Rainfall estimation from an operational S-band dual-polarization radar: Effect of radar calibration

The calibration biases of reflectivity (Z_H) and differential reflectivity (Z_DR) from an operational Mt. Bisl S-band dual polarization radar are derived to improve the accuracy of rainfall estimation. The effect of radar calibration in rain estimation is examined by using data from the dense rain gauge network. The calibration biases of Z_H are calculated by using the self-consistency constraint between Z_H and specific differential phase shift (K_DP ). This procedure is performed every 2.5 min. The biases are varied from −3.3 dB to 0.8 dB during the period between July 2010 to October 2011. The Z_DR calibration biases are obtained by two methods: 1) vertically pointing measurements, and 2) comparison of observed data with the average Z_H–Z_DR relationship derived from disdrometric data. The Z_DR biases are varied from 0.25 dB to 0.7 dB and both methods show similar results. This Z_H–Z_DR technique can be applied for a volume scan and does not require a special scan. The rainfall relationships, R(Z_H), R(Z_H, Z_DR) and R(Z_H, ξ_DR), where ξ_DR = 10^0.1ZD_R , are derived from measured disdrometer data and then adjusted with gauge data. The verification of rainfall estimation is performed by applying 1) average Z_H and Z_DR calibration biases for the entire period and 2) adaptive calibration biases that vary each rain event. The application of adaptive calibration biases is more effective for R(Z_H, Z_DR ) and R(Z_H, ξ_DR) than that for R(Z_H), thus indicating the necessity of frequent calibration of Z_H and Z_DR.