Test of the Specific Differential Propagation Phase Shift (KDP) Technique for Rain-Rate Estimation with a Ku-Band Rain Radar

The variation in drop size distribution (DSD) and the attenuation at higher frequencies are the two major impairments for quantitative rain-rate estimation. The sensitivity of rain-rate estimators (such as reflectivity factor Z, differential reflectivity Z(DR), and the specific differential propagation phase shift KDP), to the variations in DSD, raindrop shape parameter, and also to the variation in temperature, is examined at 13.8 GHz using the T-matrix procedure. It has been found that KDP is not only less sensitive to the variations in these physical quantities but is also linearly related to rain rate. The degree of deviation in KDP due to raindrop shape variation is almost comparable to that due to the DSD variations. The computed phase shift upon backscattering, delta is a very large quantity at 13.8 GHz (e.g., delta = 21 degrees for a raindrop with 6.5 mm diameter). It has been noticed that delta is almost comparable to KDP and even higher than KDP, especially at lower rain rates. Nevertheless, through proper utilization of the theoretically observed limits of delta, a scheme is presented that includes a suitable smoothing filter combined with averaging over space and time techniques for estimating the KDP from the profile of the differential propagation phase shift phi(DP). The KDP thus derived is found to be a better rain-rate estimator than reflectivity alone. The observed good agreement between the KDP estimated rain rates and those measured by the disdrometer indicates that KDP can be a better estimator at least for the uniform as well as intense rainfall (>40 mm h(-1)) at Ku-band frequencies.