Dual-pol Radar Calibration and Correction of the Bias Introduced by Non Uniform Radome Wetting

The increasing interest for polarimetric measurements in operational radar systems requires careful consideration of calibration issues. Arpa Piemonte manages two C-band polarimetric and Doppler radars, located near Torino in Bric della Croce at 0.74 km MSL and near Savona in Monte Settepani over the Apennines at 1.40 km MSL, respectively. Both radar antennas are covered by radome, which is considered an essential protection to guarantee continuous operation during severe weather conditions. The Settepani site, where strong winds often coupled with freezing rain or snow are likely to occur from November to April is particularly critical. Besides periodical electronic calibration, we exploit properties of natural scatterers in order to calibrate ZDR and ZH, using data from the operational volume scans. The self-consistency principle (Gorgucci et al. 1992, Scarchilli et al. 1996) is here considered extensively, not only for absolute radar calibration, but also for estimating the bias introduced by radome wetting. It is found that rain above the radar may cause a highly anisotropic attenuation depending on the wind intensity and direction, which tend to accumulate water preferably on the upwind side of the radome. The effect, especially relevant at the Settepani site at 1.4 km MSL, frequently interested by intense winds, may severely affect reflectivity measurements. Results of radar calibration procedure and azimuth-dependent bias corrections are presented.