Three-Parameter Rain Drop Size Distributions From GPM Dual-Frequency Precipitation Radar Measurements: Techniques and Validation With Ground-Based Observations

Raindrop size distribution (DSD) parameters need to be estimated to determine rainfall from radar measurements of backscattered signals in the precipitating medium and to study microphysics of precipitation under varying atmospheric conditions. A technique is proposed to estimate the DSD parameters from Global Precipitation Measurement (GPM) dual-frequency products at Ku and Ka bands, which has been validated with ground-based disdrometer measurements obtained during the five-year period 2014–2018 over a tropical location, Kolkata ( $22^{\circ }54^{\prime} \text{N}$ , $88^{\circ }35^{\prime} \text{E}$ ), India. The present method yields the three parameters of the DSD model in terms of a gamma function, namely, intercept ( $N_{0}$ ), shape ( $\mu $ ), and slope ( $\Lambda$ ). Based on a proposed optimization technique, the most appropriate $\mu $ – $\Lambda $ relationship is determined during a GPM satellite pass to obtain the DSD parameters. The derived parameters have yielded mass-weighted mean diameter ( $D_{m}$ ) data that show much better agreement with ground-based DSD measurements than that provided by GPM dual-frequency precipitation radar (DPR) product. This method has also been used to obtain the height profiles of DSD that broadly correlate with ground-based micro rain radar (MRR) observations, showing how DSD varies at different altitudes. The present results demonstrate that the proposed technique is effective in obtaining three-parameter DSDs over a tropical location and offers a useful method to study the microstructure of rain using space-borne radar.