Validation of the GPM Version-5 Surface Rainfall Products over Great Britain and Ireland

Instantaneous surface rain rate estimates from the Global Precipitation Measurement (GPM) Mission’s dual-frequency precipitation radar (DPR) and combined DPR & multi-frequency microwave imager (CMB) version-5 products are compared to those from the Met Office Radarnet 4 system’s Great Britain and Ireland (GBI) radar composite product. The space-borne and ground-based rainfall products are collocated spatially and temporally, and compared at 5 km and 25 km resolutions over GBI during a three year period (May 2014 – April 2017). The comparison results are evaluated as a function of both the intensity and variability of precipitation within the DPR field of view and are stratified spatially and seasonally. CMB and DPR products underestimate rain rates with respect to the Radarnet product by 21% and 31%, respectively, when considering 25 km resolution data taken within 75 km of a ground-based radar. Large variability in the discrepancies between space-borne and ground-based rain rate estimates are the result of limitations of both systems and random errors in the collocation of their measurements. The Radarnet retrieval is affected by issues with measuring the vertical extent of precipitation at far ranges, whilst the GPM system struggles in properly quantifying orographic precipitation. Part of the underestimation by the GPM products appears to be a consequence of an erroneous DPR clutter identification in the presence of low freezing levels. Both products are susceptible to seasonal variations in performance and decreases in precision with increased levels of heterogeneity within the instruments’ field of view.