Characteristics Of Extreme Precipitation Events In Anhui Province

Based on the daily precipitation of 21 stations from 1960 to 2013 in Anhui Province and the NCEP/NCAR reanalysis data,the temporal and spatial distribution characteristics of extreme precipitation indices were analyzed by using linear trend,M-K test,wavelet analysis and EOF methods.According to the extreme precipitation abnormal periods,the anomalous characteristics of atmospheric circulation of extreme precipitation events were studied by using composite analysis,the results are as follows:(1)The consecutive dry days(CDD)showed a weak downward trend in the past 54 years,while other indices were rising,and the decreasing trend of CDD in north of the Huaihe river was significant.However,other extreme precipitation indexes which are the very wet days(R95p),the Max 5-day precipitation amount(RX5day)and the simple daily intensity index(SDII)were increasing in south of the Yangtze River,while the change trend of those indices was slight in Jianghuai.In spatial,the R95p gradually increased from north to south in Anhui Province,the RX5 day was similar to the R95 p except for Mengcheng and Chuzhou stations,while the CDD gradually decreased from northwest to southeast,and the SDII was higher in the north and southwest of Anhui and lower in the middle of Anhui;there were three main distribution types of the R95 p,which were the spatial consistency,opposite variation in the north and south,and adverse variation trend in Huaibei,Jiangnan and Jianhuai.(2)The amount of summer extreme precipitation are in the state of fluctuation in 54 years,and there are three periods that 4a,8-10a,10-14 a.The ratio of summer extreme precipitation to the summer total precipitation are more than 40% and it's spatial distribution increased gradually from the north to the south.What's more,it is more than 60% to annual extreme precipitation and it is in a contrary trend.(3)The thresholds of the daily extreme precipitation events were 25.52 mm/d on the 95 th percentile and 39.06 mm/d on the 99 th percentile in Anhui,and the days of extreme precipitation were 389d and 79d,the annual variation fluctuated greatly with weak upward trend.Additionally,the daily extreme precipitation events tended to occur in May to August,especially from late June to middle July.(4)In year of more summer extreme precipitation,there is a "two groove ridge" type circulation situation in the high-latitude of Eurasia at 500 hPa height field which the subtropical high is strength and west ridge point by west;what's more,and there is “+-+” abnormal distribution from Ural Mountains to northwest pacific ocean.At 850 hPa horizontal wind anomaly field,there are cyclonic circulation anomalies in the area north of 30°N and anti-cyclonic circulation anomalies in the area south of 30°N,hence,the extreme precipitation is increase.(5)In year of more summer extreme precipitation,the regional distribution of SST anomaly in previous winter is similar to spring and mainly Distribution in Northwest pacific ocean,the eastern of Australia and the Eastern Pacific of the Equator;the high correlation areas of extreme precipitation days,extreme precipitation and SST were northwest Pacific ocean,north Indian ocean and the eastern Pacific of the Equator,especially in north Indian ocean.What's more,it is more in-depth that SST influence on summer extreme precipitation in previous winter.(6)In spring,the blocking high in Ural Mountains and Okhotsk Sea affected the occurrence of extreme precipitation events in Anhui,and the convergence in low-level circulation field over Anhui was obvious,the subtropical high was weaker.In summer,the circulation situation in low-level was similar to that in spring,but the subtropical high was stronger.As a result,the water vapor from the anomalous anticyclone periphery in the Bay of Bengal and the South China Sea was transported northward into Anhui area,and intersected with warm moist air flows in Jianghuai region with the cooperation of the cold air southward.In autumn,the subtropical high was strong,but area is small,the eastern of China is in the cyclonic circulation anomaly center and there are ascending motion at low-level.