A ZDR Calibration Check Using Hydro-Meteors in the Ice Phase

Monitoring the calibration of ZDR during real-time operations is now recognised as a necessary data-quality component in the use of a dual-polarization radar for quantitative purposes, for example for QPE. Certain classes of hydrometeor have reasonably well-known ZDR signatures - drizzle drops at specified reflectivities are an example. Similarly, a study of the particles in the ice phase of convective storms and MCSs during the PECAN field project revealed that the underlying distribution of ZDR appears predictable, and it may therefore be used as a check on ZDR calibration. Using the NCAR S-Pol radar allowed us to check the results of the technique against those from other ZDR calibration methods, such as vertical pointing scanning and the cross-polar power technique. The method has the advantage that it does not disrupt normal scanning of the radar. Furthermore, since the technique may be applied to radar volumes over a long period of time, it permits the study of variability in ZDR bias as the temperature at the radar site changes, which is useful since it is now known that ZDR bias has some dependence on temperature at the radar. In addition to the PECAN data sets, we have applied the method to the NCAR S-Pol radar data from DYNAMO, and to NEXRAD data sets during OLYMPEX. The results are presented.