Intercomparison study and optical asphericity measurements of small ice particles in the CERN CLOUD experiment
Leonid Nichman1, Emma Järvinen2, James Dorsey1,3, Paul Connolly1, Jonathan Dupplisy4, Claudia Fuchs5, Karoliina Ignatius6, Kamalika Sengupta7, Frank Stratmann6, Ottmar Möhler2, Martin Schnaiter2, and Martin Gallagher11School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK 2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Postfach 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, LS2-9JT Leeds, UK
Received: 17 Jun 2016 – Accepted for review: 25 Jul 2016 – Discussion started: 03 Aug 2016
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 sub-saturated 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 microphysical properties of small quasi-spherical ice particles in deep convection simulations and small hexagonal ice particles typical for in situ cirrus. Ice crystal asphericity and a degree of submicron complexity 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 images 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 shown a rather low sensitivity to the crystal complexity for small ice cloud particles that were grown under typical atmospheric conditions. Bulk averaged path depolarisation measurements of these clouds showed higher correlation to single particle measurements at high concentration and small diameters of cloud particles. These results have implications for the interpretation of atmospheric measurements and parametrisations for modelling, particularly for low particle number concentration clouds. This ensemble of optical instruments, using both averaged path and single particle detection presented here, in conjugation with the CLOUD chamber, reveals the possible discrepancies in comparisons of airborne and remote sensing measurements.
Nichman, L., Järvinen, E., Dorsey, J., Connolly, P., Dupplisy, J., Fuchs, C., Ignatius, K., Sengupta, K., Stratmann, F., Möhler, O., Schnaiter, M., and Gallagher, M.: Intercomparison study and optical asphericity measurements of small ice particles in the CERN CLOUD experiment, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2016-205, in review, 2016.