The quasi-biweekly oscillation characteristics of persistent severe rain and its general circulation anomaly over southeast China from May to August.

The low frequency characteristics of persistent severe rain from May to August over southeast China (SC) and its associated low frequency general circulation are studied using the daily rainfall station data in China, the OLR data and the NCEP/NCAR reanalysis data. The source and propagation of the low frequency signal are also investigated by the composite analysis. The results show that the summer rainfall over SC has the characteristics of quasi-biweekly oscillation. The number of persistent severe rain events gets its peak around June 10 and July 1. The precipitation of five-day low frequency rain events takes up the largest ratio of all low frequency events. In the lower atmosphere a strong low frequency cyclonic circulation is located over SC, while a strong low frequency anticyclone is over the South China Sea (SCS) to the western North Pacific region. The persistent low frequency water vapor is supplied from the Bay of Bengal, SCS and the Philippine Sea, and converges to the south of the Yangtze River. At the same time, a low frequency anticyclone controls northeast China in the upper troposphere. This circulation coordinating with a low frequency anticyclone west of it and a low frequency cyclone located around the coastal waters, is favorable to a divergence environment over SC. Thus, the upward motion is enhanced. About 7 days before the low frequency rainfall, there was a low frequency anticyclone over about 150°E in the lower levels and it strengthened and propagated to the coastal waters of southeast China, while the low frequency cyclone over the SCS propagated northwestward to SC. At the same time, the western Pacific subtropical high had an apparently westward extension.The lower and upper circulations coordinating with each other resulted in the low frequency rainfall over SC.