Differential power calibration for weather radar

Dual polarized weather radars, radars that can transmit both horizontal (H) and vertical (V) polarized waves, are becoming more common in the world. Dual polarization is accomplished via fast alternate transmission of H and V polarization or simultaneous H and V polarization transmission. The simultaneous H and V transmit mode is becoming quite popular since a high power, costly, polarization switch is not required. In either case, such radars can measure copolar differential reflectivity (Zdr) and copolar differential phase (φdp) which have been shown to significantly improve identification and quantification of precipitation, if measured accurately. In fact, The United States National Weather Service (NWS) network of NEXRAD radars is scheduled to be upgraded to the simultaneous H and V dual polarization mode with in the next few years. Critical to improved rainfall estimation with dual polarization radar is maintaining radar instrument calibration uncertainty of Zdr to about 0.1 dB. NCAR (National Center for Atmospheric Research) has been tasked by the NWS (National Weather Service) to investigate methods to calibrate Zdr using S-Pol, NCAR’s S-band polarimetric radar. Three methods for Zdr calibration are presented and operational implementation is discussed. In addition, a novel way to track the change in Zdr due to drift of the active portion of the receiver paths is given.