Abstract:Using the NCEP/NCAR reanalysis data, the circulation background and the large scale water vapor transport characteristics of a rarely seen persistent heavy rainfall that occurred in Hunan Province in late June to early July 2017 was analyzed first and then the trajectory model was used to simulate the trajectory of the air mass. The characteristics of water vapor transport and the regional water vapor budget were quantitatively analyzed according to the three stages of rainfall process. The results showed that the effective disposition and stable maintenance of the weather system were the main causes for the persistence of heavy rainfall. Persistent heavy rainfall was linked with global water vapor transport and convergence, and the evolution of low level jet directly affected the rainfall area and intensity of heavy rain. There were mainly three water vapor passages corridors to the heavy rain process. The first was the Somali jet stream through the Bay of Bengal and Southwest China into the heavy rain area, the second was the cross equatorial flow from the Southern Hemisphere of central and eastern Indian Ocean through the Bay of Bengal and northern South China Sea into the heavy rain area, and the third was the cross equatorial flow through South China Sea into the heavy rain area. In the third stage, there was another passage from the equatorial Pacific across the Philippines into the South China Sea and then into heavy rain area. During the first two stages, the water vapor transportation was mainly from the Bay of Bengal, and then from the South China Sea, and in the third stage, the water vapor from the Bay of Bengal and the South China Sea (including the Western Pacific) were about equal. Affected by the terrain, the water vapor from the Bay of Bengal was mainly transported to the storm zone at 700 hPa, and the water vapor from other passages was mainly transported to the 850 hPa and the lower levels. The water vapor transportation came mainly from the low level of southern and western boundaries which converged in the form of horizontal water vapor flux convergence over the low level of the rainstorm area, and was transported to the middle and upper troposphere through strong vertical ascending movement,condensing and resulting in precipitation. The intensity of precipitation was well related to the strength of the water vapor inflow on the boundaries and the regional water vapor convergence.