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Climatology of Low-Level Jets and Their Impact on Rainfall over Southern China during the Early-Summer Rainy Season

Low-level jets (LLJs) are a key factor regulating the early-summer rainfall over southern China. Their detailed activities and impact are examined using 21-yr ERA5 and TRMM rainfall data. The LLJs typically consist of boundary layer jets (BLJs) and synoptic-system-related LLJs (SLLJs). The BLJ is usually characterized by a southerly wind maximum at 950 hPa over the northern area of South China Sea, whereas the SLLJ features a southwesterly wind maximum at 850–700 hPa located more north on land. Meanwhile, the BLJ (SLLJ) has a maximum occurrence in April–June (May–July) and at late night (in the early morning), indicating the differences in seasonal and diurnal variations. The two types of LLJs are found to influence the rainfall distribution via terrain effects, synoptic disturbances, and moisture transport. During the BLJ events, rainfall is mainly confined to the south side of the Nanling and Wuyi Mountains and Yun-Gui Plateau (south region), whereas during the SLLJ events rainfall occurs both in the coastal region and to the north of the mountains (north region). The difference is caused by the southerly BLJ that induces strong orographic lifting on the windward side of the mountains, while the elevated SLLJ can pass over the mountains driving an additional upward motion more north. Active synoptic disturbances accompanied by SLLJs are also favorable for the rainfall in the north region. The moisture transportation by LLJs is another important factor regulating rainfall distribution. Rainfall in the south (north) region is mainly attributed to the net moisture flux in the boundary layer (more elevated layers) due to the BLJ (SLLJ).