Evaluation of IMERG climate trends over land in the TRMM and GPM eras

The Integrated Multi-satellitE Retrievals for global precipitation measurement (GPM) mission (IMERG) is a global precipitation product suite consisting of both near-real-time and research-grade products with high spatiotemporal resolution. However, the IMERG developers note that it is designed as a high resolution precipitation product (HRPP), not a Climate Data Record, and its ability to capture climate trends remains uncertain. Therefore, it is imperative to explore and quantify IMERG's capability in capturing climate trends for our community. This study examines the climatological performance of IMERG Final (Version 06) by analyzing annual precipitation trends over land from 2001 to 2019 using the Mann-Kendall analysis, taking gauge records as ground truth. Three different matching strategies are applied: at gauge locations, at 0.1 degrees pixels, and at 1.0 degrees pixels. Additionally, this study compares the performances during the TRMM (2001-2014) and GPM eras (2015-2019). Our results find: (1) IMERG daily data exhibits high spatial consistency with gauge records at both gauge locations and 0.1 degrees resolution, consistent with its Global Precipitation Climatology Centre calibration, with a conversion rate of approximately 89.3%; (2) IMERG performs much better overall in the GPM era than in the tropical rainfall measuring mission (TRMM) era, evidenced by a lower proportion of unreliable samples (similar to 10.2% vs. similar to 21.2%); (3) The proportion of samples showing consistent trends with gauge data is 86.7% in the GPM era, much higher than the 70.5% and 75.3% shown by the entire record and the TRMM era, respectively. This improvement in the GPM era suggests that the within-mission consistency of IMERG is higher than the between-mission consistency, likely due to residual differences in the calibration methods used during the TRMM and GPM missions. This study broadens the perspective on IMERG, showcasing its additional potential for analyzing climate trends despite its design only as a HRPP. Crucially, it highlights and reconfirms how the GPM era has enhanced IMERG's capacity for accurately tracking global precipitation trends.