Monitoring the differential reflectivity and receiver calibration of the German polarimetric weather radar network

It is a challenge to calibrate differential reflectivity ZDR to within 0.1-0.2 dB uncertainty for dual-polarization weather radars that operate 24/7 throughout the year. During operations, a temperature sensitivity of Z(DR) larger than 0.2 dB over a temperature range of 10 degrees C has been noted. In order to understand the source of the observed Z(DR) temperature sensitivity, over 2000 dedicated solar box scans, two-dimensional scans of 5 degrees azimuth by 8 degrees elevation that encompass the solar disk, were made in 2018 from which horizontal (H) and vertical (V) pseudo antenna patterns are calculated. This assessment is carried out using data from the Hohenpeissenberg research radar which is identical to the 17 operational radar systems of the German Meteorological Service (Deutscher Wetterdienst, DWD). Z(DR) antenna patterns are calculated from the H and V patterns which reveal that the Z(DR) bias is temperature dependent, changing about 0.2 dB over a 12 degrees C temperature range. One-point-calibration results, where a test signal is injected into the antenna cross-guide coupler outside the receiver box or into the low-noise amplifiers (LNAs), reveal only a very weak differential temperature sensitivity (< 0.02 dB) of the receiver electronics. Thus, the observed temperature sensitivity is attributed to the antenna assembly. This is in agreement with the NCAR (National Center for Atmospheric Research) S-Pol (S-band polarimetric radar) system, where the primary Z(DR) temperature sensitivity is also related to the antenna assembly (Hubbert, 2017). Solar power measurements from a Canadian calibration observatory are used to compute the antenna gain and to validate the results with the operational DWD monitoring results. The derived gain values agree very well with the gain estimate of the antenna manufacturer. The antenna gain shows a quasi-linear dependence on temperature with different slopes for the H and V channels. There is a 0.6 dB decrease in gain for a 10 degrees C temperature increase, which directly relates to a bias in the radar reflectivity factor Z which has not been not accounted for previously. The operational methods used to monitor and calibrate Z(DR) for the polarimetric DWD C-band weather radar network are discussed. The prime sources for calibrating and monitoring Z(DR) are birdbath scans, which are executed every 5 min, and the analysis of solar spikes that occur during operational scanning. Using an automated Z(DR) calibration procedure on a diurnal timescale, we are able to keep Z(DR) bias within the target uncertainty of +/- 0.1 dB. This is demonstrated for data from the DWD radar network comprising over 87 years of cumulative dual-polarization radar operations.