Monitoring Radar System ZDR Accuracy: S and C-Band Operational Experiences in the US and Germany

The US and German weather radar networks are using scans of external targets to estimate radar system bias in support of Differential Reflectivity (ZDR) calibration. While the C-Band German radars can use vertical scanning in rain to measure bias, the US S-Band WSR-88D system is limited to 360 degree azimuthal scans below 20 degrees elevation. The US system uses these operational volume scans to find regions containing suitable scatterers, (e.g., light rain below the melting layer, dry snow above the melting layer, and Bragg scatter in clear air) and compares the return with expected intrinsic ZDR values. The German system employs purposeful vertical scans on a frequent basis, taking advantage of the symmetry of scattering principle that the mean ZDR in light rain with a vertical scan should be zero. The statistics of returns, including bias and variability, need to be understood in order for scans of hydrometeors to be used for accurate bias estimation. Comparisons of vertical and non-vertical incidence scanning methods may prove useful in pursuit of these statistics. The US and German radars both employ daily sunspike monitoring to assess ZDR bias and to monitor antenna position accuracy. This paper reviews efforts from both countries related to the utility and variability (including seasonal and geographical availability) of these methods for the S-band WSR-88D and the German C-band polarimetric radars. The relevance of the external methods for diagnosing hardware problems will also be discussed.