Monitoring Radar Calibration Using Ground Clutter

The WSR-88D radars feature hardware and a supporting program package to calibrate reflectivity (Z) and differential reflectivity (Z_DR). To verify the Z_DR calibration procedure, three methods are used on the network: measurements in light rain, in snow areas above the melting layer, and in areas of Bragg scatter. The process also takes advantage of occasional solar noise power estimates (sun spikes) to monitor the combined antenna/receiver path bias. These methods have their limitations, the strongest of which is the presence of appropriate weather conditions that limits continuous verification of Z_DR calibration. Ground clutter is observed always that potentially can be used for radar calibration, if some stable parameters of clutter can be found.

Radar returns from ground clutter in a spectral interval -0.5 to +0.5 m s^-1 are studied before applying ground clutter filters. The mean powers of these signals in the polarimetric channels exhibit weak time variations and can be used to monitor stability of a radar system in fair weather. If the radars are in a good technical state, the variations of mean clutter powers in the channels are within ±1 dB and Z_DR values are within an interval of ±0.1 dB. These variations weakly depend on temperature and the wind near the ground. Therefore these signals can be used for continuous monitoring of radar calibration. This approach could be used to verify the calibration procedures in radars having built-in equipment (WSR-88Ds) and in radars without such equipment.

Examples of deviations in the mean signal parameters from ground clutter caused by radar hardware issues are discussed. It is shown that a wet radar radome decreases the mean power from ground clutter and increases its Z_DR. Correlation between the power drop and the rain rate on the radar site is demonstrated, which is attributed to attenuation of the radar signal for a wet radome. The drop in signal from ground clutter could be used to estimate a decrease in Z from weather objects.