VARIATIONS OF TORRENTIAL RAIN DURING FLOOD SEASON IN XIJIANG RIVER REGION AND THEIR RELATIONSHIPS WITH THE BIWEEKLY OSCILLATION OF 500 HPA KEY REGION

Using the methods of wavelet analysis,correlation analysis,power spectrum analysis and Lanczos filter,the variation characteristics of the torrential rain during the flood season(from April to September) in the Xijiang River region and their relationships with biweekly oscillations of a key region at the geopotential height of 500 hPa are investigated.The results show that the number of torrential rainfall days in the season has a single-peak distribution,which begins to increase significantly in May and reaches the peak in June.The total number of torrential rain days exhibits significant quasi-periodic oscillations of 4 years and 22 years.It is more from the end of the 1960s to the start of the 1980s and from the mid-1990s to the start of 2000s,and less during the remaining period.The area(22.5～27.5 °N,102.5～112.5 °E) in the 500 hPa geopotential height is in significantly negative correlation with the daily precipitation in the season in the Xijiang River region,named as the "500 hPa key region".The daily precipitation during the season in the Xijiang river region and "500 hPa key-area" exhibit significant quasi-weekly(6～9 day) and quasi-bi-weekly(10～30 day) oscillations,but less significant 30～60 day oscillations.They have the closest relation on the quasi-biweekly timescale.For April to September and June to August over the 48 years,the probability is 72.5% and 76.4% respectively for torrential rain to occur within three or four(in fewer cases) days before or after the valleys of the quasi-biweekly oscillations in the 500 hPa key region.Therefore,it can be used as reference for medium-term forecast of torrential rain in the Xijinag River region.