A Case Study of the Effects of a Synoptic Situation on the Motion and Development of Warm-Sector Mesoscale Convective Systems over South China

The effects of a synoptic situation on the motion and development of warm-sector mesoscale convective systems (MCSs) in a heavy rainfall event occurred in South China on 8 May 2014 are investigated using high-resolution observational data and ERA-Interim data. The results show that the blocking of an eastward moving low-level trough over southwestern China by a stable anticyclone over eastern China induced strong southerly winds ahead of the trough and an eastward vertical slant of the wind speed maximum. Consequently, a southeastward low-level wind shear formed in the region under the southerly jet, which caused a southeastward turning of the motion of a large inland MCS. Meanwhile, a northeastward/eastward low-level wind shear formed in the region under the east side of the southerly jet, leading to northeastward/eastward movement of some small MCSs in the coastal region. The continuous merging of the southeastward-moving MCS with the northeastward/eastward-moving MCSs resulted in heavy rainfall along the coastal region. On the other hand, the intensification and expansion of associated low and high mean-sea-level pressure (MSLP) centers caused southeastward migration of strong boundary-layer moisture flux convergence and high convective available potential energy zones, which promoted the persistent development of the large MCS and the later development of the small MCSs. Moreover, the strong boundary-layer southeasterly winds associated with the high MSLP center continuously enhanced the frontal ascent of the large MCS. These southeasterly winds also triggered the small MCSs through the convergence induced by differential surface friction between the land and sea or the convergence of them with the southwesterly winds from the South China Sea.