A diagnostic study of two heavy rainfall events in South China

South China is prone to heavy rainfall which may occur both in the pre-monsoon and the monsoon season. The responsible synoptic systems and the water-vapor sources, however, can be substantially different for different seasons. In this study, we aim to develop conceptual models for typical heavy rainfall events in South China through diagnostic case studies. A number of events have been analyzed using the NCEP/NCAR data, but the discussions here are focused on two representative events, one for the pre-monsoon season and the other for the monsoon season. Both events are found to be associated with extensive moist convective instability in the lower part of the troposphere. For the pre-monsoon case the instability was much weaker and the uplift of the warm moist air was provided by the cold air intrusion related to a weak cold front. The moist potential vorticity theory can be used to explain the increase of vorticity and vertical velocity in the lower part of the troposphere. For the monsoon event, the lower troposphere was extensively and strongly unstable and the upward motion was provided by the shear of a southwesterly low-level jet (LLJ) which appeared to be driven, at least partially driven, by the upper-level jet. In both events, LLJ played a major role in not only providing the dynamic conditions but also the supply of water vapor for heavy rainfall in South China. The diagnostic results presented in this study provide a useful guidance for future numerical simulations.