General Features of Squall Lines in East China

Based on mosaics of composite radar reflectivity patterns during the 2-yr period of 2008–09, a total of 96 squall lines were identified in east China with a maximum frequency of occurrence in north China near the boundaries between Shandong, Henan, Anhui, and Jiangsu Provinces. The squall lines form from March to October with a peak in July. Their diurnal variation shows a major peak in the early evening and two minor peaks in the early morning and early afternoon. The time between squall-line formation and the first echo is about 4.8 h. The squall lines have a dominant southwest–northeast orientation, an eastward motion at a speed of 14.4 m s−1, a maximum length of 243 km, a maximum intensity of 58–63 dBZ, and a duration of 4.7 h on average. The squall lines commonly form in a broken-line mode, display a trailing-stratiform pattern, and dissipate in a reversed broken-line mode. Composite rawinsonde analyses show that squall lines in midlatitude east China tend to form in a moister environment with comparable background instability, and weaker vertical shear relative to their U.S. counterparts. The rawinsondes were also composited with respect to different formation and organizational modes. The environmental flows of the squall lines in the area with high frequency of formation were classified into six synoptic weather patterns: pre–short trough, pre–long trough, cold vortex, subtropical high, tropical cyclone (TC), and posttrough. About one-third of the squall lines form in the dominant pre-short-trough pattern. Favorable conditions of various patterns were examined in terms of moisture supply, instability, vertical wind shear, low-level jet, etc.