Urban Flood Resilience in Climate Transition Zones: Diagnosing a Precipitation Regime Shift Towards Re-Benchmarking Design Standards—Evidence from Zhengzhou, China

Mid-latitude climate transition zones (30–35° N), where monsoon, westerly, and orographic forcing interact, face reorganizing precipitation regimes; yet, urban flood design standards there remain calibrated to stationary historical statistics. Using Zhengzhou, China, as a representative case, we reconstruct its precipitation regime from a homogenous 40-year monthly record (1985–2024) and diagnose the July 2021 “7·20” event through a thermal saturation–water-vapor surplus–terrain locking (THTL) framework built on ERA5 environmental fields (used only diagnostically, strictly separated from ground-truth rainfall), combined with the official design-benchmark audit and a four-event intensity–impact record (2021–2025) used as initial anchors. Pettitt and Buishand tests consistently locate a regime transition around 2002/2003: the mean annual precipitation rose by 23.7% and the extreme-month frequency nearly doubled, while excluding 2021 shows the variability surge is dominated by that single year. The “7·20” event (624.1 mm in 24 h; record 201.9 mm h−1) overloaded the drainage system by 1.6–2.5 times its design basis, with failures cascading through the metro, tunnel, and reservoir systems; peak accumulations occurred over the foothill–urban transition belt, consistent with terrain locking. The study does not re-compute design return periods; rather, it assembles the evidence base towards re-benchmarking design standards in transition zones—regime-aware, non-stationary design updating coupled with mechanism-based early warning—with quantitative non-stationary return-period reassessment defined as the essential next step.