Formation of Low-Level Jets over Southern China in the Mei-yu Season

This study examines the diversity of low-level jet (LLJ) formation and related physical processes over southern China. A total of 171 LLJ formation events with enhanced daily southwesterlies and early-morning maximum wind speeds were observed during the mei-yu seasons of 1989-2018. The LLJs can be further categorized into four types based on the increases in the daily mean and diurnal amplitude of the low-level winds. Analysis of the synoptic-scale disturbances shows that the two types of LLJ formation (Q1 and Q4), which feature large increases of daily southerly wind components, are mainly induced by west-east dipole patterns of pressure change, in association with the enhanced southwest vortex and/or the western Pacific subtropical high (WPSH). In contrast, the other two types (Q2 and Q3), which feature relatively large increases in their daily westerly components, are related to a northwest-southeast dipole pattern of pressure change due to the mid-latitude trough and the WPSH. We further analyze the considerable variations in the diurnal thermal forcing among the LLJ formation events. The strong (weak) daytime heating of solar radiation leads to relatively large (small) increases in the diurnal amplitude of low-level winds in Q1 and Q2 (Q3 and Q4) types. Therefore, different combinations of synoptic-scale disturbances and diurnal thermal forcings are found to account for the diversity in LLJ formation and associated differences in downstream rainfall patterns. These results help to improve our understanding and prediction of the formation of LLJs.