A Study On The Changes And Mechanisms Of Modern Extreme Precipitation In The Tarim Basin

Under the background of global warming,the frequency and intensity of extreme events increase.Tarim Basin,which is located in the inland arid and semi-arid area,has a serious impact on the local people’s life,ecological and environmental security and the sustainable development of economy and society due to the frequent occurrence of extreme precipitation events in recent years.Therefore,clarifying the extreme precipitation change and its dynamic mechanism in Tarim Basin is of great significance to deal with the risk of climate disasters and promote economic development and social stability.However,the current research on extreme precipitation in Tarim Basin mainly focuses on the case analysis of a certain precipitation process,and lacks the dynamic analysis of all extreme precipitation processes in the sense of climate statistics.Based on Aphrodite data,daily value data set of China’s surface climate data(V3.0),ERA5 reanalysis data and GDAS data,the extreme precipitation events in Tarim Basin from1960 to 2018 are selected by using relative threshold method and percentile threshold method,the variation characteristics of extreme precipitation and background circulation types are analyzed,and the main water vapor sources are evaluated by HYSPLIT backward trajectory model.The results show that:(1)The precipitation in Tarim Basin is scarce,the total annual precipitation is less than 100 mm,and it is mainly concentrated in summer,which accounts for more than 50% of the annual precipitation,and the precipitation in the southwest of Tarim Basin is greater than that in the East;From 1960 to 2018,1010 times of precipitation occurred in Tarim Basin,the average precipitation is about 2mm,of which light rain accounted for 31%,moderate rain accounted for 45% and heavy rain accounted for 24%.The occurrence frequency of light rain,moderate rain and heavy rain accounted for 50%,40% and 10% of the total precipitation frequency respectively;The spatial distribution pattern of precipitation and precipitation frequency at all levels in Tarim Basin is similar,which decreases from four sides to the center,and the north of the basin is more than the south.(2)From 1960 to 2018,the total annual precipitation and precipitation frequency in Tarim Basin showed an increasing trend.In the four seasons,except spring,the precipitation and precipitation frequency in other seasons also showed an increasing trend;The annual precipitation of all grades in the basin showed an increasing trend,especially after 2010,the increasing trend of heavy rain was more significant.In the four seasons,only the precipitation of all grades in summer showed a consistent upward trend,and the contribution rate of summer heavy rain to the total precipitation increased significantly;Among the three representative stations,the total annual precipitation and total annual frequency of extreme precipitation events in Tazhong and Atush show an upward trend,while the total annual precipitation and frequency of extreme precipitation events in Yanqi show a slight downward trend.(3)According to the contribution,the low-level 850 h Pa atmospheric circulation types corresponding to extreme precipitation events in Tarim basin can be divided into the following four types: the first is affected by the Central Asian anticyclone,and the westerly flow bypasses the east of Tianshan Mountain and enters the basin;The second is affected by the Mongolian anticyclone,and the westerly flow turns from the east of the Mongolian Plateau into the interior of the basin.The third is the monsoon flow from the south,which goes deep into the arid area along the southeast edge of the Qinghai Tibet Plateau and converges with the westerly flow from the north in the east of the Tarim Basin;The fourth is that the straight westerly wind is blocked by the Mongolian Plateau,and its southern branch bypasses the east of Tianshan and enters the basin.Among them,the first,second and fourth can be attributed to the westerly circulation type,and the third is the westerly monsoon convergence type.The 500 h Pa circulation field in the middle troposphere basically presents the pattern of Central Asian vortex,indicating that the large-scale background is an important prerequisite for the generation of extreme precipitation.The main water vapor source of extreme precipitation in Tarim is westerly water vapor,and the water vapor from the Indian Ocean also contributes to it.The backward trajectory results further show that the water sources of extreme precipitation mainly include the Caspian Sea and Aral Sea in Central Asia,the north of Eurasian continent,the North Atlantic and Arctic Ocean.