Abstract: The characteristics and circulation patterns of warm-sector rainstorms with the southerly flow in Liuzhou area occurring before or after the summer monsoon over the South China Sea from 2010 to 2015 were analyzed,using observational data at conventional meteorological stations and regional automatic weather stations,as well as reanalysis data of National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) with the resolution of 1°×1°.The results indicated that 72.7% of the warm-sector rainstorms with southerly flow occur after the break of summer monsoon over the South China Sea.The warm-sector rainstorms occurring before or after the summer monsoon over the South China Sea are characterized by a transport of positive vorticity at 500 hPa and a convergence zone near or below 925 hPa.The jets at super-low altitudes and the strong convergence of southerly flow within the boundary layer are the important triggering mechanism of the warm-sector rainstorms.The topography in the northern part of Liuzhou also favors the occurrence of a warm-sector rainstorm,leading to a higher occurrence frequency of warm-sector rainstorm in this region.Before the occurrence of summer monsoon in the South China Sea,the wet layer above the rainstorm area is very shallow,the invasion of dry and cold air at the middle levels favors the instability of atmospheric at upper levels of the rainstorm area,and the subtropics westerly jet is the major mechanism of divergence at upper levels.While after the occurrence of the summer monsoon in the South China Sea,the high pressure at South Asia is the mechanism of divergence at upper levels and the divergence is stronger at upper levels than lower levels.The strong "suction" action sustains the ascending motion at the upper levels of the rainstorm area.The deep wet layer in the rainstorm area is beneficial to enhance the precipitation efficiency and the formation of quasi-stationary mesoscale convective systems.

Key words: summer monsoon over the South China Sea, Early-flood season, Warm-sector rainstorm, Suction action, Topography