Two types of low-level jets in China during the warm season: Observational climatology from radiosonde data

Low-level jets (LLJs) are widespread across China and significantly shape warm-season precipitation. This study provides the first observational climatology of two LLJ types—boundary-layer jets (BLJs) and synoptic-system-related LLJs (SLLJs)—using nationwide high-vertical-resolution radiosonde data with from April to September during 2008–2017. LLJs occur most frequently in Northeast and South China, with BLJs concentrated in mountainous and coastal regions influenced by strong thermal contrasts, and SLLJs primarily in Southwest and South China under active synoptic disturbances. BLJs and SLLJs reach their maximum occurrence at ∼0.7 km and 1.9 km above ground level, respectively. The diurnal peak in LLJ frequency shifts earlier with increasing latitude, consistent with inertial oscillation theory. Statistical analyses of spatial distributions, wind directions, and ERA5 diagnostics indicate that both BLJs and SLLJs are jointly modulated by topography, monsoon background flows, and synoptic systems. These nationwide observational results provide a comprehensive and realistic view of LLJ variability and their synoptic context across China. 低空急流(LLJ)在中国分布广泛, 对暖季降水有显著影响.本研究利用2008—2017年4—9月全国高分辨率探空数据, 首次针对边界层急流(BLJ)和天气系统相关低空急流(SLLJ)两类LLJ开展观测气候学研究.结果表明, LLJ在东北和华南地区发生频率最高, 其中BLJ集中在受强热力差异影响的山区与沿海区域, SLLJ则多见于受活跃天气系统影响的西南和华南地区.LLJ频率的日变化峰值随纬度升高而提前.对空间分布,风向和ERA5数据的分析表明, BLJ和SLLJ都受到地形,季风背景气流和天气系统的共同调控.