Monitoring radar ZDR calibration using ground clutter

Differential reflectivity (ZDR) is one of the main products of dual polarization weather radars. The accuracy of ZDR measurement should be 0.1 dB which is needed for accurate measurements of light rain and snow. The WSR-88Ds have hardware and software to calibrate ZDR with a potential to achieve 0.1-dB accuracy. To verify the system calibration, external targets such as drizzle, snow, and Bragg scatter are used. The Bragg scatter method is the most accurate one as shown by data from operational WSR-88Ds. The observations also show that Bragg scatter can be masked by atmospheric biota and cannot be used all the time. ZDR calibration using light rain and snow also depends on the presence of appropriate weather. To constantly monitor the radar calibration, a method that could be used at any time is needed. Returns from ground clutter have been studied for such a purpose.

A method of signal processing to monitor the system ZDR using ground clutter is discussed. It is shown that few spectral components of ground clutter are sufficiently stable for the monitoring. The performance of the method under various weather conditions such as the wind near the ground, rain, wetness of the ground, seasonal foliage, and insect echoes have been analyzed. The implementation of the method within the operational WSR-88D's Volume Coverage Patterns is discussed.