Observation And Simulation Studies On The Influence Of Wind Shear Layers On Topographic Cloud Precipitation On The Northern Slopes Of The Central Tian Shan

The topographic cloud precipitation process in the Tianshan Mountains has a complex cloud system structure,and the local thermal conditions and vertical wind shear play a key role in the occurrence and development of the cloud precipitation process.In this paper,we use two combining observations of topographic cloud precipitation on July 27,2019 and January 14,2020,and combine them with mesoscale numerical model WRF simulations to analyze the influence of topographic cloud macrostructure characteristics,thermal instability structure and wind shear on the development of small-and medium-scale cloud masses on the northern slopes of Tianshan Mountains.The main conclusions are as following:(1)The precipitation cloud of 27 th,July,2019 is summer convective cloud precipitation process,The Ka-band cloud radar observation results show that there is a vertical wind shear at 4km,and there is a significant difference in the cloud structure above and below it.The radar reflectivity factor shows a bimodal type with height.With the combination of local convective cloud clusters and high-altitude cloud clusters under the guidance of North and South air currents,the distribution range of reflectivity factor narrows and the large value area of frequency distribution gradually moves down,indicating that the particles in the cloud grow and fall rapidly to form precipitation,after precipitation,the cloud cluster is limited below the wind shear layer and the cloud top is flat.The precipitation is mainly low-level warm cloud precipitation process,and the wind shear layer has an inhibitory effect on the precipitation.(2)January 14,2020 is the snowfall process of winter stratiform cloud.The observation results of Ka band Cloud Radar show that the middle and low layers are westerly,and there is a shallow easterly layer in the middle layer at 4km.The stratiform cloud is in a two-layer or three-layer structure,and the middle dry layer appears with easterly airflow.The structures in the upper and lower clouds of the windshear layer are similar,and the radar reflectivity factor shows a bimodal or three peak type with height.Compared with the convective clouds in summer,the peak changes at different heights of the stratus clouds in winter are smaller.When the snowfall is enhanced,the highlevel cloud spreads to the low-level cloud,and the reflectivity factor of the dry layer at4 km in the middle layer increases significantly,but the lack of supercooled water in the low-level cloud has no obvious effect on the enhancement of snowfall.When the west air flow in the upper layer is replaced by the East air flow,the water vapor transport decreases and the snowfall tend to end.(3)In the two observations,the north air flow in the middle and low layer is blocked and lifted by the Tianshan Mountains,and turns to the South air flow near the ridge at the height of 4km,forming a wind shear layer,which has an important impact on the development of precipitation clouds and the distribution of water condensates.In the summer case,the advection transport of the high-altitude southerly wind to the equivalent potential temperature makes the upper part of the windshear layer more prone to thermal instability and the lower part more prone to thermal stability,so as to suppress the low-level convection and promote the high-level convection.In winter,the weak updraft and gravity wave act together,which is conducive to the retention and accumulation of snow crystals in the middle layer over the north slope of Tianshan Mountain.