Double Low‐Level Jets Over South China in the Warm Season: Diversity and Impacts

Abstract In South China, boundary layer jets (BLJs) often coexist with synoptic‐system‐related low‐level jets (SLLJs) aloft, forming double low‐level jets (DLLJs). This study examines the structural diversity of DLLJs and their impacts on rainfall. DLLJs are classified by their horizontal positioning into three types: Type‐C (Connected, comprising 66% of all DLLJ events) with partial overlap, Type‐O (Overlapped, 31%) with nearly coincident cores, and Type‐D (Displaced, 3%) without overlap. Type‐C develops mainly between continental low pressure and South China Sea high‐pressure systems during April–June. Type‐O occurs primarily south of land‐based low‐pressure anomalies during June–August, and Type‐D is frequent during April–May along high‐pressure flanks. All BLJs and SLLJs exhibit nocturnal wind maxima, primarily caused by the combined effects of enhanced pressure gradient force from pressure tides and weakened friction. In Type‐O, SLLJs display an afternoon increase in wind speed, as weak oceanic friction at 750 hPa cannot offset daytime acceleration from pressure tides. DLLJs strongly modulate rainfall patterns through low‐level lifting that depends on wind structures and local terrains. Type‐C generates two rainbands: one on the windward side of Nanling Mountains, driven by the orographic lifting and BLJ‐SLLJ coupling, and another over the Yangtze River Basin, associated with SLLJ hydraulic jumps on the leeside of Nanling Mountains and low‐level shear convergence. Type‐O enhances coastal rainfall by deep‐layer convergence from overlapping BLJs and SLLJs combined with coastal orographic lifting. Type‐D yields the weakest rainfall related to weaker BLJs. These results highlight the structural diversity of DLLJs and provide new insights for improving rainfall forecasts.