A Simulation Study Of Moisture Sources For Precipitation In Xinjiang,China

Xinjiang Uygur Autonomous Region is the largest province in China,accounting for approximately one-sixth of China's land territory,with complex terrain and climate conditions.As a typical arid and semi-arid region,Xinjiang is short of water resources.Under the background of global warming,the change of water cycle in Xinjiang has an important impact on the local climate,ecology,human survival and development.Therefore,it is of great scientific significance to study the water cycle in Xinjiang.Based on the datasets of the Climate Forecast System Reanalysis(CFSR)from the National Center for Environmental Prediction(NCEP)and the observational precipitation data,this study addresses moisture sources for each season,the interannual variation of moisture sources,and the different types of precipitation(including precipitation under different intensities and different meteorological patterns)moisture sources in Xinjiang by using the Flexible Particle Dispersion Model(FLEXPART),machine learning,and other statistical methods.The FLEXPART is employed to simulate the trajectories of global air parcels from1979 to 2018.The results show that more than 90% of moisture sources of precipitation over Xinjiang can be tracked when the backtracking time is set to 10 days.There are three similar moisture transport paths in winter and summer,namely,the Mediterranean–Caspian Sea–Balkhash Lake path,the Eastern European Plain–Western Siberian Plain–Balkhash Lake path,and the Arabian Peninsula–Iranian Plateau–Pamir Plateau path.Besides,there is one more path: the Indian Ocean–Indian Peninsula–Pamir Plateau path in summer.The paths of moisture transport in spring(autumn)are similar to those in winter(summer).In summer,the contribution rate of Xinjiang itself and Central Asia accounts for more than 80%,in which the local contribution accounts for 52%.In the other three seasons,Xinjiang,Central Asia,North Africa–Arabian Peninsula,and South Asia are the main sources of moisture.The contribution rates of moisture uptake in these source regions apart from Central Asia vary laregely with the seasons.Regionally,the moisture source distribution for southern Xinjiang resembles those of the whole Xinjiang,and,for the northern Xinjiang,Central Asia becomes the major source region.The seasonal precipitation over Xinjiang shows an obvious interannual variation,and so does the moisture contribution amount of each moisture source region.The interannual variation of moisture sources is the strongest in summer and then in spring,autumn,and winter,respectively.The regions with the strongest interannual variation of moisture contribution in each season are Xinjiang and Central Asia,which are the case in all four seasons.The interannual variation of the moisture contribution rate in each source region is small,and there is no obvious interannual conversion of the main moisture sources.Further comparative investigation of the moisture sources between rainy and drought years in Xinjiang shows that Xinjiang is the most obvious source region in contribution amount differences.In summer,the moisture contribution rate of Xinjiang in rainy years is significantly higher than that in drought years,while the contribution rate of moisture in Central Asia is on the contrary;there is no significant change in other source regions.This increase of local contribution moisture in rainy years is mainly caused by the enhancement of local evaporation.In all,the interannual variation of moisture contribution in different source regions is mainly in the same direction,supplemented by the oppoiste trend of moisture contribution in Xinjiang and Central Asia.The interannual variation of Xinjiang precipitation is closely related to the local water cycle.The main moisture sources of Xinjiang summer precipitation in different intensities are similar to its summer climatological precipitation moisture source,and they are the Xinjiang and Central Asia.Besides,there is no obvious difference in the process of moisture accumulation before precipitation in different intensities.In detail,the moisture is largely accumulated near the time of precipitation;the farther away from the occurrence of precipitation,the less moisture accumulated.Furthermore,the self-organizing maps(SOM)method is used to make cluster analysis of the heavy precipitation in Xinjiang in wet season(April to September)according to the difference of meteorological patterns,and there are four main meteorological patterns,namely N1,N2,N3,and N4.They match the strength and position of the negative geopotential height anomalies in middle-high levels over Central Asia,which indicate the anomalous activities of the Central Asia trough and vortex.In N1 and N3 patterns,the heavy precipitation is concentrated in the Tianshan and Kunlun Mountains;in N2 and N4 patterns,the heavy precipitation is more concentrated in the Tianshan Mountains.There are four main moisture source regions for Xinjiang heavy precipitation in the wet season,which are listed in descending order by their contribution rates as southern Xinjiang(29–37%),northern Central Asia(19–27%),northern Xinjiang(14–19%),and southern Central Asia(13–16%).The contribution rate of each source region to Xinjiang heavy precipitation varies with the meteorological patterns.For each source region,the contribution rates of each source region match well with the precipitation-related particle aggregation before heavy precipitation days.These results provide a better understanding of the moisture source of Xinjiang heavy precipitation.