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  • Atmos. Meas. Tech., 9, 37393754, 2016www.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 pristineice crystals derived from polarimetric cloud radar observationsduring the ACCEPT campaignAlexander Myagkov1,a, Patric Seifert1, Ulla Wandinger1, Johannes Bhl1, and Ronny Engelmann11Leibniz Institute for Tropospheric Research (TROPOS), Permoserstr. 15, 04318, Leipzig, GermanyaRadiometer Physics GmbH (RPG), Werner-von-Siemens-Str. 4, 53340 Meckenheim, Germany

    Correspondence to: Alexander Myagkov (alexander.myagkov@radiometer-physics.de)

    Received: 24 November 2015 Published in Atmos. Meas. Tech. Discuss.: 15 January 2016Revised: 5 June 2016 Accepted: 15 June 2016 Published: 12 August 2016

    Abstract. This paper presents first quantitative estimationsof apparent ice particle shape at the top of liquid-toppedclouds. Analyzed ice particles were formed under mixed-phase conditions in the presence of supercooled water andin the temperature range from 20 to 3 C. The estima-tion is based on polarizability ratios of ice particles measuredby a Ka-band cloud radar MIRA-35 with hybrid polarimetricconfiguration. Polarizability ratio is a function of the geomet-rical axis ratio and the dielectric properties of the observedhydrometeors. For this study, 22 cases observed during theACCEPT (Analysis of the Composition of Clouds with Ex-tended Polarization Techniques) field campaign were used.Polarizability ratios retrieved for cloud layers with the cloud-top temperatures of 5, 8, 15, and 20 Cwere 1.6, 0.9, 0.6, and 0.9, respectively. Such values corre-spond to prolate, quasi-isotropic, oblate, and quasi-isotropicparticles, respectively. Data from a free-fall chamber wereused for the comparison. A good agreement of detected ap-parent shapes with well-known shapetemperature depen-dencies observed in laboratories was found. Polarizability ra-tios used for the analysis were estimated for areas locatedclose to the cloud top, where aggregation and riming pro-cesses do not strongly affect ice particles. We concludedthat, in microwave scattering models, ice particles detectedin these areas can be assumed to have pristine shapes. It wasalso found that even slight variations of ambient conditionsat the cloud top with temperatures warmer than5 C canlead to rapid changes of ice crystal shape.

    1 Introduction

    Mixed-phase clouds are a crucial component of the Earthsclimate system. Their long-lasting nature impacts the radia-tive budget and the thermodynamic structure of the atmo-sphere (Sun and Shine, 1995) and microphysical processesoccurring in mixed-phase clouds are the main source of pre-cipitation (Mlmenstdt et al., 2015).

    Ground-based remote sensing has shown a large potentialfor improving the understanding of the life cycle of mixed-phase clouds (Hogan et al., 2003; Ansmann et al., 2009; DeBoer et al., 2009; Delano and Hogan, 2010; Kanitz et al.,2011; Westbrook and Illingworth, 2013). Even though micro-physical retrieval techniques based on ground-based remoteobservations are a valuable source of information for the in-vestigation of mixed-phase clouds, further developments arerequired in order to increase the accuracy of these retrievals.From the remote-sensing perspective, mixed-phase cloudswith a single supercooled liquid layer at the top and ice vir-gae below are of special interest (Wang et al., 2004; Smithet al., 2009). Below we denote such clouds as single-layerclouds. Single-layer clouds have less complex microphysicaland dynamical properties (Fleishauer et al., 2002; Ansmannet al., 2009; Zhang et al., 2012) compared to convective cloudsystems where more than 25 different transfer processes maytake place (Seifert and Beheng, 2006; Tao and Moncrieff,2009). Thus, studying ice formation in single-layer cloudsis key to obtaining a comprehensive picture of the formationof pristine ice crystals under ambient conditions.

    Long-term polarimetric lidar observations showed that themajority of ice crystals in mixed-phase clouds are formed

    Published by Copernicus Publications on behalf of the European Geosciences Union.

  • 3740 A. Myagkov et al.: Shapetemperature relationship of pristine ice crystals

    heterogeneously within a supercooled liquid layer (De Boeret al., 2011). Westbrook and Illingworth (2011) reportedthat about 95 % of ice particles at temperatures warmer than20 C originated from liquid-water particles. Thus, ambientconditions at the top of single-layer clouds play a crucial rolein the formation of ice particles. Microphysical properties ofpristine ice crystals under controlled ambient conditions havebeen intensively investigated in laboratories. In situ measure-ments in free-fall chambers provide information about mass,size, shape, apparent density, and fall velocity of ice crystalsat different stages of their development (Fukuta, 1969; Taka-hashi et al., 1991; Fukuta and Takahashi, 1999; Takahashi,2014). Such studies provide extremely accurate informationthat can be used for the interpretation of remote observationsand validation of retrieval techniques.

    Important, yet barely explored, parameters are the shapeand apparent density of an ice crystal population. Estimatesof ice mass, area, or number concentration require accurateknowledge of particle shape (Westbrook and Heymsfield,2011; Delano et al., 2014). Radar polarimetry is knownto be a powerful tool for the classification of microphysicalproperties of hydrometeors such as ice crystals under ambi-ent conditions. In recent publications of Bhl et al. (2016)and Oue et al. (2015), vertically pointed cloud radars withlinear depolarization ratio (LDR) mode were used for quali-tative discrimination between columnar-shaped ice particlesand those of other types. In LDR mode a radar transmits ahorizontally polarized wave and receives horizontal (copo-larized) and vertical (cross-polarized) components of the re-turned signal. LDR is calculated as a ratio of the power inthe cross-polarized channel over the power in the copolar-ized channel. Quantitative shape estimations in LDR modeare limited by the strong dependence of polarimetric ob-servations on canting angles of cloud particles (Matrosovet al., 2001). Melnikov and Straka (2013) proposed an al-gorithm for the estimation of shape and orientation of par-ticles based on differential reflectivity ZDR and correlationcoefficient HV from a polarimetric weather radar with hy-brid mode. This mode employs a simultaneous transmissionof horizontally and vertically polarized components of theelectromagnetic wave and simultaneous reception of signalsin the horizontal and vertical channels. ZDR is calculated as aratio of the power received in the horizontal channel over thepower received in the vertical channel. HV is the correlationbetween the complex amplitudes of the received pulse se-quences in the horizontal and vertical receiving channel. Thedifferential reflectivity and the correlation coefficient are sen-sitive to shape, orientation, and dielectric properties of scat-terers (Straka et al., 2000). For the estimation, the authorsapproximated scattering properties of columnar-shaped andplate-like ice particles by prolate and oblate spheroids, re-spectively. Dielectric properties of ice particles depend onapparent ice density (Oguchi, 1983), which characterizes theratio of ice and air within the approximating spheroidal parti-cle. Observational weather radars are often operating nearly

    horizontally (at low elevation angles of the antenna). The au-thors claimed that under such conditions prolate and oblateparticles can have similar polarimetric signatures and cannotbe reliably distinguished. As a result the method proposed byMelnikov and Straka (2013) is only applicable for data withvalues of ZDR higher than 4 dB. Such ZDR can be only in-duced by strongly oblate particles, which, therefore, can beundoubtedly separated from prolate particles. In contrast toweather radars, cloud radars are often operated in modes withscanning in azimuth and elevation directions (Kollias et al.,2014; Lamer et al., 2014; Ewald et al., 2015). Matrosov et al.(2012) showed observational evidence that elevation depen-dencies of polarimetric variables can be efficiently used todiscriminate between different types of ice particles.

    In a previous study, Myagkov et al. (2016) combined thetwo mentioned approaches of Melnikov and Straka (2013)and Matrosov et al. (2012) into an algorithm for a quantita-tive characterization of shapes and orientations of ice parti-cles based on polarimetric observations from a newly devel-oped 35 GHz cloud radar with hybrid polarimetric configura-tion. This mode is widely used in polarimetric weather radarsbut is rarely implemented in cloud radars. The used cloudradar allowed for a calculation of the differential reflectivityand the correlation coefficients separately for every compo-nent of a Doppler spectrum (spectral line). Such informationcan be used for a characterization of particles moving withdifferent velocities. Note that within this study we only usepolarimetric variables calculated for the spectral line with themaximum amplitude (spectrum peak).

    In the case of a monostatic radar, polarimetric scatter-ing properties of scatterers are often described by a 2 2backscattering matrix. Elements of the backscattering ma-trix are scattering coefficients relating incident and scat-tered electric fields with certain polarizations. In the caseof Rayleigh scattering, i.e., when geometrical dimensions ofa particle are much smaller than the wavelength, the coeffi-cients of the backscattering matrix are functions of polariz-abilities of the particle along its axes. Values of polarizabili-ties depend on sizes along corresponding axes and dielectricproperties of the scatterer (Bringi and Chandrasekar, 2001).A spheroid is usually describe