Research On The Vertical Structure And Macro-micro Characteristics Of Typical Cloud Precipitation Processes On The Qinghai-Tibet Plateau

The plateau vortex and shear line are not only the main precipitation system in the plateau,but also bring about the severe weather in the downstream areas.This paper uses the third generation of weather radar data millimeter-wave cloud radar data and raindrop spectrometer data from TIPEX-?,combined with other conventional data,a plateau vortex weather in Na Qu region from August 5 to 7,2015 and a plateau shear line weather from August 26 to 28,2015 were comprehensively analyzed,including macro-microscopic structure and evolution characteristics of the cloud-precipitation.The following conclusions were obtained:(1)The NaQu plateau vortex used in the study is caused by the blocking effect of the downstream high pressure.The Na Qu area has a low-pressure center and produces a relatively obvious cyclonic circulation.Later,the eastward movement has an impact on the downstream area.The case of the shear line is formed by the mid-high latitude low vortex splits the trough line and the trough line extends to the plateau area.Both cases are mixed precipitation processes caused by the weather system and accompanied by local thermal effects.Adequate water vapor transport and low-altitude convergence of high-altitude divergence vertical flow field structure provide favorable benefits for cloud and precipitation generation.At night,the depression of dew point is low and the water vapor saturation is high.Most cloud precipitation processes of two cases occur at night.In addition,the plateau vortex system has a stable water vapor and dynamic conditions during the day on August 6 and maintains stable precipitation for 8 hours.(2)From the macroscopic structure,the formation of a distinct vortex cloud system of the plateau vortex can be seen in the satellite image and affect the Na Qu area.In weather radars,the plateau vortex process evolves from weak convective clouds to cumulo-stratus mixed clouds.In the stage of convective clouds,the echoes are mostly sparsely distributed in a small area with a short duration of only a few hours.In the mixed cloud stage,the echoes are dominated by neat slices,which contain scattered small-scale convective monomers.The area increases and lasts for more than a dozen hours.In the case of the shear line,there are occasional small-scale convective clouds appearing on the satellite cloud map near the shear line.In the weather radar,the cloud system is always dominated by mixed clouds.Unlike the plateau vortex,in the process of shear line,there are often multiple scattered convective cells on the cumulo-stratus mixed clouds.There is corresponding upwind zones which position correspond to the shear line.Compared with the plain area,the local convection of plateau area in the two cases is not strong at the midday time(12:00-14:00).At 15:00-20 hours,scattered small-scale convective monomers begin to appear and gradually develop,after the night,the convective systems begin to weaken.(3)From the perspective of microscopic structure,in the millimeter-wave cloud radar,in the early stage of two types weather processes,the intensity echoes show the characteristics of convective clouds,and the cloud-top with "fire-like" is concentrated in the 10-12 km,which means a strong ascending movement.There is a large value area of depolarization ratio above about 1km from ground,showing the location of the melting layer.It makes up for the lack of a zero-thick layer bright bands in the convective weather and can not judge the melting layer.The atmosphere above the melting layer is characterized by a strong ascending motion.The positive velocity zones in the cloud are mostly strip-like or thin-belt,and the maximum positive velocity is about 81-sm×.Intense air movement causes the roll movement of particles,which makes the orientation advantage of the particles weakens and the depolarization ratio is small.There is a large value area of about 1.21-sm×in the spectral width,indicating that the turbulent motion is strong.The precipitation duration is short in this stage,the raindrop spectrum is widely distributed(0.5-4.2mm),and the concentration of small raindrops(<2mm)is large.At the end of the development of the weather system,the convective movements was significantly weakens.The depolarization ratio corresponding to the melting layer appears the large value area above 1km far from the ground,and the zero-level bright band appeared on the intensity echo.Below the melting layer,the reflectivity factor is reduced due to the breakage of the raindrops and the increase in the velocity of falling particles.At this stage,the precipitation in Na Qu area is relatively stable,with a long duration,and the distribution of raindrops is narrow,concentrates between 0.5-3.0mm,and the proportion of small raindrops to total raindrops is still large.Comparing the cloud radar and raindrop spectral characteristics of two weather cases,compared with the plateau vortex process,the intensity of the reflectivity factor of the shear line is significantly higher,indicating that the number of particles per unit area in the cloud is large.The maximum instantaneous rain intensity of convective precipitation induced by the shear line(28.61hmm-×)is much larger than that of the plateau vortex(9.891hmm-×).It is related to the high unstable energy in the process of shear line.The intensity of stable precipitation is similar,maintains within 41hmm-×.In addition,the plateau vortex has a significant plume structure in the cloud top,indicating that the cloud top has a strong “chucking effect”,and the cloud is involved in the outside air or loses the internal updraft.In the case of shear line,there are several small-scale systems transit,which correspond to the influence of several continuous convective cells at the shear line position on the weather radar,and bring several changes of precipitation.(4)There are significant differences in radar echo structure and precipitation characteristics between different types of weather processes and stages in the Nagqu area of the Qinghai-Tibet Plateau.Therefore,this study can help to understand the formation mechanism of plateau clouds and precipitation,on another hand it is beneficial to test and improve the simulation process of cloud and precipitation in the plateau region,and provide reference for the development of numerical models.