Validation of the Rain Profiling Algorithm “ZPHI” from the C-Band Polarimetric Weather Radar in Darwin

Abstract

An extensive application of a rain profiling algorithm (ZPHI) employing a C-band polarimetric radar (the C-POL radar of the Australian Bureau of Meteorology Research Centre in Darwin) is presented. ZPHI belongs to the class of rain profiling algorithms that have been developed for spaceborne or airborne radars operating at attenuating frequencies. By nature, these algorithms are nonlocal: the full profile of the measured radar reflectivity is inverted to derive a retrieved profile of the rainfall rate. The retrieval accuracy lays in the imposition of an ?external constraint? in the inversion procedure. In this case, that is supplied by the differential phase shift ΦDP. The primary products of ZPHI are the profile along the beam of the specific attenuation A, and the ?normalized? intercept parameter N*0. The rainfall rate is further estimated through an R?A relation adjusted for N*0. ZPHI solves automatically two problems met when operating at C band: the along-path attenuation and the variability of the raindrop size distribution. Moreover, its robustness with respect to radar statistical error allows ZPHI to operate with short dwell times, important for operational applications. To provide high quality rain-rate retrieval, ZPHI requires careful radar calibration. Two techniques of calibration checking are investigated; both provide a calibration estimate to within 0.1 and 0.2 dB. One is based upon the climatological stability of the N*0 histogram. The second, which is purely radar based, uses a consistency test between the current rain-rate estimate by ZPHI and an estimate combining the specific attenuation A and the differential reflectivity ZDR. Comparisons of rain rate, including an extensive dataset in the month of January 1998, show a remarkable agreement between rain gauge data and the ZPHI estimate, whereas the ?classical? estimate (standard Z?R relation applied without consideration of the attenuation) appears severely biased with respect to the rain gauges. In these comparisons, evidence for the crucial role of an N*0 determination to improve the rain-rate estimate is provided.