| Precipitation over the Qilian Mountains is greatly affected by atmospheric circulation systems and local topography.Formation mechanism of precipitation in the region is not very clear because of the scarcity of observed data.For this,the observations collected by the northwest regional weather modification project-“Qilian Mountain Topographic Cloud Artificial Rain(Snow)Technology Research Experiment”during 2018-2019 were used to analysis on characteristics of precipitation and numerical simulations over Qilian Mountains.Considering the continuity of data,observations from June to September,2019 from the intensive surface stations were employed to analyze horizontal distribution characteristics of accumulated precipitation,diurnal variation characteristics of precipitation and other meteorological elements(e.g.,Temperature,relative humidity and 10m wind speed et.al),as well as multi-time scales characteristics of precipitation time series at the four meteorological stations over the Qilian Mountains.Subsequently,comparative studies of synoptic pattern were performed between two precipitation processes that occurred in the Qilian Mountains in summer 2018,and the formation mechanism of precipitation was analyzed by using numerical simulation experiments with WRF model(V3.8.1).The main conclusions are as follows:(1)Analyses of intensive observational data from June to September in 2019show that accumulated precipitation was distributed along the Hexi Corridor in a northwest-southeast direction over the Qilian Mountains,and the large values were located near the Qilian Mountains.There were two precipitation centers in the middle and eastern sections at the northeastern side of the mountain which located near Zhangye and Wuwei cities,Gansu Province respectively.The heavy precipitation center on the southwest side of the mountain was located in Qilian County and Menyuan Hui Autonomous County,Qinghai Province.This means that the formation and development of precipitation in this area is greatly affected by topography.(2)The Analysis results of observed data from the four meteorological stations that located in different regions of the Qilian Mountains(Minle Woma Station,Tianzhu Dachaigou Station,Menyuan Station and Qilian Station)show that precipitation at the four meteorological stations can occur in almost all periods of the day.The main precipitation period at Woma Station,Qilian Station and Menyuan Station is from night to early morning,with obvious night rain characteristics,and the peak of precipitation at Dachaigou Station is earlier than that at the other stations.Distribution of temperature and relative humidity in the four stations is unimodal,and characteristics of inversion phase between temperature and relative humidity are significant.(3)Analyses of multi-scale characteristics for the four stations precipitation time series with the EEMD method indicate that precipitation has closely relationship with weather systems whose time scale is about several hours and 1-2 days.The topographic cloud has significant diurnal variation characteristic,which means the development of topographic clouds plays an important role in precipitation at this region.Signal with a time scale of 3-7 days is more significant only for precipitation processes with larger rainfall and relatively long duration.It suggests that this type of precipitation processes is related to the evolution of synoptic-scale system,while it has little relation with synoptic-scale systems for precipitation processes with smaller rainfall.There are monthly-scale signals at the four stations precipitation time series,but their oscillation periods are slightly different.There are some differences in the trends for four stations time series according to the terms of RES.Precipitation at Qilian station shows a decreasing trend from June to September.Precipitation at Tianzhu,Dachaigou station shows a slow-increase tendency,which is opposite to the trend at Qilian station.(4)Two precipitation processes that occurred over the Qilian Mountains in summer 2018 belong to different types of precipitation processes,and there are obvious differences in characteristics of pattern and convective cloud clusters for the two precipitation processes.CASE1(July 19-20),a large-scale continuous heavy precipitation process,occurred under sufficient water vapor supply and favorable conditions of high-low-altitude circulation.During development process of CASE1,the convection of cloud clusters was strong,and the brightness temperature of cloud top was lower(below-48?),and precipitation had longer duration and larger amount as well as higher precipitation efficiency,which was caused by the combined results of favorable large-scale circulation features and local topography;while the precipitation process(CASE2)that occurred on August 28-29 was caused by the development of orographic clouds induced by local topography.The precipitation only occurs in the areas near the Qilian mountains.During this precipitation process,range of convective clouds is smaller and intensity is weaker than that of CASE1(the minimum brightness temperature of cloud top is-36?),and precipitation efficiency was relatively lower with a shorter duration.(5)Diagnostic analyses of the two types of precipitation cases over the Qilian Mountains using the 3 km resolution data from the WRFV3.8.1 model show that CASE1 had more water vapor supply than CASE2,and a stronger vertical upward movement,and larger cloud and rain water content centers are located in the middle and upper level of atmosphere.While the large value of cloud and rain content in CASE2 only appears in the maximum updraft area on the top of the mountain,and development height is relatively low.The maximum values of MCAPE,vertical wind shear,and the difference of potential pseudo-equivalent temperature between stratification are 1064.84J·kg-1,27.6m·s-1,36.15K at initial stage of precipitation of CASE1,while the corresponding maximum values of CASE2 are much smaller than those of CASE1,which are 546.15J·kg-1,17.37m·s-1,18.25K,respectively.The minimum value of Ri number in CASE1 is less than-1,while that in CASE2 is always greater than 0.It means there are obviously unstable atmospheric stratification,great vertical shear of the horizontal wind,strong convective motion,and a large amount of unstable energy which can be released in CASE1,which causes the formation of larger area and persistent precipitation process.However,the atmospheric stratification in CASE2 is relatively stable,vertical shear of the horizontal wind is relatively small,a weak convective precipitation process and precipitation rate is relatively smaller as well as and relatively short duration. |
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