The Synoptic Conceptual Model of 6-hour Tropical Cyclone Rainfall and Its Application in Guangxi

Every 6-hour rainfalls of 9 tropical cyclones (TC) that influence Guangxi Autonomous Region in three years from 2001 to 2003 with the number of 0103, 0104, 0107, 0212, 0214, 0218, 0220, 0307 and 0313 are selected for the study. The characteristics and the mechanism of the heavy rainfall caused by landing tropical cyclones are analyzed in detail, using the information from Tropical Cyclone Annual, observational precipitation data, NCEP/NCAR reanalysis data and black-body temperature (TBB). Based on the study, methods for diagnosing and forecasting the intensity and distribution of precipitation in 6-hour over Guangxi are developed.Every 6-hour rainfalls within the 9 tropical cyclones are analyzed. The regions with 6-hour rainfall amount over 5 mm are picked up, especially those areas of over 50 mm rainfall caused by TC are divided into four types: zonal, meridio nal, NE—SW and NW —SE orientated kinds. Then, the detailed analyses on the main physical factors at upper-level , middle-level and low-level related to the heavy rainfall in each type are made and the relationship between the rainfall in future 6-hour and each factor is investigated. The result shows that the rainfall distribution and intensity in 6-hour relate closely to the physical factors such as divergence field at upper-level, the field of vorticity at middle-level, convergence and ascending motion and the low-level jet, the moisture flux convergence and so on. On the other hand, different type of rainfall distribution and precipitation intensity is formed by the different allocation of these factors. With this understanding, the conceptual models of 6-hour precipitation forcasting and its main transforming direction are summarized. And the forcasting method on the rainfall distribution and intensity in 6-hour is presented. The method can be simply described as below. The location of TC center, the trend of low-level jet and the physical factors such as divergence field at upper-level, the field of vorticity at middle-level, convergence and ascending motion at low-level can help to find out the rainfall type. Then with the location of low-level jet, the distribution of TBB and the moisture flux convergence, themain area of rainfall can be fixed on. Finally, based on the results above and combined with the evolution of cloud systems, the allocation of physical factors and the negative variate zone of TBB, the intensity of precipitation can be worked out.The method is tested on two TCs in 2006. The test results indicate that the false fo recasts are made up of more false-alarm than miss, and the fo recasting of heavy rainfall is not so good .