Statistical Analysis And Research On Water Vapor And Cloud Water Conditions In The Three River Headwaters Region

Using the satellite inversional data of hourly precipitation from CMORPH,the high temporal-spatial resolution data of monthly-average liquid cloud water,ice cloud water,and water vapor from ECMWF(ERA-Interim),and the NCEP / NCAR reanalysis data,etc.,all collected from 1998 to 2017,and the empirical orthogonal decomposition(EOF)method,Mann-Kendall(MK)mutation detection method,the wavelet analysis method,the singular value decomposition(SVD)method,and flux analysis and other relative methods are all used to comprehensively analyze the temporal and spatial distribution characteristics of liquid-phase cloud water and ice-phase cloud water in the air,and the relationship between water vapor transport and precipitation formation in the Sanjiangyuan region and its surrounding areas in the eastern Tibetan Plateau.The Sanjiangyuan region refers to the source area of three rivers,i.e.the Yangtze River,the Yellow River and the Lancang River,and it is known as "China's water tower".The main results are shown as follows:(1)The precipitation amount(PA),total water vapor amount(WVA)and total cloud water amount(CWA)over the Sanjiangyuan region have basically the same average climatic characteristics,and their spatial distributions are more in the east than in the west,and they all show an increasing trend with time.The spatial distributions of the fields show different phase changes,which are respectively the middle-fourth inverted phase type,the overall phase consistent type,and the northwest-southeast inverted phase type;the corresponding average fields of cloud water and precipitation have a obviously downward trend with time,while the average field of the water vapor is slightly rising with time.The analysis using the MK detection method indicates that the precipitation amount and total water vapor amount in summertime suddenly changed in 2000 and 2012,respectively,and the total liquid water path(LWP)in summertime changed suddenly in 1999,2006-2008,2011,and 2014,and the total ice water path(IWP)in summer underwent mutations in2002,2006-2008 and 2012.Through the analysis of wavelet analysis,it is found that during 1998-2005,the total summer precipitation has a significant interannual variation characteristic of a 3-10 year cycle,in which the main cycle is 6 years,and the amount of water vapor is 5-9 years.There are 3 oscillations on the scale;there are quasi-5 oscillations on the water vapor volume on the 2-4 year time scale.LWP appears quasi-two oscillations on the 4-9 year scale;IWP appears quasi-two oscillations on the 4-8 year scale.The oscillation cycle and main cycle of water vapor and precipitation are the most similar.The 6-year time scale is the main cycle of oscillation.The oscillation period and main period of LWP and IWP are the most similar,and the period oscillation with the time scale of about 5 years is the strongest.(2)The sources of water vapor in the Sanjiangyuan region mainly come from the water vapor conveyor belts both in the northwest direction and in the southwest direction,and thus the Sanjiangyuan locates in the "atmospheric water sink" area.Further through analyzing the stratified characteristics of horizontal water vapor flux and the profile characteristics of vertical water vapor flux,and diagnosing the effect of vertical velocity field on precipitation,it is found that the low-level water vapor south of the plateau transports north to the plateau area in the summer.The water vapor is forced lift in the steep south slope of the plateau and the horizontal transport of water vapor gradually weakens upward from the 600 h Pa level.The vertical section analysis of moisture along the longitude shows that a high relative humidity appears in the southern slope of the Tibetan Plateau,and the water vapor also has a forceful rise,accompanied by a strong vertical updraft.The vertical speed can cause the formation of cloud water in the Sanjiangyuan area.The confluence of the water vapor conveyor belts corresponds to the maximum value of precipitation,and the strength of the precipitation changes with the movement of the confluence of water vapor conveyor belts.The formation of precipitation in the Sanjiangyuan area is not only related to the water vapor transport in the surrounding area,but also related to the amount of cloud water in the area.Therefore,the causes of precipitation depends on the transport of water vapor and the formation of cloud water.(3)The SVD analyses of the precipitation field and water vapor field,the precipitation field and liquid cloud water field,the precipitation field and ice cloud water field,the water vapor field and liquid cloud water field,the water vapor field and ice cloud water field,the liquid cloud water field and ice cloud water field from the ECMWF ERA-Interim reanalyzed data indicate that the covariance contribution rate of the first mode of them all showed a fast convergence rate,and the correlation coefficients between the two meteorological fields(corresponding to the left and right fields respectively)that participated in the analysis all passed the significance test with ith significance level = 0.001.This reflects the main characteristics and coupling relationship between left and right fields.The analyses of the spatial distribution patterns and time coefficients of the first modes of the six couples of left and right fields show that the variation trend of the circulation variable fields is almost consistent with the trend of the precipitation fields,and they are all positively correlated,so the precipitation fields can be predicted by these factor fields.(4)The reasons for the heavier precipitation in the source area of the three rivers can be summarized as follows: The first is the topographic factors.The terrain in the southwest of the source area of the three rivers blocks the gradual transportation of water vapor to the north,resulting in the gradual weakening of the precipitation in the source area of the three rivers to the north.The second is the formation of cloud water.In places with a high vertical speed,a large amount of water vapor will be forced to rise to form cloud water over the places,and the cloud water will be converted into precipitation under certain physical conditions.Through MK method,it was found that both precipitation and total cloud water were decreasing in the last 20 years.Wavelet analysis indicated that the oscillation periods of precipitation and total water vapor were consistent with the main periods.The SVD method showed that the total water vapor field,total cloud water field and precipitation field were positively correlated.These results support the conclusion that water vapor and cloud water have important influence on precipitation in the Sanjiangyuan area.