Analysis On Time Series Variation Characteristics Of Precipitation In Ansai District Of Yan'an City From 1951 To 2019

Climate change is a hot topic in the world research.Accurate estimation of climate trend change and abrupt change in the study area can play an important role in the development of the region.The Loess Plateau is an area with fragile ecological environment in China.The natural disasters occurred in the Loess Plateau in recent decades have had a great impact on people's life,property safety and economic development in the region.Under the background of global warming,the analysis of climate change in this region can predict and prevent the occurrence of natural disasters,and provide a certain scientific basis for the improvement of local ecological environment and the development of economy.Precipitation,as a common natural phenomenon,is also a basic feature of the climate in this region.In this paper,Ansay District of Yan 'an is selected as the study area,and the monthly precipitation data from 1951 to 2020 and the daily precipitation data from 1980 to 2020 are selected.Using EEMD decomposition analysis,Mann-Kendall non-parametric test,Morlet wavelet analysis,RClimdex model and R/S analysis to study the characteristics of precipitation time series change in the region from 1951 to 2019,the main conclusions are as follows:(1)From EEMD decomposition method,each component and trend component represent the change of precipitation in different time scales.The oscillation periods of each IMF component in annual precipitation are 2.8A,5.1a,22 a,33A and 69 A,respectively;The cumulative variance rate of precipitation components with periods of 2.8A and 5.1a reaches 80.5%,which indicates that the precipitation change in next year is mainly dominated by interannual change;From the res component,it can be concluded that the annual precipitation is generally in a downward trend.The periods of each component of spring precipitation are 3.8 a,8.6 a,17.3 a,34.5 A and 66 A,respectively;The component with a period of 3.8a fluctuates greatly;The component with a period of 8.6 years had a smaller amplitude before 1980 s,but increased after 1980s;The precipitation is generally in a downward trend.The component periods of summer precipitation are 2.9a,11.5a,17.3a,34.5a and 69 A,among which the component periods of 2.9a,11.5a and17.3a are relatively stable in recent decades;It can be seen from the trend component that there is a downward trend.The cumulative variance rate of IMF components with periods of 2a,6.9a and11.5a in autumn precipitation reaches 39.3%,which indicates that the precipitation is mainly determined by short periods,and the final trend component indicates that there is a downward trend in autumn precipitation.The cumulative variance rate of IMF components with periods of2.2A,6.3A and 17.3a reaches 27.9%;From the final res component,it can be seen that the winter precipitation shows a downward trend as a whole.There were two abrupt changes in annual precipitation in Ansai District.Through the M-K mutation test,it was concluded that the overall precipitation showed an upward trend after the first intersection of the curve in 1952,and the upward trend was significant in 1960,which passed the significance test.After the intersection of the second quadratic curve in 1992,the general trend was declining,and the abrupt time points were found in 1992 and 2000 by test.The M-K mutation test of spring precipitation showed that the precipitation fluctuated at every intersection of the UF curve.In 1990 s,there was an intersection point of the UF curve,and the overall precipitation showed a downward trend.According to the sliding t-test,the abrupt time point of spring precipitation was 1962.The abrupt change of summer precipitation in Ansai District occurred in 1995,after which the precipitation showed a decreasing trend.The M-K mutation test of autumn precipitation had multiple intersection points,and it was proved that the abrupt time points of autumn precipitation were1958 and 1998.The abrupt change of precipitation in winter was proved to be in 1975.(2)Annual precipitation in Ansai area experienced a lot of changes in the time scales of 3-5a and 8-15a;According to the wavelet square difference,there are four main periods of annual precipitation,which are 30 a,25a,10 a and 4a.There are four main cycles of spring precipitation,which are 29 a,18a,10 a and 5a respectively,and there are many alternating changes of spring precipitation around the main cycle.The summer precipitation has experienced several trend changes on the time scales of 3-5a,12-15 a and 21-28 a,and is still in the period of less rainfall,with the main periods of 25 a,14a and 4a,respectively.The fall precipitation experienced several changes in the time scales of 3-4a,9-12 a,14-18 a and 25-31 a,with the main periods of 29 a,16a,10 a and 3a,respectively.During the 6-10 year time scale,the winter precipitation fluctuated frequently,while during the 19-29 year time scale,the winter rainfall experienced several changes,with the main period of 24 a and 8a.(4)The extreme precipitation index of Ansai District in the past 40 years showed that most of the extreme precipitation indexes showed a decreasing trend,but with different changes.The number of continuous dry days was the largest in 1998,and the slope of the trend line of continuous dry days was 0.85 d·(10a)-1.The year with the highest number of persistently wet days was 2014,with a trend line slope of 0.39 d·(10a)-1 indicating a slight upward trend.The maximum number of days above moderate rain was 21 days and the minimum was 6 days,and the slope of the trend line was-0.86d·(10a)-1.On the whole,the number of days above heavy rain showed an obvious downward trend,with a slope of-0.65d·(10a)-1,which passed the significance test.The number of days above the rainstorm reached the maximum of 8 days,and its tendency slope was-0.46 d·(10a)-1.The year with the highest total anomalous precipitation was 2013,with a total anomalous precipitation of 400 mm.The climatic tendency slope of 1-day maximum precipitation and 5-day maximum precipitation were-0.16 d·(10a)-1 and-0.37 mm·d-1·(10a)-1,respectively.The average daily precipitation intensity showed a general downward trend,and its climatic trend slope was-0.32 mm·d-1·(10a)-1.(5)Obtained from the index correlation of extreme precipitation and extreme precipitation indexes except for dry days was significantly positive correlation with annual rainfall,and continuous dry days a low correlation with other indicators and annual precipitation,continuous wet days,at the same time,the four above moderate rain days,heavy precipitation total extreme precipitation index showed a negative correlation relationship.Most H values of extreme precipitation index are greater than 0.5,which indicates that the changes of these indexes will continue to maintain the current trend in the following period of time.