3248, 2017

Special issue: The CERN CLOUD experiment (ACP/AMT inter-journal SI)

Atmos. Meas. Tech., 10, 3231–3248, 2017

Research article 04 Sep 2017

Research article | 04 Sep 2017

Intercomparison study and optical asphericity measurements of small ice particles in the CERN CLOUD experiment

Leonid Nichman1,a,b, Emma Järvinen2, James Dorsey1,3, Paul Connolly1, Jonathan Duplissy4, Claudia Fuchs5, Karoliina Ignatius6, Kamalika Sengupta7, Frank Stratmann6, Ottmar Möhler2, Martin Schnaiter2, and Martin Gallagher1 Leonid Nichman et al.
  • 1School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK
  • 2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, P.O. Box 3640, 76021, Germany
  • 3National Centre for Atmospheric Science, Manchester, UK
  • 4Department of Physics, P.O. Box 64, 00014 University of Helsinki, Helsinki, Finland
  • 5Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland
  • 6Leibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany
  • 7University of Leeds, School of Earth and Environment, Leeds LS2-9JT, UK
  • anow at: Department of Chemistry, Boston College, Chestnut Hill, MA, USA
  • bnow at: Aerodyne Research, Inc, Billerica, MA, USA

Abstract. Optical probes are frequently used for the detection of microphysical cloud particle properties such as liquid and ice phase, size and morphology. These properties can eventually influence the angular light scattering properties of cirrus clouds as well as the growth and accretion mechanisms of single cloud particles. In this study we compare four commonly used optical probes to examine their response to small cloud particles of different phase and asphericity. Cloud simulation experiments were conducted at the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at European Organisation for Nuclear Research (CERN). The chamber was operated in a series of multi-step adiabatic expansions to produce growth and sublimation of ice particles at super- and subsaturated ice conditions and for initial temperatures of −30, −40 and −50 °C. The experiments were performed for ice cloud formation via homogeneous ice nucleation. We report the optical observations of small ice particles in deep convection and in situ cirrus simulations. Ice crystal asphericity deduced from measurements of spatially resolved single particle light scattering patterns by the Particle Phase Discriminator mark 2 (PPD-2K, Karlsruhe edition) were compared with Cloud and Aerosol Spectrometer with Polarisation (CASPOL) measurements and image roundness captured by the 3View Cloud Particle Imager (3V-CPI). Averaged path light scattering properties of the simulated ice clouds were measured using the Scattering Intensity Measurements for the Optical detectioN of icE (SIMONE) and single particle scattering properties were measured by the CASPOL.

We show the ambiguity of several optical measurements in ice fraction determination of homogeneously frozen ice in the case where sublimating quasi-spherical ice particles are present. Moreover, most of the instruments have difficulties of producing reliable ice fraction if small aspherical ice particles are present, and all of the instruments cannot separate perfectly spherical ice particles from supercooled droplets. Correlation analysis of bulk averaged path depolarisation measurements and single particle measurements of these clouds showed higher R2 values at high concentrations and small diameters, but these results require further confirmation. We find that none of these instruments were able to determine unambiguously the phase of the small particles. These results have implications for the interpretation of atmospheric measurements and parametrisations for modelling, particularly for low particle number concentration clouds.

Short summary
Optical probes are frequently used for the detection of cloud particles. The detected microphysical properties may affect particle growth and accretion mechanisms and the light scattering properties of cirrus clouds. In the CLOUD chamber study at CERN, we compared four optical measurement techniques. We show that shape derivation alone is not sufficient to determine the phase of the small cloud particles. None of the instruments were able to unambiguously determine the phase of small particles.