Temporal differential reflectivity Z(dr) bias variations are investigated using the National Center for Atmospheric Research (NCAR) S-band dual-polarization Doppler radar (S-Pol). Using data from the Multi-Angle Snowflake Camera-Ready (MASCRAD) Experiment, S-Pol measurements over extended periods reveal a significant correlation between the ambient temperature at the radar site and the Z(dr) bias. Using radar scans of the sun and the ratio of cross-polar powers, the components of the radar that cause the variation of the Z(dr) bias are identified. It is postulated that the thermal expansion of the antenna is likely the primary cause of the observed Z(dr) bias variation. The cross-polar power (CP) Z(dr) calibration technique, which is based on the solar and cross-polar power measurements, is applied to data from the Plains Elevated Convection at Night (PECAN) field project. The Z(dr) bias from the CP technique is compared to vertical-pointing Z(dr) bias measurements, and the uncertainty of the Z(dr) bias estimates is given. An algorithm is derived to correct the radar data for the time-and temperature-varying Z(dr) bias. Bragg scatter measurements are used to corroborate the CP technique Z(dr) bias measurements.
