Influence of Coastal Marine Boundary Layer Jets on Rainfall in South China

Coastal marine boundary layer jets (CMBLJs) play an important role in coastal and inland rainfall in South China. Using 21 years of ERA5 and CMORPH rainfall data, two main CMBLJs are found, one on each side of Hainan Island (named BLJ-WEST and BLJ-EAST), which are always strengthened jointly. Both CMBLJs often occur in the pre-summer rainy season and exhibit an evident diurnal cycle with a maximum at night. With the emergence of the CMBLJs, rainfall is significantly enhanced in South China, particularly downstream of each CMBLJ. The response of rainfall to the CMBLJs is mainly attributed to convergence at the terminus of each CMBLJ, terrain-induced lifting, and relevant atmospheric stratification. Coastal rainfall downstream of the BLJ-WEST is much weaker than that downstream of the BLJ-EAST because of higher CIN over the Beibu Gulf, which is caused by lower temperature lapse rates and adiabatic heating in the lee of the Annamite Range. The inland rainfall increases along with CMBLJ intensity, whereas coastal rainfall reaches a maximum in the presence of moderate CMBLJs rather than stronger CMBLJs. Stronger CMBLJs induce stronger dynamic lifting but higher CIN near the coastal area. Additionally, CAPE near the coast does not become highest with strongest CMBLJs, because the CAPE generation contributed by coastal dynamic lifting can be offset by the negative contribution caused by the horizontal advection of cold and dry air from the Indochina Peninsula. Therefore, anomalous dynamic lifting, moisture flux convergence, and CAPE/CIN associated with CMBLJ intensity jointly result in anomalous rainfall.