Relationship between temperature and apparent shape of pristine ice crystals derived from polarimetric cloud radar observations during the ACCEPT campaign
Atmos. Meas. Tech., 9, 3739–3754, 2016
www.atmos-meas-tech.net/9/3739/2016/
doi:10.5194/amt-9-3739-2016
© Author(s) 2016. CC Attribution 3.0 License.
Relationship between temperature and apparent shape of pristine
ice crystals derived from polarimetric cloud radar observations
during the ACCEPT campaign
Alexander Myagkov1,a , Patric Seifert1 , Ulla Wandinger1 , Johannes Bühl1 , and Ronny Engelmann1
1 Leibniz Institute for Tropospheric Research (TROPOS), Permoserstr. 15, 04318, Leipzig, Germany
a Radiometer Physics GmbH (RPG), Werner-von-Siemens-Str. 4, 53340 Meckenheim, Germany
Correspondence to: Alexander Myagkov ()
Received: 24 November 2015 – Published in Atmos. Meas. Tech. Discuss.: 15 January 2016
Revised: 5 June 2016 – Accepted: 15 June 2016 – Published: 12 August 2016
Abstract. This paper presents first quantitative estimations
of apparent ice particle shape at the top of liquid-topped
clouds. Analyzed ice particles were formed under mixedphase conditions in the presence of supercooled water and
in the temperature range from − 20 to −3 ◦ C. The estimation is based on polarizability ratios of ice particles measured
by a Ka-band cloud radar MIRA-35 with hybrid polarimetric
configuration. Polarizability ratio is a function of the geometrical axis ratio and the dielectric properties of the observed
hydrometeors. For this study, 22 cases observed during the
ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) field campaign were used.
Polarizability ratios retrieved for cloud layers with the cloudtop temperatures of ∼ −5, ∼ −8, ∼ −15, and ∼ −20 ◦ C
were 1.6, 0.9, 0.6, and 0.9, respectively. Such values correspond to prolate, quasi-isotropic, oblate, and quasi-isotropic
particles, respectively. Data from a free-fall chamber were
used for the comparison. A good agreement of detected apparent shapes with well-known shape–temperature dependencies observed in laboratories was found. Polarizability ratios used for the analysis were estimated for areas located
close to the cloud top, where aggregation and riming processes do not strongly affect ice particles. We concluded
that, in microwave scattering models, ice particles detected
in these areas can be assumed to have pristine shapes. It was
also found that even slight variations of ambient conditions
at the cloud top with temperatures warmer than ∼ −5 ◦ C can
lead to rapid changes of ice crystal shape.
1
Introduction
Mixed-phase clouds are a crucial component of the Earth’s
climate system. Their long-lasting nature impacts the radiative budget and the thermodynamic structure of the atmosphere (Sun and Shine, 1995) and microphysical processes
occurring in mixed-phase clouds are the main source of precipitation (Mülmenstädt et al., 2015).
Ground-based remote sensing has shown a large potential
for improving the understanding of the life cycle of mixedphase clouds (Hogan et al., 2003; Ansmann et al., 2009; De
Boer et al., 2009; Delanoë and Hogan, 2010; Kanitz et al.,
2011; Westbrook and Illingworth, 2013). Even though microphysical retrieval techniques based on ground-based remote
observations are a valuable source of information for the investigation of mixed-phase clouds, further developments are
required in order to increase the accuracy of these retrievals.
From the remote-sensing perspective, mixed-phase clouds
with a single supercooled liquid layer at the top and ice virgae below are of special interest (Wang et al., 2004; Smith
et al., 2009). Below we denote such clouds as single-layer
clouds. Single-layer clouds have less complex microphysical
and dynamical properties (Fleishauer et al., 2002; Ansmann
et al., 2009; Zhang et al., 2012) compared to convective cloud
systems where more than 25 different transfer processes may
take place (Seifert and Beheng, 2006; Tao and Moncrieff,
2009). Thus, studying ice formation in single-layer clouds
is key to obtaining a comprehensive picture of the formation
of pristine ice crystals under ambient conditions.
Long-term polarimetric lidar observations showed that the
majority of ice crystals in mixed-phase clouds are formed
Published by Copernicus Publications on behalf of the European Geosciences Union.
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A. Myagkov et al.: Shape–temperature relationship of pristine ice crystals
heterogeneously within a supercooled liquid layer (De Boer
et al., 2011). Westbrook and Illingworth (2011) reported
that about 95 % of ice particles at temperatures warmer than
−20 ◦ C originated from liquid-water particles. Thus, ambient
conditions at the top of single-layer clouds play a crucial role
in the formation of ice particles. Microphysical properties of
pristine ice crystals under controlled ambient conditions have
been intensively investigated in laboratories. In situ measurements in free-fall chambers provide information about mass,
size, shape, apparent density, and fall velocity of ice crystals
at different stages of their development (Fukuta, 1969; Takahashi et al., 1991; Fukuta and Takahashi, 1999; Takahashi,
2014). Such studies provide extremely accurate information
that can be used for the interpretation of remote observations
and validation of retrieval techniques.
Important, yet barely explored, parameters are the shape
and apparent density of an ice crystal population. Estimates
of ice mass, area, or number concentration require accurate
knowledge of particle shape (Westbrook and Heymsfield,
2011; Delanoë et al., 2014). Radar polarimetry is known
to be a powerful tool for the classification of microphysical
properties of hydrometeors such as ice crystals under ambient conditions. In recent publications of Bühl et al. (2016)
and Oue et al. (2015), vertically pointed cloud radars with
linear depolarization ratio (LDR) mode were used for qualitative discrimination between columnar-shaped ice particles
and those of other types. In LDR mode a radar transmits a
horizontally polarized wave and receives horizontal (copolarized) and vertical (cross-polarized) components of the returned signal. LDR is calculated as a ratio of the power in
the cross-polarized channel over the power in the copolarized channel. Quantitative shape estimations in LDR mode
are limited by the strong dependence of polarimetric observations on canting angles of cloud particles (Matrosov
et al., 2001). Melnikov and Straka (2013) proposed an algorithm for the estimation of shape and orientation of particles based on differential reflectivity ZDR and correlation
coefficient ρHV from a polarimetric weather radar with hybrid mode. This mode employs a simultaneous transmission
of horizontally and vertically polarized components of the
electromagnetic wave and simultaneous reception of signals
in the horizontal and vertical channels. ZDR is calculated as a
ratio of the power received in the horizontal channel over the
power received in the vertical channel. ρHV is the correlation
between the complex amplitudes of the received pulse sequences in the horizontal and vertical receivin (...truncated)
