Low-Level Jet and Its Effect on the Onset of Summertime Nocturnal Rainfall in Beijing

Low-level Jets (LLJs) play a critical role in triggering nocturnal rainfall in many areas on our planet. Nevertheless, little is known regarding the vertically resolved evolution of LLJs preceding rainfall. Here, the high-resolution wind measurements from a radar wind profiler network in Beijing are used to monitor the continuous evolution of boundary-layer jets (BLJs) and synoptic-system-related LLJs (SLLJs) before nocturnal rainfall under the southwesterlies synoptic pattern for the summers of 2020-2023. The results show that inertial oscillations precede LLJ-induced nocturnal rainfall. The typical northward propagation of LLJ is driven primarily by horizontal advection. Starting 48 min before rainfall, the BLJ nose rapidly strengthens and moves downward from 0.8 km above ground level (AGL), while the SLLJ nose moves upward to 2.4 km AGL. This leads to enhanced moisture transport and convergence, which triggers nocturnal rainfall. The findings are of great significance for improving the forecast skill of nocturnal rainfall. The role of low-level jets (LLJs) in triggering nocturnal rainfall remains elusive, largely due to the dearth of vertical atmospheric observations. To address this issue, we used high-resolution vertical wind measurements from a radar wind profiler (RWP) network near Beijing to characterize both boundary-layer jets (BLJs) and synoptic-system-related LLJs (SLLJs) preceding nocturnal summer rainfall under the influence of synoptic-scale southwesterly winds. The LLJs propagate northward ahead of the nocturnal rainfall. The noses of both types of LLJs are found to intensify dramatically within 48 min before nocturnal rainfall, along with warm and moist advection. This implies that the high-resolution LLJs obtained from the RWP network exert a significant impact on the onset of summertime nocturnal rainfall in Beijing. Radar wind profiler network captures the fine-scale evolution of low-level jet (LLJ) that triggers the nocturnal rainfall Boundary-layer (synoptic-system-related) jet nose enhanced sharply 48 min ahead of rainfall onset The surge in LLJ is key to enhancing moisture transport within 30 min preceding nocturnal rainfall