Validation of the rain profiling algorithm "ZPHI'' from the C-band polarimetric weather radar in Darwin

An extensive application of a rain profiling algorithm (ZPHI) employing a C -band polarimetric radar (the C-POL radar of the Australian Bureau of Meteo rology Research Centre in Darwin) is presented. ZPHI belongs to the class o f rain profiling algorithms that have been developed for spaceborne or airb orne radars operating at attenuating frequencies. By nature, these algorith ms are nonlocal: the full profile of the measured radar reflectivity is inv erted to derive a retrieved profile of the rainfall rate. The retrieval acc uracy lays in the imposition of an "external constraint'' in the inversion procedure. In this case, that is supplied by the differential phase shift P hi (DP). The primary products of ZPHI are the profile along the beam of the specific attenuation A, and the "normalized'' intercept parameter N-o*. Th e rainfall rate is further estimated through an R-A relation adjusted for N -o*. ZPHI solves automatically two problems met when operating at C band: t he along-path attenuation and the variability of the raindrop size distribu tion. Moreover, its robustness with respect to radar statistical error allo ws ZPHI to operate with short dwell times, important for operational applic ations. To provide high quality rain-rate retrieval, ZPHI requires careful radar ca libration. Two techniques of calibration checking are investigated; both pr ovide a calibration estimate to within 0.1 and 0.2 dB. One is based upon th e climatological stability of the N-o* histogram. The second, which is pure ly radar based, uses a consistency test between the current rain-rate estim ate by ZPHI and an estimate combining the specific attenuation A and the di fferential reflectivity Z(DR). Comparisons of rain rate, including an extensive dataset in the month of Ja nuary 1998, show a remarkable agreement between rain gauge data and the ZPH I estimate, whereas the "classical'' estimate (standard Z-R relation applie d without consideration of the attenuation) appears severely biased with re spect to the rain gauges. In these comparisons, evidence for the crucial ro le of an N-o* determination to improve the rain-rate estimate is provided.