Two-decades of GPM IMERG early and final run products intercomparison: Similarity and difference in climatology, rates, and extremes

Precipitation is an essential climate and forcing variable for modeling the global water cycle. Particularly, the recent Integrated Multi-satellite Retrievals for GPM (IMERG) product retrospectively provides an unprecedented two decades of high-resolution satellite precipitation estimates. The primary goal of this study is to examine the similarities and differences between the two latest and also arguably the most popular, GPM IMERG Early and Final Run (ER and FR) products across the globe. The results reveal that: (1) ER systematically estimates 12.0% higher annual rainfall than FR, particularly over land (16.7%); (2) ER and FR show significant differences in instantaneous rates (Root Mean Squared Difference: RMSD = 2.38 mm h⁻¹ and normalized RMSD: RMSD_norm = 1.09), especially in Africa (RMSD = 2.40 mm h⁻¹) and hot, arid regions (RMSD_norm = 1.11), but less so in Europe (RMSD = 2.16 mm h⁻¹) and cold areas (RMSD_norm = 0.87); and (3) ER measures 33.0% higher extreme rainfall rates than FR over the globe. The exploration of their similarities and differences provides a first-order global assessment of various hydrological utilities: FR is designed to be more suitable for retrospective hydroclimatology and water resource applications, while the earliest available ER product, though not bias-corrected by rain gauges, still shows potential utility for operational modeling of rainfall-triggered hazards. The findings of this study can provide an assessment to product developers and broader data users and practitioners to address the inherent issues in hardware limitations, retrieval algorithms, and uncertainty quantification for research and applications.