Analysis Of Cloud Characteristics And The Relationship To The Precipitation In The Semi-arid Area Around Qilian Mountains

Clouds are the water vapor regulators in the atmosphere and the main object of weather modification operations.The arid and semi-arid regions of northwest China have limited precipitation and water resources,which have constrained the sustainable development of the regional economy.Studying the physical characteristics of clouds in semi-arid areas and the relationship between cloud characteristics and precipitation is of great significance for the scientific development and utilization of cloud water resources in the air,and for alleviating the water resource shortage in the region.Based on the CL51 laser ceilometer data,ERA5 reanalysis data,and conventional ground observation precipitation data,from 2019 to 2021 the spatiotemporal variation characteristics of cloud physical parameters in the semi-arid area around Qilian Mountains were analyzed,with a focus on the vertical structure characteristics of clouds and their temporal variations characteristics in Wushaoling region.At last,typical precipitation cases were selected to discuss and analyze the changes in cloud characteristics during precipitation events,as well as the relationship between cloud parameters and precipitation.The main results are as follows:(1)The regional average values of cloud base height,high cloud cover,medium cloud cover,low cloud cover,total cloud cover,cloud liquid water content,and cloud ice water content in the study area(35°N-39°N,95°E-105°E)are 2504.9 m,30.82%,37.16%,16.38%,52.49%,19.56 g/m~2 and 24.73 g/m~2,respectively.The height of cloud base develops higher in spring,and there are also more medium and high cloud cover in spring.In summer,the cloud water content is relatively abundant,with the highest levels of low and total cloud cover,while in winter,it is the opposite.(2)The central region of Gansu in the southeastern part of the study area and the area near Wushaoling are characterized by high cloud cover,low cloud base height,and abundant cloud water content.The high value areas of cloud base height in four seasons are located near Minqin and the Qaidam Basin;The high value areas of high cloud cover are located in the southeastern part of the study area and the Qaidam Basin;The high value area of medium cloud cover is located in the southwest of the study area;The low value area of low cloud cover is located near Minqin and the Qaidam Basin,with an additional low value area in winter,located in the southern part of the study area and the vicinity of Gangcha;The high value area of total cloud cover is located in the southernmost part of the study area;The high value area of cloud liquid water content is located in the southern part of Gansu Province;The high value area of cloud ice water content is located in the southeast of the study area.There are certain differences in the spatial distribution of cloud parameters in different seasons,and winter is the season with the greatest variations in the location and numerical values of high and low value areas of various cloud parameters.(3)The frequency with which multi-layer clouds occur in the Wushaoling area is much lower than that of single layer clouds.The frequency of cloud occurrence in a day is highest around 20:00 and lowest around 11:00.The cloud base height of single-layer clouds is mainly distributed between 0-4 km in the vertical direction,and the frequency of medium and low clouds is higher in the afternoon and night,while it is lower in the morning;The frequency of high clouds is relatively low and mainly occurs at night.For multi-layer cloud systems,the frequency of low clouds is the highest in the lower layer,and the frequency of medium clouds in the middle and upper layers is the highest.The frequency of low and medium clouds is higher during the day and lower at night;High clouds mainly appear at night and early morning.From the perspective of seasonal changes,single-layer clouds have the highest cloud base height in summer and the lowest in spring.Among double-layer clouds,the cloud base height is higher in summer and winter,and lower in spring and autumn.The cloud base height of each layer of three-layer clouds reaches its maximum in summer.(4)Several cases study of typical cloud precipitation at Wushaoling Station reveals that there is a certain correspondence and indicative significance between changes in cloud parameters and precipitation.Before precipitation occurs,cloud cover,cloud water content increases,and cloud base height decreases.During precipitation,the frequency of multi-layer clouds increases,the cloud base height is lower,and the total cloud cover is at a higher level,with varying degrees of fluctuations in high,medium,and low cloud cover;There are significant fluctuations in cloud liquid water content and cloud ice water content.The variation of cloud types over time during precipitation process can reflect the generation and disappearance of precipitation cloud systems.During precipitation process,the fitting effect between cloud parameters and precipitation is good in the process of heavy precipitation,with the highest correlation coefficient between cloud liquid water content and precipitation.