Urban rainfall trends in IMERG datasets

Abstract Satellite precipitation records are increasingly used to assess whether urbanisation modifies local rainfall. However, it remains unclear whether reported urban signals reflect physical processes or artefacts from evolving satellite observing systems. The Integrated Multi-satellitE Retrievals for GPM (IMERG) merges observations from a growing constellation of microwave and infrared sensors, meaning that long-term trends derived from this product may partly reflect changes in the observing system rather than true precipitation changes. Here we analyse rainfall frequency and intensity over 15 major global cities spanning diverse climate regimes using IMERG Version 07B and show that urban areas exhibit a consistent increase in rainfall event frequency with a weaker enhancement in intensity relative to surrounding rural areas. Both signals are dominated by microwave-based retrievals, while infrared-dominated periods largely suppress the urban hotspot pattern, highlighting the sensitivity of detected urban signals to the underlying retrieval type. To isolate physical rainfall changes from observational artefacts, we develop a synthetic time series approach that quantifies the contribution of systematically increasing microwave sampling frequency to apparent long-term trends. We find that sampling artefacts explain up to 20% of observed long-term trends, with locally higher contributions in some cities. After accounting for these effects, the urban rainfall enhancement persists across cities, demonstrating that IMERG captures a robust and physically consistent urban signal in both rainfall frequency and intensity. These findings have direct implications for the reliability of satellite-based urban climate assessments, the interpretation of long-term precipitation trends, and the design of future observing systems.