| Qilian Mountain is located in the arid and semi-arid climate region of northwest China.Under the condition of increasing population and economic development,the restoration of ecological environment in Qilian mountain area is an urgent task.The problem of water shortage has become increasingly prominent,it has seriously hampered the sustainable development of economic and social in the region.To solve the problem of water resources shortage is the key step to realize the strategy of western development and the goal of "Belt and Road" initiative.It is one of the effective means to increase precipitation by weather modification project when the natural weather process can not provide enough precipitation.Based on the ERA5 data?precipitation data and ground conventional data from 1996 to 2016,the suitable conditions and influencing factors of artificial precipitation enhancement in Qilian Mountain region were analyzed.Including precipitation,atmospheric water content,water vapor flux,freezing level height(FLH),land surface temperature and so on,and the correlation between the variables as well.The main results are as follows:(1)There is an increasing trend of precipitation from 1996 to 2016 over Qilian Mountain.The temporal distribution of precipitation is affected by the movement of rain belt and monsoon.The precipitation is mainly concentrated in summer,and the order of the amount of precipitation is in summer > autumn > spring > winter.The difference of precipitation of the same month is more than 60 mm in different year.The spatial distribution of precipitation is affected by terrain and circulation factors,and the spatial distribution of precipitation in Qilian Mountain Area In summer,the highest precipitation area is located in the inner Qilian Mountains,and the lowest is located in the Golmud area in the southwest corner of the study area,the difference between the maximum and minimum precipitation is up to 401 mm.(2)The order of temporal variation of Precipitable Water Vapor(PWV)is summer > autumn > spring > winter,in which 95.6 mm can be achieved in July,the lowest in winter and only 14 mm in January.The spatial distribution of PWV is uneven,and the high value area is mainly located in the northeast of the study area.The high value area of PWV does not correspond to the high value area of precipitation one by one.The relative stability of Atmospheric Precipitable Water is that the difference of PWV under different precipitation types is not more than 1 mm.Therefore,PWV should not be used as the basis for judging whether it is possible to rain.(3)The transport of water vapor in the study area is mainly latitudinal,in which the west boundary is the main water vapor input direction,and the latitudinal water vapor budget is negative,the meridional water vapor budget is positive,the total water vapor budget is negative,and the water vapor input in the area is less than the output.The water vapor over the area of 100-103 °E and 36-37.5 °N is abundant in summer.In addition to the water vapor brought by the west wind,there is also the water vapor brought by the monsoon.(4)The annual mean height of the freezing level(FL)in summer is generally fluctuating and rising,but the annual trend of change in different months is not the same.The variation of FL height in some peak years is related to El Nino event.The height of FL changes in various time scales,reaching the maximum value of the whole year from July to August,with an average value of 5100 m.The increase is less than the decrease.The daily variation of FL height in each month is also a fluctuating rise(in spring)and a fluctuating decline(in autumn).This kind of cyclical change forms the annual change cycle of FL height.(5)The spatial distribution of FL height is not uniform due to the influence of complex terrain,and the distribution pattern is consistent with the trend of Qilian Mountain,that is,northwest southeast trend,which reflecting the characteristics of the terrain.The absolute height of FL corresponds well with the altitude distribution of the terrain,while the relative height is low in the high altitude area and high in the low altitude area.The absolute height of FL rises gradually from south to North in the south slope,while in the north slope,the change is opposite to the south slope,and the gradient of the north slope is larger than the south slope.(6)Surface temperature and air pressure are the main factors that affect the height of FL.The influence of surface thermal condition on the height of FL is more direct in the relatively low area(high altitude),which belongs to the short-range limited influence.The influence of surface temperature on the height of FL has a certain lag,the correlation coefficient of 2 hours behind can reach 0.8;the distribution of the surface pressure field is also related,mainly reflected in the low altitude area.By combining the two high correlation regions,the correlation coefficient can be more than 0.5,so they can be used as predictors to predict FL height.(7)There is a certain correlation between the height of FL and the u and v components of the surface wind in summer.The u component is mainly reflected in the western part of the region,while the v component has a greater impact on the middle and middle-east part of the region.Compared with the spatial distribution of regional precipitation,it is found that monsoon system is the main factor affecting the height of FL and regional precipitation in summer.Based on the information of regional meteorological factors,such as the spatial and temporal distribution and evolution of precipitation,Water Vapor Transport,mesoscale atmospheric pressure system and temperature field,and the correlation between these factors and the height of FL,a decision-making model for determining the suitable time of precipitation enhancement operation can be constructed to provide scientific reference for carrying out precipitation enhancement operation in Qilian Mountain. |
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