Increasing synchronized hourly heavy rainfall over South China reflects enhanced multi-cluster and multi-regional coactivity

Short-duration heavy rainfall occurring simultaneously across multiple subregions is more likely to trigger severe flooding than events confined to a single local area, yet long-term changes in hourly precipitation synchrony remain poorly quantified. We analyse hourly rain-gauge observations over South China during the 1981–2020 pre-summer rainy season and ERA5 environmental diagnostics. Using a simplified station-based definition, we find a significant increase in synchronized heavy-rain hours of 0.24 percentage points decade −1 , equivalent to about +16.7% of the 40-year mean per decade. This increase is expressed mainly as more frequent multi-cluster and multi-regional coactivity, rather than expansion of a single coherent rain area. Synchronized-event days occur within circulation and thermodynamic windows characterized by broader convective instability, stronger low-level moisture transport and convergence, more frequent low-level jets, and a favorable western North Pacific subtropical high configuration. The increasing recurrence of these event-like windows is consistent with the observed rise in synchronized heavy-rain hours. These results identify rainfall synchrony as a spatial connectedness dimension of short-duration extremes and highlight its relevance for compound flood-hazard assessment in humid monsoon regions.