Calibration of Linearly Polarized Weather Radar for Accurate Measurements of Circular Depolarization Ratio

The circular depolarization ratio (CDR) measurements have recently regained interest within the weather radar community because of their potential insight into the physical interpretation of polarimetric radar measurements above the melting layer. The acquisition of CDR measurement using a linearly polarized weather radar with simultaneous transmission and reception has recently been proposed by scientists from the National Severe Storms Laboratory (NSSL) and implemented in C-band and S-band radar systems manufactured by Enterprise Electronics Corporation (EEC). An obvious advantage of the proposed CDR measurements over the legacy linear depolarization ratio (LDR) measurements is that the high-power mechanical waveguide switches necessary to enable the LDR mode of operations are no longer required. As an added benefit, CDR measurements are made in the simultaneous transmission mode, eliminating the need for a special LDR mode where the measurements of differential reflectivity and cross-correlation coefficient are disabled. This innovation in weather radar would bring many benefits, including the reduction of cost, simplification of system architecture, higher reliability, and more efficient data acquisition strategies.

It is important that the accurate CDR measurements require dedicated efforts in the system calibration. The major two issues include the calibration of differential phase upon transmission in H/V transmitters and the calibration of differential phase and power upon reception in H/V receivers. The current paper addresses the theoretical basis of the CDR measurements and the necessary calibration requirements. The effect of miscalibration is analyzed using theoretical simulations. EEC has applied a special method and process for their radar systems to implement the CDR calibration. The practical procedure is also detailed for the entire calibration process. The radar calibration for CDR measurements is demonstrated with the EEC's in-house C-band polarimetric weather radar. Analysis results show that the calibration process is effective and the proposed CDR measurements reveal important features of the storm microphysics.