HAPpy Hour-The Impact of Marine Boundary Layer Jet on heavy Rainfall in South China

The characteristics of boundary layer jets (BLJs) over South China Sea (SCS) and their impacts on heavy rainfall are examined using scientific-research-ship observations, reanalysis and numerical simulations. Typically, the BLJs exhibit a maximum intensity at night and reach their peak at 950 hPa. They are formed by the large-scale diurnal land-sea thermal contrasts under the background of the monsoon flow. Two main BLJs are found, one on each side of Hainan Island (named BLJ-WEST and BLJEAST), which are always strengthened jointly. The emergence of the BLJs leads to increased rainfall in South China, particularly in areas downstream of each BLJ. The strengthened BLJs drive mesoscale ascent on their northwestern edge and terminus, creating enhanced convergence zones and promoting convection initiation and upscale convective growth dynamically. From a thermodynamic perspective, the BLJ-driven mesoscale ascent combined with high sea surface temperature strengthens the warm moist tongue over the SCS, allowing it to farther extend northward and reach the coast of South China. The inland rainfall increases along with BLJ intensity, whereas coastal rainfall reaches a maximum in the presence of moderate BLJs rather than stronger BLJs. Stronger BLJs induce stronger dynamic lifting but higher CIN near the coastal area.