Mean Features of Continuing Heavy Rain Process in Sichuan Basin During 2—6 September 2004

The persistent precipitation which lasts a long period of time, concentrates on an area and induces the most serious disasters is the essential concern of the researchers and weather reporters. Previous researches have mainly paid close attention to the mechanism of the formation and development of heavy rain, but not attached much importance to the analysis of mean features of heavy rain process.A continuing heavy rain process occurs in Sichuan Basin during 2—6 September 2004. The composite analysis is used in order to analyze mean features of this heavy rain process. The composite technique of meteorological data is as the follow: , where F (p, k) represents mean value of meteorological factor at k-station on p-isobaric surface; i represents time level of observation; p represents depth of isobaric surface; k is the ordinal number of meteorological station. Diagnostic analysis on time averaging values F (p, k) of meteorological factors from 20:00 on 3 September to 08:00 5 September 2004 is made. The results show that: ① "Sanda" typhoon moves westward, causing the mainland high cell stable and meanwhile the cold trough moving southeastward along the northeast side of the Tibetan Plateau blocking on the north of Sichuan Basin. The cold trough forms and dissipates repeatedly over this area, creating a favorable environment for the persistent torrential rain. ② The divergence intensity is higher than the vorticity in lower troposphere. Atmospheric motion is characterized by convergence of air current. The relationship between them is quite the contrary in upper troposphere. ③ The vertical coupling of meso-scale convergence belt in low layer with divergence belt in high layer constructs a meso-scale system with long lifespan and deep upward current. ④ The sources of watervapour over heavy rain area are the South China Sea and the Bay of Bengal. The moisture flux transferring to heavy rain area from the South China Sea is more than the Bay of Bengal, but there is not significant difference between them. To analyze the intensity of the moisture transfer from different sources contributes to the reveal of the moisture source of continuing heavy rain process. ⑤ The coupling between meso-scale convergence belt in lower troposphere and meso-scale divergence belt in upper troposphere constructs a deep and long life meso-scale system. It directly contributes to the maintenance of heavy rain. ⑥ Atmospheric motion is strongly unbalanced during heavy rain. Force faction of the unbalance leads to the maintenance of the strong convergence in low layer and is also the main dynamic factor leading to the continuing heavy rain.