Characteristic Classification Of Air Flow Field And Synoptic Background Analysis Of Typical Precipitation Process In Qilian Mountains

Social and economic development puts forward higher and higher requirements for ecological construction,water resources exploitation and sustainable utilization.As an important means of utilizing atmospheric water resources,weather modification has played a positive role in increasing rainfall and drought resistance,increasing water storage in rivers and lakes,and protecting ecological environment.In order to meet the needs of social development,it is urgent to strengthen the capacity construction of weather modification in northwest China.Before the shadow operation,it is very important to determine the timing and scope of rainfall enhancement,and the analysis of operating conditions is essential.Water vapor is the source of natural precipitation and artificial precipitation,and air flow is an important factor affecting water vapor transport and precipitation.The comprehensive observation of air flow field is particularly important for artificial precipitation operation,and the influence range can be predicted according to the change of air flow field.Therefore,before the implementation of shadow operation,it is necessary to have a full understanding of the airflow field.Qilian Mountain is an important ecological safety barrier in western China.In order to strengthen the construction of artificial rainfall demonstration area in Qilian Mountain and scientifically develop and utilize the cloud water resources in the air,the characteristics of air flow field and precipitation process in Qilian Mountain should be studied deeply.Based on ERA5 reanalysis data from January 2018 to December 2020 and a large number of observations,Meanwhile,the precipitation data of regional and cryptic stations over the Qilian Mountains and its surrounding areas from July 2019 to December 2020,as well as the divergence,vorticity,vertical velocity,cloud water content and water vapor flux divergence of ERA5 reanalysis data during the same period,were used to conduct characteristic classification of the high and low air flow fields over the Qilian Mountains.At the same time,the weather situation and physical quantity field variation characteristics of precipitation process are analyzed on the basis of air flow field classification.The research shows that:(1)The air flow field on the surface of Qilian Mountains and in the upper 500 h Pa can be divided into five types according to the air flow direction: southerly air flow type,northwest and north air flow type,southwest and west air flow type,convergence and divergence air flow type,northeast and east air flow type.The most frequent surface air field in the qilian Mountains is southwest and west-oriented air flow,accounting for 48.88% of the annual frequency.Among them,the frequency of northwest and northerly airflow and southerly airflow is the highest in May,convergence and divergence airflow and northeast and easterly airflow are the highest in August,and southwest and westerly airflow are the highest in December.The 500 h Pa air flow field with the highest frequency was the northwest and north air flow pattern,accounting for 46.70% of the whole year.Among them,the frequency of southwest and west-to-west airflow is the highest in January,convergence and divergence airflow is the highest in May,northeast and east-to-east airflow and south-to-south airflow is the highest in July,and northwest and north-to-north airflow is the highest in December.(2)In the precipitation process that occurs during the test,precipitation will occur in all the five surface air field types.Five types of airflow field of circulation situation there are four,respectively "westerlies much wave type","two ridges a trough type","two groove ridge" and "west and low in model",four are conducive to the precipitation of circulation situation: "westerlies wave type" cold eddy at the bottom of the short-wave trough carrying cold waves cause unstable stratification;Under the influence of high pressure ridge,cold air moved southward and accumulated in the middle part of Gansu province,forming unstable stratification with cold air at the top and warm air at the bottom.The positive vorticity advection in front of the "two-trough and one-ridge" southern branch is beneficial to the lifting of warm and humid air in the lower layer.The positive vorticity advection in front of the short-wave trough of "high in the west and low in the east" is conducive to the lifting of warm and humid air in the low layer.The physical quantity characteristics of the five surface air field types are as follows: the divergence field is characterized by convergence at low altitude and divergence at high altitude;The vorticity field is characterized by positive vorticity at low altitude and negative vorticity at high altitude.The vertical velocity and cloud water content are characterized by an ascending movement area and sufficient water vapor.The weather situation and the change of physical quantity field of convergence and divergence airflow precipitation with the highest frequency have a positive effect on the precipitation.Therefore,the amount of artificial precipitation can be increased in the qilian Mountains when the convergence and divergence airflow pattern appears.