Mapping Short-Duration Heavy Rainfall across Diverse Climatic Zones and Terrains in China Using a Regionally Adaptive Index

Short-duration heavy rainfall (SHR) is a major hazard, but conventional methods using fixed thresholds or percentile extremes struggle to identify locally impactful events and preclude objective cross-regional comparisons. This paper introduces a novel SHR intensity index that quantifies an SHR event's local extremity relative to its climatic background. This is achieved by normalizing the exceedance beyond the historical 95th percentile precipitation using the standard deviation of values above that percentile. Applied to hourly data from 2422 stations (2010–2022), the index identifies three intensity tiers, with Tier 2 and above representing higher-impact SHR events. Key findings include: (1) The new index reveals SHR hotspots not only in humid southeastern China but also in arid/semi-arid northwestern and northern regions, which are largely missed by the conventional 20 mm h⁻¹ threshold. (2) SHR exhibits distinct regionality: frequency peaks in South China, while extreme-intensity events (Tier 3) are most frequent over the mid–lower Yangtze River basin. Diurnal cycles vary from nocturnal peaks in the Sichuan basin to afternoon maxima in North China. (3) Long-duration SHR events (≥ 4 h) also impact dry western and northern regions. (4) Underlying surfaces modulate SHR: within the same region, hills promote higher afternoon SHR frequency, while plains exhibit greater average intensity. For similar surface types, wetter southern regions show higher frequency and intensity than drier northern ones. This regionally adaptive index provides a transferable framework for identifying locally impactful SHR, offering enhanced utility for regional hazard assessment, forecasting, and climate adaptation planning.