Cloud radar with hybrid mode towards estimation of shape and orientation of ice crystals

摘要

This paper is devoted to the experimental quantitative characterization ofthe shape and orientation distribution of ice particles in clouds. Thecharacterization is based on measured and modeled elevation dependencies ofthe polarimetric parameters differential reflectivity and correlationcoefficient. The polarimetric data are obtained using a newly developed35 GHz cloud radar MIRA-35 with hybrid polarimetric configuration andscanning capabilities. The full procedure chain of the technicalimplementation and the realization of the setup of the hybrid-mode cloudradar for the shape determination are presented. This includes thedescription of phase adjustments in the transmitting paths, the introductionof the general data processing scheme, correction of the data for thedifferences of amplifications and electrical path lengths in the transmittingand receiving channels, the rotation of the polarization basis by45°, the correction of antenna effects on polarimetric measurements,the determination of spectral polarimetric variables, and the formulation ofa scheme to increase the signal-to-noise ratio. Modeling of the polarimetricvariables is based on existing back-scattering models assuming the spheroidalrepresentation of cloud scatterers. The parameters retrieved from the modelare polarizability ratio and degree of orientation, which can be assigned tocertain particle orientations and shapes. The developed algorithm is appliedto a measurement of the hybrid-mode cloud radar taken on 20 October 2014 inCabauw, the Netherlands, in the framework of the ACCEPT (Analysis of theComposition of Clouds with Extended Polarization Techniques) campaign. Thecase study shows the retrieved polarizability ratio and degree of orientationof ice particles for a cloud system of three cloud layers at differentheights. Retrieved polarizability ratios are 0.43, 0.85, and 1.5 whichcorrespond to oblate, quasi-spherical, and columnar ice particles,respectively. It is shown that the polarizability ratio is useful for thedetection of aggregation/riming processes. The orientation of oblate andprolate particles is estimated to be close to horizontal whilequasi-spherical particles were found to be more randomly oriented.