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

Abstract

The calibration biases of reflectivity (ZH) and differential reflectivity (ZDR) 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 ZH are calculated by using the self-consistency constraint between ZH and specific differential phase shift (KDP ). 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 ZDR calibration biases are obtained by two methods: 1) vertically pointing measurements, and 2) comparison of observed data with the average ZH–ZDR relationship derived from disdrometric data. The ZDR biases are varied from 0.25 dB to 0.7 dB and both methods show similar results. This ZH–ZDR technique can be applied for a volume scan and does not require a special scan. The rainfall relationships, R(ZH), R(ZH, ZDR) and R(ZH, ξDR), where ξDR = 100.1ZDR , are derived from measured disdrometer data and then adjusted with gauge data. The verification of rainfall estimation is performed by applying 1) average ZH and ZDR 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(ZH, ZDR ) and R(ZH, ξDR) than that for R(ZH), thus indicating the necessity of frequent calibration of ZH and ZDR.