Circular depolarization ratios of single water droplets and finite ice circular cylinders: a modeling study 1Institute for Atmospheric and Climate Science, ETH Zurich, 8092, Zurich, Switzerland
11 May 2012
2Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany
Received: 08 Oct 2011 – Published in Atmos. Chem. Phys. Discuss.: 08 Nov 2011 Abstract. Computations of the phase matrix elements for single water droplets and ice
crystals in fixed orientations are presented to determine if circular
depolarization δC is more accurate than linear depolarization
for phase discrimination. T-matrix simulations were performed to calculate
right-handed and left-handed circular depolarization ratios δ+C,
respectively δ−C and to compare them with linear ones. Ice crystals
are assumed to have a circular cylindrical shape where their
surface-equivalent diameters range up to 5 μm. The circular
depolarization ratios of ice particles were generally higher than linear
depolarization and depended mostly on the particle orientation as well as
their sizes. The fraction of non-detectable ice crystals (δ<0.05) was
smaller considering a circular polarized light source, reaching 4.5%.
However, water droplets also depolarized light circularly for scattering
angles smaller than 179° and size parameters smaller than 6 at side-
and backscattering regions. Differentiation between ice crystals and water
droplets might be difficult for experiments performed at backscattering
angles which deviate from 180° unlike LIDAR applications.
Instruments exploiting the difference in the P44/P11 ratio at a
scattering angle around 115° are significantly constrained in
distinguishing between water and ice because small droplets with size
parameters between 5 and 10 do cause very high circular depolarizations at
this angle. If the absence of the liquid phase is confirmed, the use of
circular depolarization in single particle detection is more sensitive and
less affected by particle orientation.
Revised: 25 Apr 2012 – Accepted: 27 Apr 2012 – Published: 11 May 2012
Citation: Nicolet, M., Schnaiter, M., and Stetzer, O.: Circular depolarization ratios of single water droplets and finite ice circular cylinders: a modeling study, Atmos. Chem. Phys., 12, 4207-4214, doi:10.5194/acp-12-4207-2012, 2012.