A Methodology for Estimating the Parameters of a Gamma Raindrop Size Distribution Model using Redundancy of the Polarization Observations at Attenuating Frequency

A method for estimating three parameters of a gamma raindrop size distribution (DSD) model from polarimetric radar at attenuating frequency was developed. This DSD retrieval method is fully automated and uses the consistency between radar reflectivity, differential reflectivity, and the path integral of specific differential phase to take into account the attenuation effect along the range profile including the wet radome. The proposed method does not require any assumptions of relationship among DSD parameters or simplifications of equations that describe relationship between the axis ratio and diameter of raindrops, which have been used in previous studies. Moreover, the proposed algorithm needs no external reference data such as two-dimensional video disdrometer (2DVD) measurements for attenuation corrections because it retrieves the co-polar and differential specific attenuation from the interrelation among the polarimetric measurements. In addition, this algorithm is available for dual-pol radar PPI data analysis in real time because it is fully automated.

     The performance of this algorithm was evaluated by comparison with optical disdrometers and a weighing precipitation gauge. The evaluation of the algorithm showed that the retrieved three DSD parameters of raindrops, reflectivity, and differential reflectivity from actual C-band polarimetric radar data have fairly good agreement with those obtained by surface measurements. Moreover, the rainfall rates retrieved using this algorithm have comparable precision with those estimated from the specific differential phase, and outperform those estimated through the so-called Z–R relationship, particularly during heavy rainfall.