Validation of GPM DPR rainfall and Drop Size Distribution through disdrometers in the Northeastern Iberian Peninsula

Assessing global precipitation trends with some degree of certainty under climate change is a challenge for the scientific community. Therefore, accurate and reliable observations of this variable are required on a global scale that extend to climatological time scales. For this purpose, the Dual-frequency Precipitation Radar (DPR) aboard the Core Satellite of the Global Precipitation Measurement (GPM) mission is currently the only active sensor able to provide, at global scale, three-dimensional measurements of the structure and characteristics of precipitation. In this study, the performance of the variable's precipitation intensity, radar reflectivity factor (ZKu and ZKa) and Drop Size Distribution (DSD) parameters (weighted mean diameter, Dm; intercept parameter, Nw) of the GPM DPR are evaluated. For this purpose, information from seven disdrometers (OTT Parsivels1, 2) located in different topographic areas of the northeast of the Iberian Peninsula is used as reference. Comparison of the DSD parameters show an overestimation of Dm approximately 0.1 mm at low and moderate precipitation rates (0.1-4 mm/h) and of 0.4 mm at precipitation rates higher than 4 mm/h by the dual-frequency DPR algorithm regarding the disdrometer. In contrast, the performance of Nw is underestimated by the DPR, with a maximum value close to 6 dB at moderate precipitation rates (4-8 mm/h). The lowest errors were observed regarding radar reflectivity factor and Dm, while the agreement was poorer considering the Nw and rainfall index.  This lack of accuracy of the GPM DPR rainfall index measurement in Catalonia could be due to the use of predefined constants in the relationships between the rainfall rate and the Dm on which its algorithm is based. This suggests an in-depth investigation of the retrieval algorithms adopted to improve their performance, which requires more disdrometer data to increase precipitation sampling opportunities.