Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, P.O. Box 3640, 76021 Karlsruhe, Germany
09 Nov 2012
Received: 31 May 2012 – Published in Atmos. Chem. Phys. Discuss.: 18 June 2012 Abstract. The article presents the laser scattering and depolarisation
instrument SIMONE that is installed at the large aerosol and cloud
chamber facility AIDA of the Karlsruhe Institute of
Technology. SIMONE uses a 488 nm cw laser to probe simulated
atmospheric clouds by measuring the scattered light from the
1.8° and 178.2° directions. At 178.2°, the
scattered light is analysed for the linear polarisation state to
deduce the particle linear depolarisation ratio δp which is
a common measurement parameter of atmospheric lidar
applications. The optical setup and the mathematical formalism of
the depolarisation detection concept are given. SIMONE
depolarisation measurements in spheroidal hematite aerosol and
supercooled liquid clouds are used to validate the instrument.
Revised: 28 October 2012 – Accepted: 30 October 2012 – Published: 09 November 2012
SIMONE data from a series of AIDA ice nucleation experiments at
temperatures between 195 and 225 K were analysed in terms of the
impact of the ice particle microphysics on δp. We
found strong depolarisation values of up to 0.4 in case of small
growing and sublimating ice particles with volume equivalent
diameters of only a few micrometers.
Modelling runs with the T-matrix method showed that the measured
depolarisation ratios can be accurately reproduced assuming
spheroidal and cylindrical particles with a size distribution that
has been constrained by IR extinction spectroscopy. Based on the
T-matrix modelling runs, we demonstrate that in case of small ice
crystals the SIMONE depolarisation results are representative for
the lidar depolarisation ratio which is measured at exact
backscattering direction of 180°.
The relevance of our results for the interpretation of recent lidar
observations in cirrus and contrails is discussed. In view of our
results, the high depolarisation ratios observed by the spaceborne
lidar CALIOP in the tropical upper troposphere might be a hint for
the presence of small (sublimating) ice particles in the outflows of
deep convective systems.
Citation: Schnaiter, M., Büttner, S., Möhler, O., Skrotzki, J., Vragel, M., and Wagner, R.: Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics, Atmos. Chem. Phys., 12, 10465-10484, doi:10.5194/acp-12-10465-2012, 2012.