Boundary-Layer and Low-Level Moisture Fluxes during Low-Level Jet Events in South China and Their Relationship with Early Summer Rainfall

Abstract Low-level jets (LLJs) play a key role in driving early summer rainfall in South and East China (SEC) by supplying essential moisture and energy for convective processes. LLJs in South China (SC) are classified into two distinct types: synoptic-system-related LLJ (SLLJ) and boundary layer jet (BLJ), each exerting distinct influences on rainfall mechanisms. This study investigates the individual influence of SLLJ and BLJ, as well as their interplay, on early summer rainfall over SEC, with a focus on moisture flux dynamics. The findings reveal the independent yet equally significant contributions of SLLJs and BLJs in shaping the pattern of early summer rainfall in SEC. Rainfall intensity over SEC correlates with the intensity of BLJs. Strong BLJs result in pronounced boundary layer convergence and upward moisture transport, and thus favorable for enhanced rainfall over SEC. The location of rainfall is closely associated with the pattern of SLLJs. Southwesterly SLLJs along the coast of SC enhances local low-level convergence and upward motions, leading to rainfall mainly confined to SC. By contrast, a quasi-meridional-oriented SLLJ promotes the low-level northward advection of moisture, which is elevated from the boundary layer by topography in SC. This part of moisture, along with the northward-transported moisture within the boundary layer, is elevated to upper layers at the frontal zone in East China (EC), jointly contributing to enhanced rainfall in EC. Overall, the moisture for rainfall in SC is mainly contributed by the transport of BLJs, whereas the moisture for rainfall in EC is jointly attributed to the northward moisture advection by both BLJs and SLLJs. Significance Statement Low-level jets (LLJs) are frequently observed in South China (SC) during warm seasons and play a key role in early summer rainfall over the region by providing the moisture and energy necessary for convective processes. LLJs in the coastal south and continental China are classified into two types: synoptic-system-related LLJ (SLLJ) and boundary layer jet (BLJ), each exerting distinct influences on rainfall mechanisms. Despite their distinct characteristics and impacts on rainfall, the influences of SLLJs and BLJs on rainfall pattern are interconnected with each other through their interactions. In this work, an in-depth study considering the boundary layer and low level as a whole is conducted to fully understand not only the individual role of SLLJs and BLJs but also their interaction in shaping the spatial pattern of early summer rainfall in South and East China (SEC). The results reveal the distinct yet equally crucial roles of SLLJs and BLJs in determining the location and intensity of early summer rainfall over SEC in a climatological manner and underscore the need for further investigation to quantitatively understand the individual and interactive influences of SLLJs and BLJs on early summer rainfall across various SEC regions.