Differential reflectivity calibration and antenna temperature

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 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) 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.