Studies On Precipitation Characteristics And Numerical Simulation Of Low Level Jet Effect During Precipitation In The Qilian Mountains

Low level jet refers to the region of high wind speed in the middle and low troposphere,which is one of the important influencing systems in the precipitation process.Especially for the precipitation process,the water vapor and unstable energy provided by it are very important for the occurrence and persistence of precipitation.However,for the plateau region,due to the complex topography,the precipitation process is affected by more factors.What is the role of the low-level jet stream in the process of precipitation in complex terrain at high latitudes? The research content of this paper will focus on this issue.In order to clarify the influence mechanism of low-level jet stream on precipitation in the Qilian Mountains,using the surface observation data obtained by the “Northwest Engineering Project” and the FNL reanalysis data from the National Center for Environmental Prediction(NCAR),the paper analyzed the spatial and temporal distribution characteristics of precipitation and low-level jet stream in the Qilian Mountains by combining the observation data analysis and numerical simulation.On this basis,the influence of low-level jet on precipitation in Qilian Mountains is studied.The following conclusions are obtained:(1)There are significant seasonal differences in precipitation in the Qilian Mountains.The precipitation in the wet season(from May to September)accounts for94.7% of the annual precipitation,and the precipitation in the mountainous area is greater than that in the surrounding area.(2)There are two types of low-level jet that affect the precipitation in the Qilian Mountains,namely,the northwest low-level jet and the southeast low-level jet.The northwest low-level jet is located between 700 h Pa and 850 h Pa,which is divided into two branches,one is located over Inner Mongolia and the other is located over Xinjiang.The former has greater influence range and intensity than the latter.The southeast low-level jet is mainly located at the height of 600 h Pa over the Min Mountain area,and its occurrence frequency,intensity and range are less than that of the northwest low-level jet.(3)The generation of the low level jet in northwest and southeast is affected by the subtropical high and low vortex in the west Pacific.Among them,the northwest low-level jet is located in the east of the northwest vortex,and the southeast low-level jet is located in the east of the southwest vortex.During the wet season,the low level jet in Xinjiang and Inner Mongolia migrated to the east.There is little difference between the intensity and range of the northwest low-level jet during the day and night,and the diurnal variation is small,and it is less affected by the inertial oscillation.However,the diurnal variation characteristics of the southeast low-level jet are significant,and it is stronger at night than during the day,which is consistent with the inertial oscillation mechanism of low-level jet formation.(4)The formation process of the typical southeast low-level jet and northwest low-level jet in the Qilian Mountains and their effects on the precipitation process in the Qilian Mountains are numerically simulated by using WRF model.The results show that the southeast low level jet that affects the precipitation process includes the southwest low level jet on the east side of Qinghai-Tibet Plateau and the southeast low level jet over Min Mountain.The outlet area of the southwest low level jet coincides with the entrance area of the southeast low level jet,and the precipitation area of the Qilian Mountain is just to the right of the outlet of the southeast low level jet.The southwest low-level jet carries the water vapor of low latitude into the southeast low-level jet,and the southeast low-level jet transports it to the precipitation area of the Qilian Mountains.In addition,the airflow over the precipitation area of the Qilian Mountains has clockwise shear,which is accompanied by convergence under the action of topography.Moreover,the cold air in the northwest vortex meets the warm air in the low level jet in the southeast,and the convective instability is enhanced,which is the favorable condition for precipitation,leading to the occurrence of precipitation in the Qilian Mountains.The northwest low-level jet that affects the precipitation process is generated by the coupling effect of the upper westerly jet,and there are deep troughs in both high and low levels.The northwest low-level jet stream has a larger range and stronger strength,and the jet stream axis bends northward,with obvious wind shear at the high and low levels,transporting water vapor in the northwest direction to the precipitation area.The precipitation area is located to the left of the entrance of the southeast low-level jet stream,and the convective instability is enhanced,which leads to the occurrence of precipitation.The above research results reveal the mechanism of the low-altitude jet to reduce water under the complex terrain conditions of the Qilian Mountain,and provide some scientific reference and theoretical basis for improving the precipitation prediction level of the Qilian Mountain.