Role of Low-Level Vertical Wind Shear in Modulating Orographic Extreme Rainfall Distribution over an Isolated Mesoscale Mountain: Three Distinct Patterns over Mount Tai, North China

Extreme rainfall over isolated mesoscale terrain remains difficult to forecast due to the complex interplay between orography and atmospheric forcing. Based on 67 extreme rainstorm events (EREs) around Mount Tai, North China, during 2010–2023 and using k-means clustering and composite analysis, this study identifies three distinct orographic precipitation patterns. Type I (20 events) centers in the Tai’an basin on the southern (windward) slope, with high intensity and nocturnal peak around 0100 local standard time (LST). Type II (11 events) occurs from the summit to the northern slope, exhibits frontal characteristics with prolonged duration due to topographic blocking of cold air, and lacks a pronounced diurnal cycle. Type III (36 events) covers the northern (leeward) plain, has the widest extent but the lowest intensity, and peaks near 0500 LST. The key mechanistic finding concerns the weakly baroclinic conditions that govern Type I and Type III. Under a deep southerly environmental flow, the temporal evolution of the low-level vertical wind shear (between the surface and mountain-top height) determines whether heavy rain concentrates on the windward or leeward side. When the southerly wind weakens with height (i.e., vertical shear decreases upward), the deflected easterly flow on the windward side converges with the environmental southerly wind, producing cyclonic vorticity and enhanced lifting over the southern basin (Type I). The hourly rainfall peaks around midnight, consistent with the inertial oscillation of the low-level jet. Conversely, when the southerly wind strengthens with height (vertical shear increases upward), the orographic effect generates cyclonic convergence on the leeward side, shifting the heavy rainfall to the northern plain (Type III), with peak intensity occurring in early morning as the low-level wind decelerates. Type II, in contrast, occurs under a strongly baroclinic frontal system, where the orographically blocked cold air produces stationary frontal precipitation on the northern slope, independent of the diurnal shear evolution. These results advance the understanding of how isolated mesoscale topography and low-level vertical wind shear jointly control the spatial pattern, diurnal cycle, and triggering of extreme rainfall. The identified shear-based mechanisms provide a physically based reference for refined operational forecasting over Mount Tai and analogous mountainous regions in North China.