Calibration of Zdr for an S-band polarimetric radar

The polarimetric radar variable of differential reflectivity (Zdr) has demonstrated application for increasing the accuracy of radar rainfall estimates, providing information for hydrometeor identification, and for clutter discrimination. The addition of Zdr measurement capability to the national network of WSR-88D radars is being considered. However, Zdr is a difference between two large quantities, each of limited absolute accuracy, creating a problem of compounded numerical error. An error of as little as .3 dB in Zdr can lead to precipitation estimate errors of about 17%, reducing benefits that may be realized through use of this parameter.

Reduction of the error in Zdr has received considerable recent attention from NCAR's radar systems group during the evolution of the NCAR S-Pol radar. Improvements to Zdr accuracy have resulted from 1) refinements to episodic vertical pointing methodology for Zdr bias correction, 2) use of the sun for routine, periodic monitoring of differential changes in system gains, 3) the utility of continuous monitoring of Zdr stability through use of a test pulse, and 4) a more complete consideration of hardware issues such as dual receiver and dual T/R tube configurations.

The various procedures are summarized, with examples provided from several field campaigns. The results indicate that the calibration techniques can provide absolute accuracy of Zdr to about +/- 0.05 dB. The procedures can be automated, and several of these may be applied directly to any Zdr-capable system, including NEXRAD.