Abstract:Based on the conventional surface and upper-air observations, Doppler radar products, wind-profiling radar data, VDRAS and NCEP reanalysis data, the triggering and development mechanisms of multiple β-meso scale convective systems under the influence of subtropical high in North China from 5 to 6 August 2018 are analyzed. The results show that this severe torrential rain occurred under the control of subtropical high, in a high temperature and high humidity air mass, with extremely unstable atmospheric stratification. The torrential rain was caused by multiple successive developing mesoscale convective systems, namely MCS-Ⅰ, MCS-Ⅱ, MCS-Ⅲ and MCS-Ⅳ, which were quasi-stationary β-meso scale systems on the windward slope of Taihang Mountains in southwest-northeast direction, in Baoding Area of North China Plain in north-south direction, near Baoding to Bazhou in the southwest-northeast direction and centered on Xiong’an New Area in east-west direction， respectively. In a similar environment, different mesoscale convective systems had significant differences in triggering mechanisms. MCS-Ⅰ on the windward slope of Taihang Mountains was triggered by near-surface easterly warm and humid airflow converging and lifting on the windward slope with mountain wind. After being influenced by MCS-Ⅰ, the local cold pool formed by radiation difference and earlier-stage severe precipitation was strengthened again. Its outflow formed two surface convergence lines with environmental wind, which triggered MCS-Ⅱ and MCS-Ⅲ and organized convection along convergence line into band-like development. The enhanced easterly wind at ultra-low altitude superimposed the outflow of the cold pool, promoting further uplift of the warm and humid air mass along the mountain under the effect of terrain uplift, triggering MCS-Ⅰ reconstruction. MCS-Ⅳ developed most vigorously and lasted longest, and was the direct maker of the severe torrential rain center. On one hand, the two merger processes of MCS-Ⅱ and MCS-Ⅲ, MCS-Ⅰ and MCS-Ⅳ were important reasons for the strengthening and persistence of MCS-Ⅳ. On the other hand, the boundary layer easterly jet provided favorable conditions such as water vapor 〖JP2〗and unstable energy for MCS-Ⅳ’s development, and also〖JP〗 promoted the development of mesoscale vortex in its left front side, resulting in a significant increase of cyclonic vorticity in MCS-Ⅳ’s location, enhancing the vertical secondary circulation development centered on jet axis, causing MCS-Ⅳ’s continuous maintenance, and forming an east-west severe torrential rain belt centered at Xiong’an New Area in North China Plain.