Dual-polarization radar retrievals of coastal Pacific Northwest rain drop size distribution parameters using random forest regression

This study presents and evaluates novel radar retrievals of drop size distribution (DSD) parameters over the mountainous Olympic Peninsula of Washington State. The observations in this study were collected during the 2015-16 Olympic Mountain Experiment (OLYMPEX) and included the NASA S-band NPOL radar and a collection of PARSIVEL2 disdrometers over the windward slopes of the barrier. Nonlinear and random forest regressions were applied to the PARSIVEL2 data in order to develop retrievals for median volume diameter and liquid water content. Modest improvement in DSD retrieval accuracy can be obtained by the random forest model compared to nonlinear regression. Evaluation of disdrometer observations and the retrievals derived from NPOL radar data indicate that the radar retrievals can accurately reproduce observed DSDs in this region, including the common wintertime regime of small but numerous rain drops. NPOL retrievals during the OLYMPEX period are further evaluated using three 2DVD disdrometers and two vertically-pointing Micro Rain Radars. Results indicate that radar retrievals accurately capture the DSD characteristics below the melting level when compared against these independent observing systems.