资源环境科技发展态势分析平台

A heavy rainfall event associated with a low-level shear line, a mid-level easterly wave, and a marine boundary layer jet occurred from 17 to 18 July 2017 along the coastal area of South China. The shear line was found associated with a low-level disturbance on the northwestern edge of the Luzon Island, while the easterly wave exerted significant influences on its maintenance and development over the South China Sea (SCS). Using the Global Forecast System (GFS) analysis and numerical simulations, the mechanism for the disturbance development is investigated. Along with its development, the disturbance extended upwards to the mid-level easterly wave and established an eastward-tilting structure under the effect of the vertical wind shear. When it approached the coast of South China, the height of the disturbance was lowered down due to the intensified divergence above it, and a slanting updraft was formed. A quasi-closed circulation within the easterly wave helped to protect the moist air inside and was favorable for sustained convection. The westward component of the divergent flow, which was induced by the downdraft below the easterly wave, converged in the disturbance that proved to be the primary mechanism for the disturbance development, while the cold core formed in the downdraft intensified the local baroclinicity and contributed to convection development. Although the simulation reproduced the disturbance evolution, it failed to reproduce the vertically tilting structure of the disturbance. In addition, the convection intensity in the simulation was overestimated due to the wet biases in the low troposphere. Sensitivity experiments showed that the disturbance moved faster and more westwards, when it was weakened through terrain modification, which led to stronger boundary layer jet and heavier precipitation.