华北两类产生极端强天气的线状对流系统分布特征与环境条件

华北线状对流系统精细气候分布及其所产生的极端天气特征尚不清楚,本研究利用雷达拼图资料和客观识别方法普查2013—2018年华北171例线状对流系统的时、空分布特征,根据其所致强对流天气的统计结果,发现华北地区至少有2类线状对流系统,分别产生极端强雷暴大风和极端强降水。分析了这2类线状对流系统的环流形势、环境条件、地形作用和关键中尺度系统地面冷池等的特征。主要结论如下:华北线状对流系统的空间分布尤其是初始形成位置与大地形关系密切,京津冀的太行山和燕山山脚区域为其中的一个高发区;2类线状对流系统发生月份、空间尺度、移动速度、形成时刻和维持时间等都具有显著差异;2类线状对流系统的环流背景、环境条件和冷池也差别明显。强雷暴大风型线状对流系统的环境大气斜压性强,中层干和大的垂直减温率造成的最优对流有效位能、下沉对流有效位能大值区是产生极端大风的重要环境条件,地面强冷池以及0—3 km风垂直切变对前向传播起到了重要作用。强降水型线状对流系统产生的降水极端性较前一类型更为凸出,天气尺度强迫相对较弱,水汽条件极其充沛,地面弱冷池或地形与低层南风气流相互作用维持的后向传播是其发展和缓慢移动的主要机制,也...

Climatology and environmental conditions of two types of quasi-linear convective systems with extremely intense weather in North China

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.