System ZDR of the WSR-88D: Hardware Issues and Temperature Dependence

Dual polarization radar measurements require well calibrated radars. Radar quantitative precipitation estimates using differential reflectivity (Z_DR) requires measurements of this variable with an accuracy of 0.1 dB. The WSR-88D radar features built-in calibration devices located in a climate controlled box. Some of the radar devices, e.g., the low noise amplifiers, waveguides, antenna are out of climate control and experience ambient temperature/humidity impacts. Results of a study of the system Z_DR at various ambient temperatures are presented. Some methods that potentially can compensate for the temperature variations are discussed.

It is also shown that the IF Digital Receiver can introduce a Z_DR bias larger than desired 0.1 dB. Non linearity of the low noise amplifiers also contributes to the system Z_DR bias. A potential calibration procedure to eliminate these biases is proposed. Automatic procedures for refining the system Z_DR bias due to the above issues are discussed.