Abstract:
The fine climatological distribution and extreme weather of QLCSs (Quasi-linear convective systems) in North China are still not clear. In this paper, the spatial and temporal distribution characteristics of 171 QLCS cases in North China identified by an objective identification method for the period 2013—2018 are analyzed. According to the statistics of intense weather produced by them, at least two types of QLCSs exist: one type with strong thunderstorm wind gusts and the other with extreme heavy rainfall. Furthermore, the characteristics of circulation pattern, environmental condition, terrain effect, and surface cold pool as a key type of mesoscale system are given for the two types of QLCSs. The findings are presented as follows. The spatial distribution of QLCSs in North China, especially their formation positions, are closely related to the Taihang Mountains and Yanshan Mountains, and the foothills of the two mountains are the highest occurrence frequency areas in Beijing-Tianjin-Hebei region. This is a feature revealed for the first time. There are significant differences in the month of occurrence, the spatial scale, the moving speed, the formation time and maintenance period between the two types of QLCSs. The circulation background, environmental conditions and cold pool are also obviously different. The atmospheric baroclinicity is relatively obvious for QLCSs with strong thunderstorm wind gusts. The large value area of BCAPE (best convective available potential energy) and DCAPE (downdraft convective available potential energy) caused by the dry middle layer and large temperature reduction rate are important environmental conditions for the generation of strong convective wind gusts. The strong cold pool and vertical wind shear within the layer of 0—3 km altitude play an important role in the forward propagation of the QLCSs. Extreme precipitation caused by QLCSs with heavy rainfall is more prominent for this type of QLCSs than that for the previous type. The second type of QLCSs usually occur in weak synoptic-scale forcing systems with sufficient water vapor supply. The back propagation maintained by the interaction between the weak cold pool or the windward slope and the low level southerlies is the main mechanism for the development and slow movement of the QLCS, which is also directly responsible for extreme heavy rainfall.