A 35-GHz cloud radar with the hybrid polarimetric configuration

Radar polarimetry is one of the important tools for the remote sensing of the atmosphere. Radar systems that are operated in the so-called hybrid polarimetric configuration allow the calculation of polarimetric variables such as differential reflectivity, correlation coefficient, and differential phase shift based on the measurement of the coherency matrix. To date this technique was mainly applied to polarimetric weather radars where it has shown a high efficiency for the detection of dangerous weather phenomena, retrieval of precipitation intensity, and classification of different atmospheric scatterers.

Modern cloud radars also have polarimetric capabilities. A number of studies is devoted to polarimetric applications in cloud radars. Nevertheless, many of operational cloud radars are operating in the so called linear-depolarization mode and permit to calculate only one polarimetric variable – LDR (linear depolarization ratio). The shape estimation in the linear-depolarization mode is limited by the strong dependence of polarimetric measurements on the orientation of ice particles and antenna properties.

To get the advantage of a set of polarimetric variables besides LDR we built a 35-GHz cloud radar of type MIRA35 with the hybrid mode. The radar transmits horizontal and vertical polarization components simultaneously. The phase shift between the components was adjusted to 0°. The standard software of MIRA35 was modified to be able to calculate spectral polarimetric parameters such as differential reflectivity, differential phase shift, correlation coefficient, slanted linear depolarization ratio, and co-cross-channel correlation coefficient. The radar was mounted on a trailer into a scanning unit permitting to make full scans in azimuth and half-scans in elevation.

The implementation of the radar was realized in a collaboration of Leibniz Institute for Tropospheric Research (TROPOS, Leipzig, Germany) and the cloud radar producer METEK GmbH (Elmshorn, Germany) within the Initial Training Network ITARS.

In this presentation we show (1) issues concerned with technical implementation including the phase adjustments, (2) polarization calibration of the radar using vertical observations in light rain, and (3) estimation of the accuracy of the measured polarimetric variables. The application of the radar to the estimation of particle shape and orientation will be shown on a separate poster.