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 in North China for the period 2010–2023, this study applies k-means clustering and composite analysis to identify three distinct orographic precipitation patterns: Type I (20 events) is centered over the Tai’an basin on the southern (windward) slope, featuring high rainfall intensity and a nocturnal peak near 0100 local standard time (LST); Type II (11 events) occurs from the mountain summit to the northern slope, showing frontal characteristics and a long-lasting duration resulting from topographic blocking of cold air, with no well-defined diurnal cycle; and Type III (36 events) occupies the northern (leeward) plain, with the largest spatial extent but the weakest rainfall intensity, as well as a diurnal peak around 0500 LST. The key mechanistic finding concerns the weakly baroclinic conditions governing 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). 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 the early morning as the low-level winds decelerate. Type II, in contrast, takes place within a strongly baroclinic frontal system, where the orographically blocked cold air produces stationary frontal precipitation along the northern slope, independent of diurnal shear evolution. These results advance the understanding of how isolated mesoscale topography and low-level vertical wind shear jointly govern the spatial pattern, diurnal cycle, and initiation of extreme rainfall. The identified shear-based mechanisms provide a physically based reference for refined operational forecasting over Mount Tai and analogous mountainous regions across North China.