From Nocturnal Warm–Rain Processes to Afternoon Mixed–Phase Precipitation: A Diurnal Transition in the Dynamical–Microphysical Pathways of Heavy Rainfall Over North China

Heavy rainfall occurs frequently over North China in summer, yet the multiscale dynamical‐microphysical processes that initiate and organize these events remain inadequately understood, limiting progress in quantitative precipitation forecasting over complex terrain. This study investigates a representative event occurred during–31 July 2024 using dual‐polarization radar observations and three‐dimensional wind fields retrieved from the Variational Doppler Radar Analysis System. We compare the triggering conditions, organizational structures, and microphysical processes of two distinct rainfall stages: a nocturnal period (P1) and an afternoon period (P2). Both P1 and P2 occurred under a similar large‐scale circulation background, which provided a favorable environment for moisture transport and large‐scale ascent in both phases. However, despite this common synoptic background, the triggering mechanisms differed markedly between the two phases. During P1, convection was influenced by the Eastern Asian trough, southwesterly flow along the periphery of the subtropical high, and terrain‐induced boundary‐layer vortices associated with the nocturnal low‐level jet near the Taihang Mountains. Weak but widespread ascent within stratiform clouds prolonged the residence time of cloud water, enhancing efficient warm‐rain production through collision–coalescence within a deep warm‐cloud layer. In contrast, afternoon convection during P2 featured an organized quasi‐linear convection system, driven by intensified low‐level convergence associated with an eastward‐propagating vortex and a daytime sea‐breeze circulation. Strong updrafts lofted supercooled liquid water into the mixed‐phase region, where graupel growth through riming and subsequent melting dominated precipitation production, albeit with lower rainfall than in P1. Overall, this study reveals a diurnal transition from nocturnal warm‐rain‐dominated to afternoon mixed‐phase‐dominated precipitation, regulated by evolution of lower‐tropospheric and boundary‐layer dynamical structures. These findings provide new observational evidence for improving cloud‐microphysical parameterization and highlight the critical role of boundary‐layer–terrain coupling in predicting heavy rainfall over complex terrain.